This file was created by the TYPO3 extension publications --- Timezone: CEST Creation date: 2026-04-23 Creation time: 19:23:57 --- Number of references 160 article Scuderi2022290 Observations of slow earthquakes and tremor have raised fundamental questions about the physics of quasi-dynamic rupture and the underlying fault zone processes. The presence of serpentinite at P-T conditions characteristic of deep tremor and slow earthquakes suggests that it plays an important role in controlling complex fault slip behaviour. Here, we report on experiments designed to investigate the frictional behaviour of serpentinite sampled from outcrop exposures (SO1 and SO2) of altered ultramafic rocks present at depth, and recovered from the SAFOD borehole (G27). XRD analyses reveal the presence of chrisotyle, lizardite, kaolinite, talc in SO1; lizardite, clinochlore and magnetite in SO2; and lizardite, quartz and calcite in G27. We sheared fault gouge in a double-direct shear configuration using a true triaxial deformation apparatus. The effective normal stress was varied from 2 to 40 MPa. We conducted velocity stepping tests and slide-hold-slide (SHS) tests in each experiment to characterize frictional stability and healing. At the end of each experiment, post-shear permeability was measured and the samples were recovered for microstructural analysis. The steady-state friction coefficient was μ = 0.17 for SO1, μ = 0.33 for SO2 and μ = 0.53 for G27. Overall, the gouges exhibit velocity strengthening behaviour, and become nearly velocity neutral at 40 MPa effective normal stress. SHS tests show positive healing rates for SO2 and G27, whereas SO1 exhibits zero or negative healing rates. Permeability decreases with increasing σn', with SO1 (k = 10-20 m2) showing the lowest values. Microstructural observations reveal a well-developed R-Y-P fabric in SO1, which is not observed in SO2 and G27. We posit that the development of shear fabric controlled by mineralogy governs frictional and hydrological properties. In this context, when serpentinite is associated with other weak phyllosilicate minerals, frictional stability and hydrological properties can vary greatly, with a potential control on the mode of fault failure. © 2022 The Author(s) 2022. Published by Oxford University Press on behalf of The Royal Astronomical Society. 2022 English 0956540X 10.1093/gji/ggac188 Geophysical Journal International 231 Oxford University Press 290-305 Department of Geosciences and Energy, Institute Center for Geomechanics Geofluids and Geohazards, Pennsylvania State University, University Park, PA 16802, United States; Dipartimento di Scienze della Terra, Università degli studi La Sapienza, Rome, 00185, Italy; School of Geosciences, University of Oklahoma, Norman, OK 73019, United States 1 Calcite; Earthquakes; Elasticity; Fault slips; Kaolinite; Magnesite; Magnetite; Shear flow, Effective normal stress; Fault gouge; Fault zone; Fault zone rheology; Frictional stability; Hydrological properties; Lizardite; Permeability and porosities; Rheology and friction of fault zones; Serpentinite, Friction, deformation mechanism; earthquake rupture; experimental study; fault gouge; fault slip; fault zone; friction; hydromechanics; microstructure; mineralogy; P-T conditions; permeability; phyllosilicate; porosity; rheology; serpentinite; structural control; triaxial test https://www.scopus.com/inward/record.uri?eid=2-s2.0-85133665960&doi=10.1093%2fgji%2fggac188&partnerID=40&md5=0c91d73e37b7e7e77d3535efcb102612 cited By 1 M.M. Scuderi B.M. Carpenter article Li2021261 In this article, we review our previous research for spatial and temporal characterizations of the San Andreas Fault (SAF) at Parkfield, using the fault-zone trapped wave (FZTW) since the middle 1980s. Parkfield, California has been taken as a scientific seismic experimental site in the USA since the 1970s, and the SAF is the target fault to investigate earthquake physics and forecasting. More than ten types of field experiments (including seismic, geophysical, geochemical, geodetic and so on) have been carried out at this experimental site since then. In the fall of 2003, a pair of scientific wells were drilled at the San Andreas Fault Observatory at Depth (SAFOD) site; the main-hole (MH) passed a ~200-m-wide low-velocity zone (LVZ) with highly fractured rocks of the SAF at a depth of ~3.2 km below the wellhead on the ground level (Hickman et al., 2005; Zoback, 2007; Lockner et al., 2011). Borehole seismographs were installed in the SAFOD MH in 2004, which were located within the LVZ of the fault at ~3-km depth to probe the internal structure and physical properties of the SAF. On September 28 2004, a M6 earthquake occurred ~15 km southeast of the town of Parkfield. The data recorded in the field experiments before and after the 2004 M6 earthquake provided a unique opportunity to monitor the co-mainshock damage and post-seismic heal of the SAF associated with this strong earthquake. This retrospective review of the results from a sequence of our previous experiments at the Parkfield SAF, California, will be valuable for other researchers who are carrying out seismic experiments at the active faults to develop the community seismic wave velocity models, the fault models and the earthquake forecasting models in global seismogenic regions. © The Seismological Society of China and Institute of Geophysics, China Earthquake Administration 2021. 2021 English 16744519 10.29382/eqs-2021-0014 Earthquake Science 34 Earthquake Science 261-285 Department of Earth Sciences, University of Southern California, Los Angeles, 90089, United States 3 https://www.scopus.com/inward/record.uri?eid=2-s2.0-85117065096&doi=10.29382%2feqs-2021-0014&partnerID=40&md5=f07d4bb6af51e95768e69f70d41816ce cited By 2 Y.-G. Li article Chavarria2021770 Vertical seismic profiling (VSP) technology is increasingly being used in the fields of earthquake seismology and tectonics. This is motivated in part by the growing number of oil field microseismic monitoring surveys, but more so by projects that involve drilling deep wells for monitoring crustal activity at depth. Examples of these projects are the San Andreas Fault Observatory at Depth (SAFOD), the Nankai Trough Seismogenic Zone Experiment, the Gulf of Corinth Rift Laboratory, and the Taiwan Chelungpu Fault Drilling Project, and other projects by the International Continental and Ocean Drilling Programs (ICDP and IODP). These projects require instrumentation and surveying in deep and possibly hot borehole environments. With higher resolution than surface seismic data, images from 2D and 3D VSP data contribute to better characterization and interpretation of complex reservoirs at smaller scales. The location of receivers in the low-noise borehole environment yields higher signal-to-noise ratios, higher frequency content due to less detrimental propagation effects from the overburden, and direct correlation of data at seismic frequencies with well logs. © 2007 Society of Exploration Geophysicists. 2021 English 1070485X 10.1190/1.2748495 The Leading Edge 26 Society of Exploration Geophysicists 770-776 Paulsson Geophysical Services (P/GSI), Brea, CA, United States 6 Boreholes; Earthquakes; Monitoring; Oil fields; Oil well drilling; Tectonics, Earthquake seismology; Fault zone; Oil field microseismic monitoring surveys; Vertical seismic profiling (VSP), Seismology, borehole; fault zone; Ocean Drilling Program; oil field; San Andreas Fault; seismic data; seismology; vertical seismic profile, Gulf of Corinth; Ionian Sea; Mediterranean Sea; Nankai Trough; Pacific Ocean https://www.scopus.com/inward/record.uri?eid=2-s2.0-84995345824&doi=10.1190%2f1.2748495&partnerID=40&md5=0c7a2416381e9b7b633371c4b9911451 cited By 13 A.J. Chavarria A. Goertz M. Karrenbach B. Paulsson P. Milligan V. Soutyrine A. Hardin D. Dushman L. LaFlame article Holmes2021 Core samples obtained from scientific drilling could provide large volumes of direct mi-crostructural and compositional data, but generating results via the traditional treatment of such data is often time-consuming and inefficient. Unifying microstructural data within a spatially referenced Geographic Information System (GIS) environment provides an opportunity to readily locate, visual-ize, correlate, and apply remote sensing techniques to the data. Using 26 core billet samples from the San Andreas Fault Observatory at Depth (SAFOD), this study developed GIS-based procedures for: 1. Spatially referenced visualization and storage of various microstructural data from core billets; 2. 3D modeling of billets and thin section positions within each billet, which serve as a digital record after irreversible fragmentation of the physical billets; and 3. Vector feature creation and unsuper-vised classification of a multi-generation calcite vein network from cathodluminescence (CL) imagery. Building on existing work which is predominantly limited to the 2D space of single thin sections, our results indicate that a GIS can facilitate spatial treatment of data even at centimeter to nanometer scales, but also revealed challenges involving intensive 3D representations and complex matrix transformations required to create geographically translated forms of the within-billet coordinate systems, which are suggested for consideration in future studies. © 2021 by the authors. Licensee MDPI, Basel, Switzerland. 2021 English 22209964 10.3390/ijgi10050332 ISPRS International Journal of Geo-Information 10 MDPI AG Department of Geography and Geosciences, University of Louisville, Louisville, KY 40208, United States 5 https://www.scopus.com/inward/record.uri?eid=2-s2.0-85107208468&doi=10.3390%2fijgi10050332&partnerID=40&md5=69c6d68479ca89fe7c4e7f3e0b60547e cited By 0 E.M. Holmes A.E. Gaughan D.J. Biddle F.R. Stevens J. Hadizadeh article Chang2020 To better illuminate structural discontinuities around the San Andreas Fault Observatory at Depth (SAFOD) site, following the previous study of Zhang et al. (2009), we have extended the Generalized Radon Transform (GRT) method to jointly use scattered P and SH waves from abundant local earthquakes recorded by a local dense seismic network. A hybrid imaging condition is applied to extract consistent structures from scattered P and SH images. In this way, more robust structure information can be determined from separate images with potential artifacts. For the transverse component waveforms, coherent S-S scattered waves after the direct SH waves can be clearly identified, and they are less interfered with by other scattered and converted waves compared to scattered P-P waves on the vertical component. Similar to Zhang, Wang, et al. (2009), near-vertical reflectors are imaged on both sides of the San Andreas Fault (SAF) with scattered SH waves, which is generally consistent with the imaging results using scattered P-P waves from local earthquakes and a wide-angle active seismic reflection profile. Compared to the P-P scattering imaging result, the imaging result using scattered SH waves has higher resolution due to shorter S wavelength and cleaner S-S scattered waveforms, and the SAF, as well as other reflectors around it, is better resolved. Although the resolution of the joint image obtained by combining separate imaging results from different scattered waves may be degraded, it is able to more robustly characterize strong structure discontinuities using passive seismic sources. © 2020. American Geophysical Union. All Rights Reserved. 2020 English 21699313 10.1029/2019JB019017 Journal of Geophysical Research: Solid Earth 125 Blackwell Publishing Ltd Laboratory of Seismology and Physics of Earth Interior, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, China; Mengcheng National Geophysical Observatory, University of Science and Technology of China, Hefei, China; CAS Center for Excellence in Comparative Planetology, University of Science and Technology of China, Hefei, China 9 P-wave; Radon transform; San Andreas Fault; seismic source; seismic tomography; SH-wave; structural geology; wave scattering, California; Parkfield; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-85091536384&doi=10.1029%2f2019JB019017&partnerID=40&md5=6132529ff74e50428f517f4d0dd86ee4 cited By 0 K. Chang H. Zhang article Yang2019212 In situ stress measurement at seismogenic depth is critically important for deciphering fault zone processes. In this study, we conducted a second active-source crosswell field experiment at the Parkfield San Andreas Fault Observatory at Depth (SAFOD) drill site to investigate the detectability of stress-induced seismic velocity changes at the top part of the seismogenic zone. We employed the same configuration of our previous experiments, which deployed a piezoelectric source and a three-component (3C) accelerometer at 1 km deep inside the pilot and main holes, respectively. We also added a hydrophone, which is attached to the source, to monitor the repeatability of the source waveforms. Over a 40-day recording period, we confirmed an ∼0:04% travel-time variation in S wave and coda that roughly follows the fluctuation of barometric pressure. We attributed this correlation to stress sensitivity of seismic velocity and the stress sensitivity is estimated to be 2:0 × 10 −7 Pa −1 , which is approximately two orders of magnitude higher than those measured in laboratory with dry rock samples, but is consistent with our previous results. Our results confirm the hypothesis that substantial cracks and/or pore spaces exist at seismogenic depths and thus may be used to monitor the subsurface stress field with active-source crosswell seismic. © 2019 Seismological Society of America. All Rights Reserved. 2019 English 08950695 10.1785/0220180080 Seismological Research Letters 90 Seismological Society of America 212-218 Department of Earth, Environmental, and Planetary Sciences, Rice University, 6100 Main Street, Houston, TX 77005, United States; Earth Science Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, United States; Berkeley Seismological Laboratory, University of California, 219 McCone Hall, Berkeley, CA 94720, United States 1 Atmospheric pressure; Seismic waves; Seismology; Shear waves; Stresses; Strike-slip faults, Barometric pressure; Continuous measurements; In-situ stress measurement; Orders of magnitude; San Andreas fault; Seismic velocities; Stress sensitivity; Sub-surface stress field, Traffic control, fault zone; in situ measurement; in situ stress; San Andreas Fault; seismic velocity; seismic zone; seismology; travel time; waveform analysis, California; Parkfield; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-85062921101&doi=10.1785%2f0220180080&partnerID=40&md5=697d56a43548215184f4470c4e6c9023 cited By 2 C. Yang F. Niu T.M. Daley T. Taira article Lellouch20196931 Structural imaging and event location require an accurate estimation of the seismic velocity. However, active seismic surveys used to estimate it are expensive and time-consuming. During the last decade, fiber-optic-based distributed acoustic sensing has emerged as a reliable, enduring, and high-resolution seismic sensing technology. We show how downhole distributed acoustic sensing passive records from the San Andreas Fault Observatory at Depth can be used for seismic velocity estimation. Using data recorded from earthquakes propagating near-vertically, we compute seismic velocities using first-break picking as well as slant stack decomposition. This methodology allows for the estimation of both P and S wave velocity models. We also use records of the ambient seismic field for interferometry and P wave velocity model extraction. Results are compared to a regional model obtained from surface seismic as well as a conventional downhole geophone survey. We find that using recorded earthquakes, we obtain the highest P wave model resolution. In addition, it is the only method that allows for S wave velocity estimation. Computed P and S models unravel three distinct areas at the depth range of 50-750 m, which were not present in the regional model. In addition, we find high VP/VS values near the surface and a possible VP/VS anomaly about 500 m deep. We confirm its existence by observing a strong S-P mode conversion at that depth. ©2019. American Geophysical Union. All Rights Reserved. 2019 English 21699313 10.1029/2019JB017533 Journal of Geophysical Research: Solid Earth 124 Blackwell Publishing Ltd 6931-6948 Department of Geophysics, Stanford University, Stanford, CA, United States; Now at Earthquake Research Institute, The University of Tokyo, Tokyo, Japan 7 acoustic survey; borehole geophysics; estimation method; geophone; P-wave; S-wave; San Andreas Fault; seismic velocity, California; San Andreas; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-85069939400&doi=10.1029%2f2019JB017533&partnerID=40&md5=ab4848f914466d2ea7532baaee9dbf79 cited By 40 A. Lellouch S. Yuan Z. Spica B. Biondi W.L. Ellsworth article Hadizadeh201814 Previous studies of the San Andreas Fault damage-zone samples from the San Andreas Fault Observatory at Depth (SAFOD) have identified a variety of tectonic microstructures including pressure solution cleavage, calcite-sealed fractures vein fabric, and pyrite and anhydrite hydrothermal fracture sealing. Understanding the deformation provenance of the damage zone rocks and operative deformation mechanism(s) based on preserved microstructures provide insight into overall deformation behavior of the entire seismogenic zone in the creeping section of this transform fault. We analysed the deformation of hydrothermal secondary pyrite in connection with network of calcite veins in a sample of foliated ultracataclasites bordering the actively creeping Southwestern Deforming Zone (SDZ), using SEM, EBSD and CL microscopy. The results show that calcite veins associated with the pressure solution cleavage are crosscut by the secondary pyrite deformed under a range of P-T conditions. Relatively undeformed secondary pyrite is found sealing implosion microbreccia. Our review of previously available data indicates that the damage zone rocks may represent a collage of structural and compositional domains from both locked and creeping sections of the SAF. This interpretation together with results of this study suggest that weak-clay frictional deformation mechanism(s) is likely to be the predominant aseismic creep mechanism at depths below the SAFOD. © 2018 2018 English 01918141 10.1016/j.jsg.2018.09.005 Journal of Structural Geology 117 Elsevier Ltd 14-26 Department of Geography & Geosciences, University of Louisville, Louisville, KY 40292, United States; Department of Earth Ocean & Ecological Sciences, University of Liverpool, Liverpool, United Kingdom Calcite; Microstructure; Pyrites; Strike-slip faults; Structural geology; Transform faults, Aseismic creep in the SAF; Calcite veins; Creep deformation mechanisms; Deformation behavior; Deformation mechanism; Pressure solution; SAFOD; Seismogenic zones, Creep, calcite; cathodoluminescence; creep; deformation mechanism; pressure solution; pyrite; vein (geology) https://www.scopus.com/inward/record.uri?eid=2-s2.0-85053423219&doi=10.1016%2fj.jsg.2018.09.005&partnerID=40&md5=f952112ec8c1631e9c0b15f84a4c9828 cited By 2 J. Hadizadeh A.P. Boyle article Moore20184515 An exposure of a creeping segment of the Bartlett Springs Fault (BSF), part of the San Andreas Fault system in northern California, is a ~1.5-m-wide zone of serpentinite-bearing fault gouge cutting through Late Pleistocene fluvial deposits. The fault gouge consists of porphyroclasts of antigorite serpentinite, talc, chlorite, and tremolite-actinolite, along with some Franciscan metamorphic rocks, in a matrix of the same materials. The Mg-mineral assemblage is stable at temperatures above 250–300 °C. The BSF gouge is interpreted to have been tectonically incorporated into the fault from depths near the base of the seismogenic zone and to have risen buoyantly to the surface where it is now undergoing right-lateral displacement. The ultramafic-rich composition, frictional properties, and inferred mode of emplacement of the BSF serpentinitic gouge correspond to those of the creeping traces of the San Andreas Fault identified in the SAFOD (San Andreas Fault Observatory at Depth) drill hole. This suggests a common origin for creep at both locations. A tectonic model for the source of the ultramafic-rich materials in the BSF is proposed that potentially could explain the distribution of creep throughout the northernmost San Andreas Fault system. Published 2018. This article is a US Government work and is in the public domain in the USA. 2018 English 02787407 10.1029/2018TC005307 Tectonics 37 Blackwell Publishing Ltd 4515-4534 U. S. Geological Survey, Menlo Park, CA, United States 12 Creep; Kaolinite; Magnesite; Metamorphic rocks; Serpentine; Strike-slip faults, Fault creep; Metasomatic rocks; SAFOD; San Andreas fault; Serpentinite, Structural geology, comparative study; data interpretation; displacement; emplacement; fault gouge; San Andreas Fault; seismic hazard; seismic zone; serpentinite, California; San Andreas; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-85058217106&doi=10.1029%2f2018TC005307&partnerID=40&md5=6527f968b3267ed5fda3ef6063d6afb5 cited By 4 D.E. Moore R.J. McLaughlin J.J. Lienkaemper article Kaneko2017162 Recent laboratory shear-slip experiments conducted on a nominally flat frictional interface reported the intriguing details of a two-phase nucleation of stick-slip motion that precedes the dynamic rupture propagation. This behavior was subsequently reproduced by a physics-based model incorporating laboratory-derived rate-and-state friction laws. However, applying the laboratory and theoretical results to the nucleation of crustal earthquakes remains challenging due to poorly constrained physical and friction properties of fault zone rocks at seismogenic depths. Here we apply the same physics-based model to simulate the nucleation process of crustal earthquakes using unique data acquired during the San Andreas Fault Observatory at Depth (SAFOD) experiment and new and existing measurements of friction properties of SAFOD drill core samples. Using this well-constrained model, we predict what the nucleation phase will look like for magnitude ∼2 repeating earthquakes on segments of the San Andreas Fault at a 2.8 km depth. We find that despite up to 3 orders of magnitude difference in the physical and friction parameters and stress conditions, the behavior of the modeled nucleation is qualitatively similar to that of laboratory earthquakes, with the nucleation consisting of two distinct phases. Our results further suggest that precursory slow slip associated with the earthquake nucleation phase may be observable in the hours before the occurrence of the magnitude ∼2 earthquakes by strain measurements close (a few hundred meters) to the hypocenter, in a position reached by the existing borehole. ©2016. American Geophysical Union. All Rights Reserved. 2017 English 00948276 10.1002/2016GL071569 Geophysical Research Letters 44 Blackwell Publishing Ltd 162-173 GNS Science, Lower Hutt, New Zealand; School of Geology and Geophysics, University of Oklahoma, Norman, OK, United States; Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy; Department of Earth Sciences, University of Durham, Durham, United Kingdom 1 Core drilling; Drills; Faulting; Friction; Geophysics; Laboratories; Nucleation; Numerical models; Phase meters; Plates (structural components); Slip forming; Stick-slip; Strike-slip faults; Tectonics; Tribology, Earthquake nucleation; Friction parameters; Frictional interface; Physics-based modeling; Rate and state friction; Repeating earthquake; Slow rupture front; Two-phase nucleation, Earthquakes, active fault; earthquake hypocenter; earthquake magnitude; earthquake rupture; experimental study; fault zone; friction; laboratory method; nucleation; numerical model; San Andreas Fault https://www.scopus.com/inward/record.uri?eid=2-s2.0-85010661370&doi=10.1002%2f2016GL071569&partnerID=40&md5=cd009227ad9af728a6846f09735f5306 cited By 7 Y. Kaneko B.M. Carpenter S.B. Nielsen article Phillips20175789 The San Andreas Fault Observatory at Depth (SAFOD) drilling project directly sampled a transitional (between creeping and locked) segment of the San Andreas Fault at 2.7 km depth. At the site, changes in strain rate occur between periods of coseismic slip (&gt;10−7 s−1) and interseismic creep (10−10 s−1) over decadal scales (~30 years). Microstructural observations of core retrieved from the SAFOD site show throughgoing fractures and gouge-rich cores within the fractures, evidence of predominantly brittle deformation mechanisms. Within the gouge-rich cores, strong phases show evidence of deformation by pressure solution once the grain size is reduced to a critical effective grain size. Models of pressure solution-accommodated creep for quartz-phyllosilicate mixtures indicate that viscous weakening of quartz occurs during the interseismic period once a critical effective grain size of 1 μm is achieved, consistent with microstructural observations. This causes pronounced weakening, as the strength of the mixture is then controlled by the frictional properties of the phyllosilicate phases. These results have pronounced implications for the internal deformation of fault zones in the shallow crust, where at low strain rates, deformation is accommodated by both viscous and brittle deformation mechanisms. As strain rates increase, the critical effective grain size for weakening decreases, localizing deformation into the finest-grained gouges until deformation can no longer be accommodated by viscous processes and purely brittle failure occurs. ©2017. American Geophysical Union. All Rights Reserved. 2017 English 21699313 10.1002/2016JB013828 Journal of Geophysical Research: Solid Earth 122 Blackwell Publishing Ltd 5789-5812 Department of Earth Sciences, University of New Brunswick, Fredericton, NB, Canada; Department of Earth and Planetary Sciences, McGill University, Montréal, QC, Canada 7 brittle deformation; coseismic process; creep; crust; deformation mechanism; fault gouge; grain size; phyllosilicate; pressure; San Andreas Fault; slip; strain rate; strength https://www.scopus.com/inward/record.uri?eid=2-s2.0-85027843395&doi=10.1002%2f2016JB013828&partnerID=40&md5=56028a704278d579d2606b77b09a0d3c cited By 5 N.J. Phillips J.C. White article Sutherland2017137 Temperature and fluid pressure conditions control rock deformation and mineralization on geological faults, and hence the distribution of earthquakes. Typical intraplate continental crust has hydrostatic fluid pressure and a near-surface thermal gradient of 31 ± 15 degrees Celsius per kilometre. At temperatures above 300-450 degrees Celsius, usually found at depths greater than 10-15 kilometres, the intra-crystalline plasticity of quartz and feldspar relieves stress by aseismic creep and earthquakes are infrequent. Hydrothermal conditions control the stability of mineral phases and hence frictional-mechanical processes associated with earthquake rupture cycles, but there are few temperature and fluid pressure data from active plate-bounding faults. Here we report results from a borehole drilled into the upper part of the Alpine Fault, which is late in its cycle of stress accumulation and expected to rupture in a magnitude 8 earthquake in the coming decades. The borehole (depth 893 metres) revealed a pore fluid pressure gradient exceeding 9 ± 1 per cent above hydrostatic levels and an average geothermal gradient of 125 ± 55 degrees Celsius per kilometre within the hanging wall of the fault. These extreme hydrothermal conditions result from rapid fault movement, which transports rock and heat from depth, and topographically driven fluid movement that concentrates heat into valleys. Shear heating may occur within the fault but is not required to explain our observations. Our data and models show that highly anomalous fluid pressure and temperature gradients in the upper part of the seismogenic zone can be created by positive feedbacks between processes of fault slip, rock fracturing and alteration, and landscape development at plate-bounding faults. © 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. 2017 English 00280836 10.1038/nature22355 Nature 546 Nature Publishing Group 137-140 GNS Science, PO Box 30368, Lower Hutt, New Zealand; SGEES, Victoria University of Wellington, PO Box 600, Wellington, New Zealand; Department of Geology, University of Otago, PO Box 56, Dunedin, 9054, New Zealand; Department of Ocean and Earth Science, University of Southampton, Southampton, SO14 3ZH, United Kingdom; School of Environmental Sciences, University of Liverpool, Liverpool, L69 3GP, United Kingdom; Department of Earth Sciences, University of California, Riverside, CA 92521, United States; Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI 48109, United States; University of Auckland, Private Bag 92019, Auckland, 1142, New Zealand; School of Geology and Geophysics, University of Oklahoma, Norman, OK 73019, United States; CNRS, Université de Montpellier, Montpellier, 34095, France; GNS Science, Private Bag 1930, Dunedin, 9054, New Zealand; Universit Grenoble-Alpes, Universit Savoie Mont Blanc, CNRS, IRD, IFSTTAR, ISTerre, Grenoble, F-38000, France; Schlumberger Fiber-Optic Technology Centre, Romsey, Hampshire, SO51 9DL, United Kingdom; Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706, United States; Department of Earth and Space Science, Osaka University, Osaka, 565-0871, Japan; Department of Geosphere Sciences, Yamaguchi University, Yamaguchi, 753-8511, Japan; Graduate School of Engineering, Kyoto University, Kyoto, 615-8540, Japan; Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology, Kochi, 783-8502, Japan; Department of Physics, University of Alberta, Edmonton, AB T6G 2R3, Canada; Department of Earth and Planetary Sciences, McGill University, Montreal, QC H3A 0G4, Canada; Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia; ScopeM, ETH, Zürich, 8093, Switzerland; Department of Geology, Shinshu University, Asahi 3-1-1, Matsumoto, Japan; Faculty of Geosciences, HPT Laboratory, Utrecht University, Utrecht, 3584 CD, Netherlands; Department of Earth Science and Technology, Akita University, Akita City, 010-8502, Japan; GFZ German Research Centre for Geosciences, Telegrafenberg, Potsdam, 14473, Germany; Geological Survey of Japan, AIST, Tsukuba, Japan; Department of Geosciences, Pennsylvania State University, University Park, PA 16802, United States 7656 active fault; deformation; earthquake magnitude; earthquake rupture; fluid dynamics; heating; hydrostatic pressure; hydrothermal activity; hydrothermal system; movement; plate boundary; pressure effect; seismic zone; temperature effect; temperature gradient; topography, Article; earthquake; environmental temperature; heat; heating; hydrostatic pressure; pressure gradient; priority journal; rock; shear stress; topography https://www.scopus.com/inward/record.uri?eid=2-s2.0-85020188195&doi=10.1038%2fnature22355&partnerID=40&md5=c62982ed0f3b74e83c993a796fbeea43 cited By 75 P. Upton J. Coussens M. Allen L.-M. Baratin N. Barth L. Becroft A. Boles N.G.R. Broderick L. Janku-Capova B.M. Carpenter B. Célérier A. Cooper A. Coutts L. Craw M.-L. Doan J. Eccles D. Faulkner J. Grieve J. Grochowski A. Gulley A. Hartog K. Jacobs T. Jeppson N. Kato S. Keys Y. Kometani W. Lin A. Lukacs D. Mallyon C. Massiot L. Mathewson B. Melosh C. Menzies J. Moore L. Morales C. Morgan H. Mori A. Niemeijer O. Nishikawa D. Prior M. Savage A. Schleicher N. Shigematsu S. Taylor-Offord D. Teagle H. Tobin R. Valdez K. Weaver T. Wiersberg N. Woodman M. Zimmer article Kim20161910 Finite-source inversions are performed using small earthquake waveforms as empirical Green's functions (eGf) to investigate the rupture process of repeating earthquakes along the San Andreas Fault in Parkfield, California. The eGf waveform inversion method is applied to a repeating Mw 2.1 Parkfield earthquake sequence using three-component velocity waveforms recorded by an array of borehole seismometers. The obtained models show a circular slip distribution with a ~20 m radius, a 3.0-4.2 cm average slip of the main asperity, and peak displacement of 10.6-13.5 cm. The static stress drop distribution shows that the main asperity has a peak stress drop of 69.5-94.7 MPa. The inversion results support an earlier finding by Dreger et al. (2007) that high-strength asperities exist in the rupture areas of the Mw 2.1 events at Parkfield. In addition, notable temporal peak slip and stress drop reduction was observed after the 2004 Parkfield event while the average value remains constant (~12 MPa) over time. These events may represent mechanically strong sections of the fault, surrounded by regions that are undergoing continuous deformation (creep), Given repeated loading of the strong asperities, it would be expected that these similar repeating earthquakes should also have very similar slip distributions since surrounding regions are deforming aseismically. There are small differences in the waveforms of these repeating earthquakes, and this could be because of rupture nucleation points not being in exactly the same location within the region of the fault that is capable of stick-slip behavior. Our result indicates that waveform slip inversion is needed to reveal spatial and temporal variations of the stress drop within the rupture area to improve understanding of fault healing and rupture mechanics. ©2016. American Geophysical Union. All Rights Reserved. 2016 English 21699313 10.1002/2015JB012562 Journal of Geophysical Research: Solid Earth 121 Blackwell Publishing Ltd 1910-1926 Department of Materials System Science, Yokohama City University, Yokohama, Japan; Berkeley Seismological Laboratory, University of California, Berkeley, CA, United States 3 earthquake magnitude; earthquake prediction; earthquake rupture; fault displacement; Green function; inverse problem; San Andreas Fault; seismograph; stick-slip; stress; wave velocity, California; Parkfield; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-84961588774&doi=10.1002%2f2015JB012562&partnerID=40&md5=eec7135f66a5aa54ce84b24bbeeedf3c cited By 29 A. Kim D.S. Dreger T. Taira R.M. Nadeau article Luetkemeyer2016174 The San Andreas Fault Observatory at Depth (SAFOD), near Parkfield, California, is a borehole drilled through two active deforming zones of the San Andreas fault, the Southwest Deforming Zone (SDZ) and the Central Deforming Zone (CDZ). These zones accommodate displacement by seismic slip and aseismic creep. Elevated fluid pressures and fluid–rock interactions have been proposed to explain the low apparent strength and aseismic creep observed, but the origin of the fluids and existence of high fluid pressures remains uncertain. We use clumped-isotope thermometry and δ18O–δ13C compositions of calcite in veins to constrain the origin of paleofluids and compare these results to the isotopic composition of modern-day pore fluids from the SAFOD borehole and nearby areas. We observe that: (1) calcite vein temperatures vary from 81 to 134 °C, which overlaps the current ambient borehole temperatures of 110–115 °C at sampled depths; (2) vein calcite is not in carbon isotope equilibrium with modern-day pore fluids; (3) the δ18O values of paleofluids close to the SDZ and CDZ, calculated from vein δ18O and temperature data, are not in equilibrium with local modern-day pore waters but approach equilibrium with modern pore waters far from these zones; and (4) syntectonic vein calcite is only in C- and O-isotopic equilibrium with their host rocks within the SDZ and CDZ. Spatial patterns of δ18O and δ13C show little evidence for across-fault fluid-flow. Clumped isotope temperatures are consistent with locally-derived fluid sources, but not with continuous or episodic replenishment of fluids from shallow sedimentary brines or deep fluid sources. Our findings are compatible with flow of meteoric fluids from the southwestern damage zone into the SDZ and CDZ, which would have favored the formation of weak phyllosilicates and contributed to the present day weakness of the two actively deforming zones. © 2016 Elsevier B.V. 2016 English 00401951 10.1016/j.tecto.2016.05.024 Tectonophysics 690 Elsevier B.V. 174-189 Department of Earth and Atmospheric Sciences, Saint Louis University, 205 O'Neil Hall, 3642 Lindell Blvd, Saint Louis, MO 63108, United States; Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195, United States; Department of Geology and Geophysics, Texas A & M University, College Station, TX 77843, United States; Department of Geology, Utah State University, Logan, UT 84322-4505, United States Calcite; Carbon; Creep; Deformation; Faulting; Fluids; Isotopes; Observatories; Silicates; Strike-slip faults; Thermometers; Water, Borehole temperature; Calcite veins; Clumped isotopes; Fluid-rock interaction; Isotopic composition; SAFOD; San Andreas fault; Syn-tectonic veins, Flow of fluids, borehole geophysics; calcite; carbonate rock; fault zone; fracture flow; hydrogeology; isotopic composition; meteoric water; San Andreas Fault; vein (geology); water-rock interaction, California; Parkfield; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-84971597832&doi=10.1016%2fj.tecto.2016.05.024&partnerID=40&md5=e110e67c29fa1be2bf4ae1ad21fa672a cited By 13 P.B. Luetkemeyer D.L. Kirschner K.W. Huntington J.S. Chester F.M. Chester J.P. Evans article Ikari2016164 The central San Andreas Fault in California is known as a creeping fault, however recent studies have shown that it may be accumulating a slip deficit and thus its seismogenic potential should be seriously considered. We conducted laboratory friction experiments measuring time-dependent frictional strengthening (healing) on fault zone and wall rock samples recovered during drilling at the San Andreas Fault Observatory at Depth (SAFOD), located near the southern edge of the creeping section and in the direct vicinity of three repeating microearthquake clusters. We find that for hold times of up to 3000 s, frictional healing follows a log-linear dependence on hold time and that the healing rate is very low for a sample of the actively shearing fault core, consistent with previous results. However, considering longer hold times up to ∼350,000 s, the healing rate accelerates such that the data for all samples are better described by a power law relation. In general, samples having a higher content of phyllosilicate minerals exhibit low log-linear healing rates, and the notably clay-rich fault zone sample also exhibits strong power-law healing when longer hold times are included. Our data suggest that weak faults, such as the creeping section of the San Andreas Fault, can accumulate interseismic shear stress more rapidly than expected from previous friction data. Using the power-law dependence of frictional healing on hold time, calculations of recurrence interval and stress drop based on our data accurately match observations of discrete creep events and repeating Mw=2 earthquakes on the San Andreas Fault. © 2016 Elsevier B.V. 2016 English 0012821X 10.1016/j.epsl.2016.06.036 Earth and Planetary Science Letters 450 Elsevier B.V. 164-172 MARUM, Center for Marine Environmental Sciences, University of Bremen, Germany; Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy; School of Geology and Geophysics, University of Oklahoma, Norman, OK, United States; Central Laboratory for Crystallography and Applied Material Sciences, Crystallography Group, Geosciences, University of Bremen, Germany Clay minerals; Earthquakes; Friction; Rock drilling; Shear stress; Strike-slip faults; Tribology, Creeping faults; Friction experiments; Linear dependence; Power law relation; Power-law dependences; Recurrence intervals; SAFOD; San Andreas fault, Structural geology, drilling; earthquake recurrence; fault zone; friction; microearthquake; recurrence interval; San Andreas Fault, California; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-84976636936&doi=10.1016%2fj.epsl.2016.06.036&partnerID=40&md5=a29c20b66df0fa14fa2ab18d16fb0939 cited By 15 M.J. Ikari B.M. Carpenter A.J. Kopf article Wojatschke20163865 The presence of smectite (saponite) in fault gouge from the Central Deforming Zone of the San Andreas Fault at Parkfield, CA has been linked to low mechanical strength and aseismic slip. However, the precise relationship between clay mineral structure, fabric development, fault strength, and the stability of frictional sliding is not well understood. We address these questions through the integration of laboratory friction tests and FIB-SEM analysis of fault rock recovered from the San Andreas Fault Observatory at Depth (SAFOD) borehole. Intact fault rock was compared with experimentally sheared fault gouge and different proportions of either quartz clasts or SAFOD clasts extracted from the sample. Nano-textural measurements show the development of localized clay particle alignment along shear folia developed within synthetic gouges; such slip planes have multiples of random distribution (MRD) values of 3.0–4.9. The MRD values measured are higher than previous estimates (MRD 1.5) that show lower degrees of shear localization and clay alignment averaged over larger volumes. The intact fault rock exhibits less well-developed nano-clay fabrics than the experimentally sheared materials, and MRD values decrease with smectite content. We show that the abundance, strength, and shape of clasts all influence fabric evolution via strain localization: quartz clasts yield more strongly developed clay fabrics than serpentine-dominated SAFOD clasts. Our results suggest that (1) both clay abundance and the development of nano-scale fabrics play a role in fault zone weakening and (2) aseismic creep is promoted by slip along clay shears with >20 wt % smectite content and MRD values ≥2.7. © 2016. American Geophysical Union. All Rights Reserved. 2016 English 15252027 10.1002/2016GC006500 Geochemistry, Geophysics, Geosystems 17 Blackwell Publishing Ltd 3865-3881 Institut of Geography and Geology, Ernst-Moritz-Arndt-University Greifswald, Greifswald, Germany; Dipartimento di Scienze della Terra, Università degli Studi La Sapienza, Rome, Italy; ConocoPhilips School of Geology and Geophysics, University of Oklahoma, Norman, OK, United States; Department of Geosciences and Center for Geomechanics, Geofluids and Geohazards, Pennsylvania State University, University Park, PA, United States 10 Clay minerals; Deformation; Friction; Nanotechnology; Quartz; Serpentine; Silicate minerals; Strike-slip faults; Structural geology; Tectonics, Different proportions; Fabric development; Fault zone; Frictional behavior; Frictional sliding; Random distribution; Shear localizations; Strain localizations, Fault slips, clay mineral; deformation; experimental study; fault gouge; fault zone; friction; petrofabric; San Andreas Fault; slip, California; Parkfield; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-84991051995&doi=10.1002%2f2016GC006500&partnerID=40&md5=6724f9bf452e160e5d118f74569e6ebc cited By 11 J. Wojatschke M.M. Scuderi L.N. Warr B.M. Carpenter D. Saffer C. Marone article Moore2016153 We compare frictional strengths in the temperature range 25-250 °C of fault gouge from SAFOD (CDZ and SDZ) with quartzofeldspathic wall rocks typical of the central creeping section of the San Andreas Fault (Great Valley sequence and Franciscan Complex). The Great Valley and Franciscan samples have coefficients of friction, μ > 0.35 at all experimental conditions. Strength is unchanged between 25° and 150 °C, but μ increases at higher temperatures, exceeding 0.50 at 250 °C. Both samples are velocity strengthening at room temperature but show velocity-weakening behavior beginning at 150 °C and stick-slip motion at 250 °C. These rocks, therefore, have the potential for unstable seismic slip at depth. The CDZ gouge, with a high saponite content, is weak (μ = 0.09-0.17) and velocity strengthening in all experiments, and μ decreases at temperatures above 150 °C. Behavior of the SDZ is intermediate between the CDZ and wall rocks: μ < 0.2 and does not vary with temperature. Although saponite is probably not stable at depths greater than ~3 km, substitution of the frictionally similar minerals talc and Mg-rich chlorite for saponite at higher temperatures could potentially extend the range of low strength and stable slip down to the base of the seismogenic zone. © 2016. 2016 English 01918141 10.1016/j.jsg.2016.06.005 Journal of Structural Geology 89 Elsevier Ltd 153-167 U.S. Geological Survey, Earthquake Science Center, 345 Middlefield Road, Mail Stop 977, Menlo Park, CA 94025, United States Landforms; Rocks; Slip forming; Stick-slip; Strike-slip faults, Franciscan Complex; Frictional strength; Great Valley sequence; SAFOD; San Andreas fault; Saponite, Minerals, creep; fault gouge; friction; rock; San Andreas Fault; saponite; seismic zone; slip; strength; strike-slip fault; temperature, California; Central Valley [California]; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-84975498333&doi=10.1016%2fj.jsg.2016.06.005&partnerID=40&md5=1a737f13c543a481f5b380fd011aa48e cited By 25 D.E. Moore D.A. Lockner S. Hickman article Morrow20158240 The San Andreas Fault Observatory at Depth (SAFOD) scientific drill hole near Parkfield, California, crosses the San Andreas Fault at a depth of 2.7 km. Downhole measurements and analysis of core retrieved from Phase 3 drilling reveal two narrow, actively deforming zones of smectite-clay gouge within a roughly 200 m wide fault damage zone of sandstones, siltstones, and mudstones. Here we report electrical resistivity and permeability measurements on core samples from all of these structural units at effective confining pressures up to 120 MPa. Electrical resistivity (~10 Ω-m) and permeability (10-21 to 10-22 m2) in the actively deforming zones were 1 to 2 orders of magnitude lower than the surrounding damage zone material, consistent with broader-scale observations from the downhole resistivity and seismic velocity logs. The higher porosity of the clay gouge, 2 to 8 times greater than that in the damage zone rocks, along with surface conduction were the principal factors contributing to the observed low resistivities. The high percentage of fine-grained clay in the deforming zones also greatly reduced permeability to values low enough to create a barrier to fluid flow across the fault. Together, resistivity and permeability data can be used to assess the hydrogeologic characteristics of the fault, key to understanding fault structure and strength. The low resistivities and strength measurements of the SAFOD core are consistent with observations of low resistivity clays that are often found in the principal slip zones of other active faults making resistivity logs a valuable tool for identifying these zones. © Published 2015. This article is a U.S. Government work and is in the public domain in the USA. 2015 English 21699313 10.1002/2015JB012214 Journal of Geophysical Research: Solid Earth 120 Blackwell Publishing Ltd 8240-8258 U.S. Geological Survey, Menlo Park, CA, United States 12 active fault; deformation; electrical resistivity; fault gouge; permeability; porosity; San Andreas Fault; sediment core; shear zone, California; Parkfield; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-84957847021&doi=10.1002%2f2015JB012214&partnerID=40&md5=6da25f74f7bc85f2db0af6ea595d49e2 cited By 8 C. Morrow D.A. Lockner S. Hickman article Jeppson20154983 The San Andreas Fault (SAF), like other mature brittle faults, exhibits a zone of low seismic velocity hypothesized to result from fluid pressure effects and/or development of a damage zone. To address the relative contributions of these mechanisms in developing low-velocity zones, we measured P and S wave velocities ultrasonically at elevated confining and pore pressures on core samples from the San Andreas Fault Observatory at Depth (SAFOD). We compared those data to wireline and seismic-scale velocities to examine the scale dependence of acoustic properties of the fault core and damage zone. Average laboratory P and S wave velocities of the fault gouge at estimated in situ conditions are 3.1 and 1.5 km/s, respectively, consistent with the sonic log from the same intervals. These data show that fault core has intrinsically low velocity, even if no anomalous pore pressure is assumed, due to alteration and mechanical damage. In contrast, laboratory average P and S wave velocities for the damage zone are 4.7 and 2.5 km/s, up to 41% greater than the sonic log in the damage zone. This scale dependence indicates that stress conditions or macroscale features dominate the damage zone's acoustic properties, although velocity dispersion could play a role. Because no pressure anomaly was detected while drilling the SAFOD borehole, we infer that damage at a scale larger than core samples controls the elastic properties of the broader damage zone. This result bolsters other independent lines of evidence that the SAF does not contain major pore fluid overpressure at SAFOD. ©2015. American Geophysical Union. All Rights Reserved. 2015 English 21699313 10.1002/2015JB012043 Journal of Geophysical Research: Solid Earth 120 Blackwell Publishing Ltd 4983-4997 Geoscience Department, University of Wisconsin-Madison, Madison, WI, United States 7 acoustic property; elastic modulus; fault zone; laboratory method; P-wave; S-wave; San Andreas Fault; seismic velocity; ultrasonics; velocity structure; wave velocity https://www.scopus.com/inward/record.uri?eid=2-s2.0-84939250077&doi=10.1002%2f2015JB012043&partnerID=40&md5=f22ddf32214342ab99ac44251dc4baab cited By 19 T.N. Jeppson H.J. Tobin article French2015827 We report the strength and constitutive behavior of gouge sampled from the Central Deforming Zone (CDZ) of the San Andreas Fault. Layers of flaked CDZ gouge were sheared in the triaxial saw cut configuration using the stress relaxation technique to measure the gouge strength over 4 orders of magnitude in shear strain rate and at rates as low as 5 × 10-10s-1 and within an order of magnitude of in situ rates. Deformation conditions correspond to the in situ effective normal stress (100 MPa) and temperature (65 to 120C) at the sampling depth of 2.7 km. Gouge was sheared dry and with brine pore fluid at 25 MPa pore pressure. Dry gouge is stronger and more rate strengthening than brine-saturated gouge. Brine-saturated CDZ gouge strengthens with increasing strain rate and decreasing temperature, and the dependencies of strength on strain rate and temperature increase at rates below ∼5 × 10-9s-1. At strain rates greater than ∼5 × 10-9s-1, the rate dependence is consistent with previous studies on the CDZ gouge conducted at even higher rates. The increase in rate dependence below ∼5 × 10-9s-1 indicates a change in the rate-controlling deformation mechanism. The magnitude of the friction rate dependence parameter, a, and the temperature sensitivity of a are consistent with crystal plasticity of the phyllosilicates. We hypothesize a micromechanical model for the CDZ gouge whereby a transition from fracture and delamination-accommodated frictional flow to crystal plasticity-accommodated frictional flow occurs with decreasing strain rate. © 2015. American Geophysical Union. All Rights Reserved. 2015 English 21699313 10.1002/2014JB011496 Journal of Geophysical Research: Solid Earth 120 Blackwell Publishing Ltd 827-849 Department of Geology and Geophysics, Center for Tectonophysics, Texas A and M University, College Station, TX, United States; Department of Geology, University of Maryland, College Park, MD, United States 2 clay; creep; deformation mechanism; fault gouge; fault zone; micromechanics; microstructure; San Andreas Fault; strain rate; stress field, California; San Andreas; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-85027948655&doi=10.1002%2f2014JB011496&partnerID=40&md5=b5b21297b826aff0b5b4b4c3fc0c386e cited By 35 M.E. French F.M. Chester J.S. Chester article Bennington20151033 We present jointly inverted models of P-wave velocity (Vp) and electrical resistivity for a two-dimensional profile centered on the San Andreas Fault Observatory at Depth (SAFOD). Significant structural similarity between main features of the separately inverted Vp and resistivity models is exploited by carrying out a joint inversion of the two datasets using the normalized cross-gradient constraint. This constraint favors structurally similar Vp and resistivity images that adequately fit the seismic and magnetotelluric (MT) datasets. The new inversion code, tomoDDMT, merges the seismic inversion code tomoDD and the forward modeling and sensitivity kernel subroutines of the MT inversion code OCCAM2DMT. TomoDDMT is tested on a synthetic dataset and demonstrates the code’s ability to more accurately resolve features of the input synthetic structure relative to the separately inverted resistivity and velocity models. Using tomoDDMT, we are able to resolve a number of key issues raised during drilling at SAFOD. We are able to infer the distribution of several geologic units including the Salinian granitoids, the Great Valley sequence, and the Franciscan Formation. The distribution and transport of fluids at both shallow and great depths is also examined. Low values of velocity/resistivity attributed to a feature known as the Eastern Conductor (EC) can be explained in two ways: the EC is a brine-filled, high porosity region, or this region is composed largely of clay-rich shales of the Franciscan. The Eastern Wall, which lies immediately adjacent to the EC, is unlikely to be a fluid pathway into the San Andreas Fault’s seismogenic zone due to its observed higher resistivity and velocity values. © 2014, Springer Basel. 2015 English 00334553 10.1007/s00024-014-1002-9 Pure and Applied Geophysics 172 Birkhauser Verlag AG 1033-1052 University of Wisconsin-Madison, Madison, WI, United States; Laboratory of Seismology and Physics of the Earth’s Interior, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, China; U.S. Geological Survey, Denver, CO, United States 5 Codes (symbols); Magnetotellurics; Seismic waves; Seismographs; Strike-slip faults; Tectonics; Transport properties; Velocity; Wave propagation, Inverse theory; Joint inversion; Magnetotelluric data; Resistivity images; Seismic tomography; Sensitivity kernels; Structural similarity; Two-dimensional profiles, Seismology, data inversion; data set; electrical resistivity; gradient analysis; inverse analysis; magnetotelluric method; San Andreas Fault; seismic data; seismic tomography; seismic zone; velocity structure https://www.scopus.com/inward/record.uri?eid=2-s2.0-84926167005&doi=10.1007%2fs00024-014-1002-9&partnerID=40&md5=e9559bc10b1a3120e538fd44cdf425fd cited By 41 N.L. Bennington H. Zhang P.A. Bedrosian article Bradbury20151053 We examine the fine-scale variations in mineralogical composition, geochemical alteration, and texture of the fault-related rocks from the Phase 3 whole-rock core sampled between 3,187.4 and 3,301.4 m measured depth within the San Andreas Fault Observatory at Depth (SAFOD) borehole near Parkfield, California. This work provides insight into the physical and chemical properties, structural architecture, and fluid-rock interactions associated with the actively deforming traces of the San Andreas Fault zone at depth. Exhumed outcrops within the SAF system comprised of serpentinite-bearing protolith are examined for comparison at San Simeon, Goat Rock State Park, and Nelson Creek, California. In the Phase 3 SAFOD drillcore samples, the fault-related rocks consist of multiple juxtaposed lenses of sheared, foliated siltstone and shale with block-in-matrix fabric, black cataclasite to ultracataclasite, and sheared serpentinite-bearing, finely foliated fault gouge. Meters-wide zones of sheared rock and fault gouge correlate to the sites of active borehole casing deformation and are characterized by scaly clay fabric with multiple discrete slip surfaces or anastomosing shear zones that surround conglobulated or rounded clasts of compacted clay and/or serpentinite. The fine gouge matrix is composed of Mg-rich clays and serpentine minerals (saponite ± palygorskite, and lizardite ± chrysotile). Whole-rock geochemistry data show increases in Fe-, Mg-, Ni-, and Cr-oxides and hydroxides, Fe-sulfides, and C-rich material, with a total organic content of >1 % locally in the fault-related rocks. The faults sampled in the field are composed of meters-thick zones of cohesive to non-cohesive, serpentinite-bearing foliated clay gouge and black fine-grained fault rock derived from sheared Franciscan Formation or serpentinized Coast Range Ophiolite. X-ray diffraction of outcrop samples shows that the foliated clay gouge is composed primarily of saponite and serpentinite, with localized increases in Ni- and Cr-oxides and C-rich material over several meters. Mesoscopic and microscopic textures and deformation mechanisms interpreted from the outcrop sites are remarkably similar to those observed in the SAFOD core. Micro-scale to meso-scale fabrics observed in the SAFOD core exhibit textural characteristics that are common in deformed serpentinites and are often attributed to aseismic deformation with episodic seismic slip. The mineralogy and whole-rock geochemistry results indicate that the fault zone experienced transient fluid–rock interactions with fluids of varying chemical composition, including evidence for highly reducing, hydrocarbon-bearing fluids. © 2014, Springer Basel. 2015 English 00334553 10.1007/s00024-014-0896-6 Pure and Applied Geophysics 172 Birkhauser Verlag AG 1053-1078 Geology Department, Utah State University, Logan, UT 84322-4505, United States 5 Deformation; Geochemistry; Kaolinite; Lithology; Minerals; Nickel; Oil bearing formations; Petroleum deposits; Serpentine; Silicate minerals; Strike-slip faults; Structural geology; Sulfide minerals; Textures; X ray diffraction, Coast range ophiolites; Fluid-rock interaction; Mineralogical compositions; Physical and chemical properties; Structural architecture; Textural characteristic; Total organic contents; Whole-rock geochemistries, Rocks, chemical composition; deformation mechanism; fault zone; hydrochemistry; mineralization; protolith; San Andreas Fault; saponite; serpentinite; water-rock interaction, California; Parkfield; San Simeon; United States, Capra hircus https://www.scopus.com/inward/record.uri?eid=2-s2.0-84926179721&doi=10.1007%2fs00024-014-0896-6&partnerID=40&md5=782699c2cf82c5ae243a0feaf235a9bc cited By 23 K.K. Bradbury C.R. Davis J.W. Shervais S.U. Janecke J.P. Evans article Pollitz20152835 A series of near-surface chemical explosions conducted at the San Andreas Fault Observatory at Depth (SAFOD) were recorded by high-frequency downhole receiver arrays in separate experiments in November 2003 and May 2005. The 2003 experiment involved ~100 kg shots detonated along a 46-km-long line (Hole–Ryberg line) centered on SAFOD and recorded by 32 three-component geophones in the pilot hole between 0.8 and 2.0 km depth. The 2005 experiment involved ~36 kg shots detonated at Parkfield Area Seismic Observatory (PASO) stations (at ~1–8 km offset) recorded by 80 three-component geophones in the main hole between the surface and 2.4 km depth. These data sample the downgoing seismic wavefield and constrain the shallow velocity and attenuation structure, as well as the first-order characteristics of the source. Using forward modeling on a velocity structure designed for the near field, both observed P- and S-wave energy for the PASO shots are identified with the travel times expected for direct and/or reflected phases. Larger-offset recordings from shots along the Hole–Ryberg line reveal substantial SVand SH energy, especially southwest of SAFOD from the source as indicated by P-to-S amplitude ratios. The generated SV energy is interpreted to arise chiefly from P-to-S conversions at subhorizontal discontinuities. This provides a simple mechanism for often-observed low P-to-S amplitude ratios from nuclear explosions in the far field, as originating from strong near-field wave conversions. © 2015, Seismological Society of America. All rights reserved. 2015 English 00371106 10.1785/0120140242 Bulletin of the Seismological Society of America 105 Seismological Society of America 2835-2851 U.S. Geological Survey, 345 Middlefield Road, MS 977, Menlo Park, CA 94025, United States 6 Explosions; Nuclear explosions; Observatories; Seismology; Strike-slip faults; Wave energy conversion, Amplitude ratios; Downhole receivers; Forward modeling; High frequency HF; Reflected phasis; San Andreas fault; Seismic wavefields; Velocity structure, Shear waves, explosion; geophone; S-wave; seismic discrimination; seismic velocity; wave generation, California; Parkfield; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-84949257201&doi=10.1785%2f0120140242&partnerID=40&md5=b965f7b62d7a2190e3172753b5cfdfe9 cited By 0 F.F. Pollitz W. Ellsworth J. Rubinstein article Carpenter20155273 We present results from a comprehensive laboratory study of the frictional strength and constitutive properties for all three active strands of the San Andreas Fault penetrated in the San Andreas Observatory at Depth (SAFOD). The SAFOD borehole penetrated the Southwest Deforming Zone (SDZ), the Central Deforming Zone (CDZ), both of which are actively creeping, and the Northeast Boundary Fault (NBF). Our results include measurements of the frictional properties of cuttings and core samples recovered at depths of ~2.7 km. We find that materials from the two actively creeping faults exhibit low frictional strengths (μ = ~0.1), velocity-strengthening friction behavior, and near-zero or negative rates of frictional healing. Our experimental data set shows that the center of the CDZ is the weakest section of the San Andreas Fault, with μ = ~0.10. Fault weakness is highly localized and likely caused by abundant magnesium-rich clays. In contrast, serpentine from within the SDZ, and wall rock of both the SDZ and CDZ, exhibits velocity-weakening friction behavior and positive healing rates, consistent with nearby repeating microearthquakes. Finally, we document higher friction coefficients (μ > 0.4) and complex rate-dependent behavior for samples recovered across the NBF. In total, our data provide an integrated view of fault behavior for the three active fault strands encountered at SAFOD and offer a consistent explanation for observations of creep and microearthquakes along weak fault zones within a strong crust. ©2015. American Geophysical Union. All Rights Reserved. 2015 English 21699313 10.1002/2015JB011963 Journal of Geophysical Research: Solid Earth 120 Blackwell Publishing Ltd 5273-5289 Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy; Department of Geosciences and Energy, Institute Center for Geomechanics, Geofluids, and Geohazards, Pennsylvania State University, University Park, PA, United States 7 active fault; creep; fault slip; fault zone; friction; microearthquake; San Andreas Fault, California; San Andreas; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-84939214772&doi=10.1002%2f2015JB011963&partnerID=40&md5=a9c0d6ff3c99ae78171feca0063fb26a cited By 64 B.M. Carpenter D.M. Saffer C. Marone article Abercrombie20154263 I use a well-recorded earthquake sequence to investigate the uncertainties of earthquake stress drops calculated using an empirical Green’s function (EGF) approach. The earthquakes in the largest (M~ 2.1) repeating sequence targeted by the San Andreas Observatory at Depth (SAFOD), Parkfield, California, are recorded by multiple borehole stations and have simple sources, well-constrained stress drops, and abundant smaller earthquakes to use as EGFs. I perform three tests to estimate quantitatively the likely uncertainties to arise in less optimal settings. I use EGF earthquakes with a range of cross-correlation values and separation distances fromthe main earthquakes. The stress dropmeasurements decrease by a factor of 3 as the quality of the EGF assumption decreases; a good EGF must be located within approximately one source dimension of the large earthquake, with high cross correlation. I subsample the large number ofmeasurements for the main earthquakes to investigate the expected stress drop uncertainties in studies where fewer stations or EGFs are available. If only one measurement is available, the uncertainties are likely to be at least 50%. The uncertainties decrease to <20% with five ormore measurements; usingmultiple EGFs is a good alternative to multiple stations. To investigate the effects of limited frequency bandwidth, I recalculate the corner frequencies after progressively decimating the sample rate. Decreasing the high-frequency limit of the bandwidth decreases the estimate of the corner frequency (and stress drop). The corner frequency may be underestimated if it is within a factor of 3 of the maximum frequency of the signal. © 2015. American Geophysical Union. All Rights Reserved. 2015 English 21699313 10.1002/2015JB011984 Journal of Geophysical Research: Solid Earth 120 Blackwell Publishing Ltd 4263-4277 Department of Earth and Environment, Boston University, Boston, MA, United States 6 https://www.scopus.com/inward/record.uri?eid=2-s2.0-84930532967&doi=10.1002%2f2015JB011984&partnerID=40&md5=7c13a8fd013160f67bff1cd32f6dc64b cited By 109 R.E. Abercrombie article Janssen2014100 The microstructures, mineralogy and chemistry of representative samples collected from the cores of the San Andreas Fault drill hole (SAFOD) and the Taiwan Chelungpu-Fault Drilling project (TCDP) have been studied using optical microscopy, TEM, SEM, XRD and XRF analyses. SAFOD samples provide a transect across undeformed host rock, the fault damage zone and currently active deforming zones of the San Andreas Fault. TCDP samples are retrieved from the principal slip zone (PSZ) and from the surrounding damage zone of the Chelungpu Fault. Substantial differences exist in the clay mineralogy of SAFOD and TCDP fault gouge samples. Amorphous material has been observed in SAFOD as well as TCDP samples. In line with previous publications, we propose that melt, observed in TCDP black gouge samples, was produced by seismic slip (melt origin) whereas amorphous material in SAFOD samples was formed by comminution of grains (crush origin) rather than by melting. Dauphiné twins in quartz grains of SAFOD and TCDP samples may indicate high seismic stress. The differences in the crystallographic preferred orientation of calcite between SAFOD and TCDP samples are significant. Microstructures resulting from dissolution-precipitation processes were observed in both faults but are more frequently found in SAFOD samples than in TCDP fault rocks. As already described for many other fault zones clay-gouge fabrics are quite weak in SAFOD and TCDP samples. Clay-clast aggregates (CCAs), proposed to indicate frictional heating and thermal pressurization, occur in material taken from the PSZ of the Chelungpu Fault, as well as within and outside of the SAFOD deforming zones, indicating that these microstructures were formed over a wide range of slip rates. © 2014 Elsevier Ltd. 2014 English 01918141 10.1016/j.jsg.2014.04.004 Journal of Structural Geology 65 Elsevier Ltd 100-116 GeoForschungsZentrum Potsdam, Telegrafenberg, Potsdam 14473, Germany; Department of Earth and Planetary Science, University of California, Berkeley, CA 94720, United States; Department of Geosciences, National Taiwan University, P.O. Box 13-318, Roosevelt Road, Taipei 106, Taiwan Chemical analysis; Deformation; Grinding (comminution); Microstructure; Minerals; Quartz; Seismology; Strike-slip faults; Tectonics, CPO; EBSD; Fault rock; SAFOD; TCDP, Structural geology, active fault; amorphous medium; calcite; clay mineral; crystal structure; crystallography; deformation; fault gouge; faulting; host rock; melting; microstructure; quartz; San Andreas Fault; slip rate https://www.scopus.com/inward/record.uri?eid=2-s2.0-84900831848&doi=10.1016%2fj.jsg.2014.04.004&partnerID=40&md5=8e0d6271781b3ba26f38c38495db3d9a cited By 26 C. Janssen R. Wirth H.-R. Wenk L. Morales R. Naumann M. Kienast S.-R. Song G. Dresen article Richard20148132 Creep processes may relax part of the tectonic stresses in active faults, either by continuous or episodic processes. The aim of this study is to obtain a better understanding of these creep mechanisms and the manner in which they change in time and space. Results are presented from microstructural studies of natural samples collected from San Andreas Fault Observatory at Depth borehole drilled through the San Andreas Fault, which reveal the chronology of the deformation within three domain types. (i) A relatively undeformed zone of the host rock reflects the first step of the deformation process with fracturing and grain indentations showing the coupling between fracturing and pressure solution. (ii) Shear deformation development that associates fracturing and solution cleavage processes leads to profound changes in rock composition and behavior with two types of development depending on the ratio between the amount of dissolution and deposition: abundant mineral precipitation strengthens some zones while pervasive dissolution weakens some others, (iii) zones with mainly dissolution trended toward the present-day creeping zones thanks to both the passive concentration of phyllosilicates and their metamorphic transformation into soft minerals such as saponite. This study shows how interactions between brittle and viscous mechanisms lead to widespread transformation of the rocks and how a shear zone may evolve from a zone prone to earthquakes and postseismic creep to a zone of steady state creep. In parallel, the authors discuss how the creeping mechanism, mainly controlled by the very low friction of the saponite in the first 3-4 km depth, may evolve with depth. ©2014. American Geophysical Union. All Rights Reserved. 2014 English 21699313 10.1002/2014JB011489 Journal of Geophysical Research: Solid Earth 119 Blackwell Publishing Ltd 8132-8153 ISTERRE, Université Grenoble Alpes, CNRS, Grenoble, France; Physics of Geological Processes, University of Oslo, Oslo, Norway 11 active fault; brittle deformation; creep; fault zone; metamorphic rock; rock microstructure; San Andreas Fault; tectonic setting; viscosity https://www.scopus.com/inward/record.uri?eid=2-s2.0-84919477317&doi=10.1002%2f2014JB011489&partnerID=40&md5=a5c962ffc25d88341709b731ce64dfaa cited By 31 J. Richard J.-P. Gratier M.-L. Doan A.-M. Boullier F. Renard article Zhang20143013 We incorporate body-wave arrival time and surface-wave dispersion data into a joint inversion for three-dimensional P-wave and S-wave velocity structure of the crust surrounding the site of the San Andreas Fault Observatory at Depth. The contributions of the two data types to the inversion are controlled by the relative weighting of the respective equations. We find that the trade-off between fitting the two data types, controlled by the weighting, defines a clear optimal solution. Varying the weighting away from the optimal point leads to sharp increases in misfit for one data type with only modest reduction in misfit for the other data type. All the acceptable solutions yield structures with similar primary features, but the smaller-scale features change substantially. When there is a lower relative weight on the surface-wave data, it appears that the solution over-fits the body-wave data, leading to a relatively rough Vs model, whereas for the optimal weighting, we obtain a relatively smooth model that is able to fit both the body-wave and surface-wave observations adequately. © 2014, Springer Basel. 2014 English 00334553 10.1007/s00024-014-0806-y Pure and Applied Geophysics 171 Birkhauser Verlag AG 3013-3022 Laboratory of Seismology and Earth’s Interior, School of Earth and Space Sciences, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, China; Earth and Environmental Sciences, Los Alamos National Laboratory, Los Alamos, NM 87545, United States; ISTerre, CNRS, IRD, Université Joseph Fourier, Saint-Martin-d’Hères, France; Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706, United States 11 Economic and social effects; Seismic waves; Shear waves; Strike-slip faults; Surface waves; Wave propagation, Optimal solutions; Optimal weighting; Relative weights; S-wave velocity structure; San Andreas fault; Seismic structure; Surface wave data; Surface wave dispersion, Dispersion (waves), arrival time; body wave; inverse problem; P-wave; S-wave; San Andreas Fault; surface wave; three-dimensional modeling; velocity structure; wave dispersion https://www.scopus.com/inward/record.uri?eid=2-s2.0-84911444202&doi=10.1007%2fs00024-014-0806-y&partnerID=40&md5=f686fc48b1cc1c18245e06c6a57c0266 cited By 35 H. Zhang M. Maceira P. Roux C. Thurber article Coble2014956 Along the central segment of the San Andreas Fault (SAF) near Parkfield, California, displacement occurs by a combination of aseismic creep and micro-earthquake slip. To constrain the strength and parametrize a constitutive relation for the creeping behaviour of the central segment of the SAF, we conducted friction experiments on clay-rich gouge retrieved by coring the Central Deforming Zone (CDZ) of the SAF at 2.7 km vertical depth. The gouge was flaked rather than powdered to preserve the natural scaly microfabric, and formed into 2-mm-thick layers that were sheared using a triaxial deformation apparatus. Experiments were conducted at in situ effective normal stress (100 MPa), pore pressure (25 MPa) and temperature (80-120 °C) conditions using brine pore fluid with the ionic composition of the in situ formation fluid. Velocity-stepping (0.006-0.6 μm s-1) and temperature-stepping experiments were conducted on brine-saturated gouge, and slide-hold-slide experiments were conducted on brine-saturated and room-dry gouge. Results are used to quantify the effects of rate, state, temperature and pore fluid on the strength of the CDZ gouge. We find that the gouge is extremely weak (μ &lt; 0.13) and rate-strengthening, consistent with findings of previous studies on the CDZ gouge. We also find that, in a rate and state friction framework, slip history has a negligible effect on strength (b ≈ 0) under both saturated and dry conditions. The CDZ gouge is temperature-weakening from 80 to 120 °C and weakens 17 per cent when saturated with brine compared to room-dry conditions. Employing the laboratory-derived friction constitutive parameters, and including the temperature weakening and the strain-rate strengthening effects, we determine an approximate in situ friction coefficient of μ ≈ 0.11. For μ ≈ 0.11, aseismic creep under normal pore fluid conditions is permitted for angles up to 79° between the maximum horizontal stress and the plane of the SAF, consistent with nearby stress orientation measurements., Copy; The Authors 2014. 2014 English 0956540X 10.1093/gji/ggu306 Geophysical Journal International 199 Oxford University Press 956-967 Center for Tectonophysics, Department of Geology and Geophysics, Texas A and M University, College Station, TX 77843-3115, United States; Institute of Earthquake and Volcano Geology, Geological Survey of Japan, Ibabraki, Japan 2 Creep; Geomechanics; Strain rate; Strike-slip faults, Constitutive parameters; Constitutive relations; Creep and deformations; Effective normal stress; Fault zone; Friction coefficients; Rate and state friction; Strain-rate strengthening effects, Friction, creep; deformation; displacement; fault gouge; fault zone; friction; geomechanics; rheology; San Andreas Fault, California; San Andreas; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-84924987089&doi=10.1093%2fgji%2fggu306&partnerID=40&md5=0ff2c059d6f16d3f9e40a5d80650d1cc cited By 22 C.G. Coble M.E. French F.M. Chester J.S. Chester H. Kitajima article Morrow201499 The San Andreas Fault Observatory at Depth (SAFOD) scientific borehole near Parkfield, California crosses two actively creeping shear zones at a depth of 2.7km. Core samples retrieved from these active strands consist of a foliated, Mg-clay-rich gouge containing porphyroclasts of serpentinite and sedimentary rock. The adjacent damage zone and country rocks are comprised of variably deformed, fine-grained sandstones, siltstones, and mudstones. We conducted laboratory tests to measure the permeability of representative samples from each structural unit at effective confining pressures, Pe up to the maximum estimated in situ Pe of 120MPa. Permeability values of intact samples adjacent to the creeping strands ranged from 10-18 to 10-21m2 at Pe=10MPa and decreased with applied confining pressure to 10-20-10-22m2 at 120MPa. Values for intact foliated gouge samples (10-21-6×10-23m2 over the same pressure range) were distinctly lower than those for the surrounding rocks due to their fine-grained, clay-rich character. Permeability of both intact and crushed-and-sieved foliated gouge measured during shearing at Pe≥70MPa ranged from 2 to 4×10-22m2 in the direction perpendicular to shearing and was largely insensitive to shear displacement out to a maximum displacement of 10mm. The weak, actively-deforming foliated gouge zones have ultra-low permeability, making the active strands of the San Andreas Fault effective barriers to cross-fault fluid flow. The low matrix permeability of the San Andreas Fault creeping zones and adjacent rock combined with observations of abundant fractures in the core over a range of scales suggests that fluid flow outside of the actively-deforming gouge zones is probably fracture dominated. © 2013. 2014 English 01918141 10.1016/j.jsg.2013.09.009 Journal of Structural Geology 64 Elsevier Ltd 99-114 U.S. Geological Survey, 345 Middlefield Road, MS 977 Menlo Park, CA 94025, United States Clay minerals; Deformation; Flow of fluids; Fracture; Mechanical permeability; Observatories; Rock pressure; Shearing; Strike-slip faults, Confining pressures; Fault gouge; Low matrix permeability; Maximum displacement; Representative sample; SAFOD; San Andreas fault; Shear displacement, Core samples, confining pressure; country rock; displacement; fault gouge; permeability; San Andreas Fault; sedimentary rock; serpentinite; shear zone, California; San Andreas; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-84901266017&doi=10.1016%2fj.jsg.2013.09.009&partnerID=40&md5=a49b4a39f14de9484af63d65a49b50b8 cited By 33 C.A. Morrow D.A. Lockner D.E. Moore S. Hickman article Moore201482 Creep in the San Andreas Fault Observatory at Depth (SAFOD) drillhole is localized to two foliated gouges, the central deforming zone (CDZ) and southwest deforming zone (SDZ). The gouges consist of porphyroclasts of serpentinite and sedimentary rock dispersed in a foliated matrix of Mg-smectite clays that formed as a result of shearing-enhanced reactions between the serpentinite and quartzofeldspathic rocks. The CDZ takes up most of the creep and exhibits differences in mineralogy and texture from the SDZ that are attributable to its higher shearing rate. In addition, a ~0.2-m-wide sector of the CDZ at its northeastern margin (NE-CDZ) is identical to the SDZ and may represent a gradient in creep rate across the CDZ. The SDZ and NE-CDZ have lower clay contents and larger porphyroclasts than most of the CDZ, and they contain veinlets and strain fringes of calcite in the gouge matrix not seen elsewhere in the CDZ. Matrix clays in the SDZ and NE-CDZ are saponite and corrensite, whereas the rest of the CDZ lacks corrensite. Saponite is younger than corrensite, reflecting clay crystallization under declining temperatures, and clays in the more actively deforming portions of the CDZ have better equilibrated to the lower-temperature conditions. © 2014. 2014 English 01918141 10.1016/j.jsg.2014.09.002 Journal of Structural Geology 68 Elsevier Ltd 82-96 U. S. Geological Survey, 345 Middlefield Road MS/977, Menlo Park, CA 94025, United States PA Corrensite; Fault gouge; SAFOD; San Andreas fault; Saponite; Shearing-enhanced reactions, corrensite; creep; damage; deformation; fault gouge; mineralogy; San Andreas Fault; saponite; sedimentary rock; serpentinite; shear zone; texture https://www.scopus.com/inward/record.uri?eid=2-s2.0-84907863573&doi=10.1016%2fj.jsg.2014.09.002&partnerID=40&md5=b98e68ae3c728aa7f1132446d9f5d20a cited By 19 D.E. Moore article Johri20142057 We analyze fracture-density variations in subsurface fault-damage zones in two distinct geologic environments, adjacent to faults in the granitic SSC reservoir and adjacent to faults in arkosic sandstones near the San Andreas fault in central California. These damage zones are similar in terms of width, peak fracture or fault (FF) density, and the rate of FF density decay with distance from the main fault. Seismic images from the SSC reservoir exhibit a large basement master fault associated with 27 seismically resolvable second-order faults. A maximum of 5 to 6 FF/m (1.5 to 1.8 FF/ft) are observed in the 50 to 80 m (164 to 262 ft) wide damage zones associated with second-order faults that are identified in image logs from four wells. Damage zones associated with second-order faults immediately southwest of the San Andreas Fault are also interpreted using image logs from the San Andreas Fault Observatory at Depth (SAFOD) borehole. These damage zones are also 50-80 m wide (164 to 262 ft) with peak FF density of 2.5 to 6 FF/m (0.8 to 1.8 FF/ft). The FF density in damage zones observed in both the study areas is found to decay with distance according to a power law F = F0r-n. The fault constant F0 is the FF density at unit distance from the fault, which is about 10-30 FF/m (3.1-9.1 FF/ft) in the SSC reservoir and 6-17 FF/m (1.8-5.2 FF/ft) in the arkose. The decay rate n ranges from 0.68 to 1.06 in the SSC reservoir, and from 0.4 to 0.75 in the arkosic section. This quantification of damage-zone attributes can facilitate the incorporation of the geometry and properties of damage zones in reservoir flow simulation models. Copyright © 2014. The American Association of Petroleum Geologists. All rights reserved. 2014 English 01491423 10.1306/05061413173 AAPG Bulletin 98 American Association of Petroleum Geologists 2057-2079 Department of Geophysics, 397 Panama Mall, Stanford UniversityCA 94305, United States; BP America, 501 Westlake Park Blvd., Houston, TX 77079, United States; ConocoPhillips Technology and Projects, 600 N. Dairy Ashford, Houston, TX 77079, United States 10 Boreholes; Decay (organic); Well logging, Density decay; Fault damage zone; Fracture density; Geologic environment; Reservoir flow; San Andreas fault; Second orders; Seismic image, Strike-slip faults, damage mechanics; fault geometry; fault plane; fault propagation; fracture zone; sandstone; seismic stratigraphy; simulation, California; California; San Andreas; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-84916912288&doi=10.1306%2f05061413173&partnerID=40&md5=781cd0f5f4e45fee5e65caee0d4bdf3a cited By 53 M. Johri M.D. Zoback P. Hennings article Warr2014234 The San Andreas Fault is one of the most studied earthquake-generating structures on Earth, but the reason that some sections are anomalously weak, and creep without apparent seismicity, remains poorly understood. Here, we present results from nanoscale (FIB-SEM) 3D microstructural observations of weak (friction coefficient of 0.095) SAFOD clay fault gouge containing serpentinite clasts, recovered from the active Central Deforming Zone at ~2.7km vertical depth. Our nanoscale observations confirm that frictional slip and extreme weakness occurvia deformation of smectite clay that forms a shear fabric within the fault zone. We infer that creep initiates by fracture-controlled, substrate growth of oriented Mg-smectite on R, P and Y shears, followed by clay smearing and ductile flow of an evolving and expanding clay matrix. At the crystal-scale, pervasive sliding occurs along hydrated smectite interlayers and surfaces occupied by exchangeable Mg- and Ca-ions, with slip typically spaced at 3-5 lattice layers apart. We conclude that the strength and seismic behaviour of major tectonic faults at shallow crustal levels evolves as clay fabric develops with accumulated fault slip. © 2014 Elsevier Ltd. 2014 English 01918141 10.1016/j.jsg.2014.10.006 Journal of Structural Geology 69 Elsevier Ltd 234-244 Institut für Geographie und Geologie, Ernst-Moritz-Arndt Universität Greifswald, F.L. Jahn-Str. 17a, Greifswald, 17487, Germany; INGV, Section, Via di Vigna Murata 605, Rome, 00143, Italy; Department of Geosciences Center for Geomechanics, Geofluids and Geohazards, The Pennsylvania State University, University Park, PA 16802, United States; Department of Earth and Environmental Sciences, University of Michigan, 1100 University Ave, Ann Arbor, MI 48109, United States PA Clay fabrics; SAFOD; San Andreas fault; SEM and TEM; Smectites, Creep, creep; deformation; fault gouge; fault zone; magnesium; San Andreas Fault; scanning electron microscopy; seismicity; smectite; transmission electron microscopy https://www.scopus.com/inward/record.uri?eid=2-s2.0-84909579317&doi=10.1016%2fj.jsg.2014.10.006&partnerID=40&md5=3afcb654bca961b37a130b3961fa5b13 cited By 24 L.N. Warr J. Wojatschke B.M. Carpenter C. Marone A.M. Schleicher B.A. Pluijm article French20141777 The strength of clay-rich gouge from the Central Deforming Zone (CDZ) of the San Andreas Fault (SAF) was measured using a high-speed rotary shear apparatus to evaluate the potential for unstable slip along the creeping segment of the SAF. Wet and dry gouge was sheared at 0.1-1.3 m/s, 0.5-1.5 MPa normal stress, and 1-20 m displacement. CDZ gouge is weaker wet than dry and exhibits displacement strengthening to peak friction followed by weakening to steady state strength that decreases with increasing velocity. A clay foliation (Unit 2) develops from the initial microstructure (Unit 1) during the first 1.5 m of slip coincident with increasing strength. Subsequent weakening occurs during shear within Unit 2, and subsequently with development of a localized foliated slip zone (Unit 4) and fluidized material (Unit 3). Displacement and dynamic weakening result from slip along clay foliation assisted by shear-heating pressurization of pore fluid in wet gouge and additional grain-size reduction and possible clay dehydration in dry gouge. Peak strength is proportional to normal stress, but steady state strength is insensitive to normal stress probably because pore pressure approaches the normal stress. As such, CDZ gouge is weak at coseismic rates relative to interseismic creep strength. The potential for sustaining rupture propagation into the CDZ from an adjacent seismic segment is sensitive to the relationship used to extrapolate the critical weakening displacement from experimental to in situ conditions. Rupture propagation from a microseismic patch within the CDZ is unlikely, but sustained propagation from a large earthquake (e.g., Parkfield event) may be possible. ©2014. American Geophysical Union. All Rights Reserved. 2014 English 21699313 10.1002/2013JB010757 Journal of Geophysical Research: Solid Earth 119 Blackwell Publishing Ltd 1777-1802 Department of Geology and Geophysics, Center for Tectonophysics, Texas A and M University, College Station, TX, United States; Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan; Kochi Institute for Core Sample Research, JAMSTEC, Nankoku, Japan 3 clay; coseismic process; dehydration; displacement; fault gouge; grain size; microstructure; pore pressure; San Andreas Fault; smectite; stress analysis, California; San Andreas; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-84923309339&doi=10.1002%2f2013JB010757&partnerID=40&md5=574bf0d05df2ef52d19933796b79bc17 cited By 32 M.E. French H. Kitajima J.S. Chester F.M. Chester T. Hirose article Schleicher20131039 To understand the role of hydrated clay minerals in active fault systems, a humidity chamber connected to an X-ray diffractometer was used to determine the adsorption of water onto and/or into the crystal structure of smectite. This new type of analysis was carried out under specific temperature and humidity conditions, using powdered clay size fractions (< 2 μm) of rock samples from the San Andreas Fault (USA) and the Nankai Trough (Japan). Pressure cannot be controlled, but does not significantly affect clay swelling at shallow conditions. Air-dried samples show a discrete smectite phase that swells after traditional ethylene glycolation to an interlayer distance of 1.5 and 1.7 nm. Using the humidity chamber, however, the samples show a shorter interlayer distance, between 1.09 and 1.54 nm. Based on our analysis, we show that (i) ethylene glycol overestimates the size of the interlayer space, and therefore water content, so is a crude maximum only; (ii) interlayer swelling occurs in smectite clay minerals at all temperatures between 25 and 95°C; and (iii) particle orientation increases with increasing humidity, indicating a higher mobility of smectite from interlayer hydration. Detailed characterization of the hydration state of smectite under original conditions is critical for understanding of clay-fluid interaction, the mechanical behavior during fault displacements, and fluid budgets at depth. We propose that humidity chamber experiments should be the new standard procedure to constrain swelling characteristics of natural and synthetic clay minerals. © 2013. American Geophysical Union. All Rights Reserved. 2013 English 15252027 10.1002/ggge.20077 Geochemistry, Geophysics, Geosystems 14 Blackwell Publishing Ltd 1039-1052 University of Michigan, Department of Earth and Environmental Sciences, 1100 N. University Ave, Ann Arbor, MI 48109-1005, United States; not available, 2-2475 West 3rd Avenue, Vancouver, BC V6K 1L9, Canada 4 Clay minerals; Ethylene; Ethylene glycol; Particle size analysis; Strike-slip faults; Tectonics, clay-hydration; Humidity chambers; NanTroSEIZE; SAFOD; Smectites, Hydration, active fault; adsorption; crystal structure; displacement; experimental mineralogy; humidity; hydration; San Andreas Fault; smectite; swelling; temperature effect; water; water content, Japan; Nankai Trough; Pacific Ocean; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-84878396633&doi=10.1002%2fggge.20077&partnerID=40&md5=350a56a8779b2f3033abd8bfd13827f2 cited By 18 A.M. Schleicher H. Hofmann article Babaie201383 Scientific drilling near Parkfield, California has established the San Andreas Fault Observatory at Depth (SAFOD), which provides the solid earth community with short range geophysical and fault zone material data. The BM2KB ontology was developed in order to formalize the knowledge about brittle microstructures in the fault rocks sampled from the SAFOD cores. A knowledge base, instantiated from this domain ontology, stores and presents the observed microstructural and analytical data with respect to implications for brittle deformation and mechanics of faulting. These data can be searched on the knowledge base's Web interface by selecting a set of terms (classes, properties) from different drop-down lists that are dynamically populated from the ontology. In addition to this general search, a query can also be conducted to view data contributed by a specific investigator. A search by sample is done using the EarthScope SAFOD Core Viewer that allows a user to locate samples on high resolution images of core sections belonging to different runs and holes.The class hierarchy of the BM2KB ontology was initially designed using the Unified Modeling Language (UML), which was used as a visual guide to develop the ontology in OWL applying the Protégé ontology editor. Various Semantic Web technologies such as the RDF, RDFS, and OWL ontology languages, SPARQL query language, and Pellet reasoning engine, were used to develop the ontology. An interactive Web application interface was developed through Jena, a java based framework, with AJAX technology, jsp pages, and java servlets, and deployed via an Apache tomcat server. The interface allows the registered user to submit data related to their research on a sample of the SAFOD core. The submitted data, after initial review by the knowledge base administrator, are added to the extensible knowledge base and become available in subsequent queries to all types of users. The interface facilitates inference capabilities in the ontology, supports SPARQL queries, allows for modifications based on successive discoveries, and provides an accessible knowledge base on the Web. © 2013 Elsevier Ltd. 2013 English 00983004 10.1016/j.cageo.2013.03.004 Computers and Geosciences 56 83-91 Department of Geosciences, Georgia State University, P.O. Box 4105, Atlanta, GA 30302-4105, United States; Department of Geography and Geosciences, University of Louisville, Louisville, KY 40292, United States Apache Tomcat server; High resolution image; Interactive web applications; OWL; RDF; SAFOD core; San Andreas fault; Semantic Web technology, Knowledge based systems; Microstructure; Query languages; Strike-slip faults; Unified Modeling Language, Core samples, brittle deformation; brittle medium; drilling; faulting; knowledge; microstructure; San Andreas Fault https://www.scopus.com/inward/record.uri?eid=2-s2.0-84876336988&doi=10.1016%2fj.cageo.2013.03.004&partnerID=40&md5=e060233aaa72aa45312d8bdca5a6984d cited By 2 H.A. Babaie M. Broda Cindi J. Hadizadeh A. Kumar article Tietze2013130 3-D inversion techniques have become a widely used tool in magnetotelluric (MT) data interpretation. However, with real data sets, many of the controlling factors for the outcome of 3-D inversion are little explored, such as alignment of the coordinate system, handling and influence of data errors and model regularization. Here we present 3-D inversion results of 169 MT sites from the central San Andreas Fault in California. Previous extensive 2-D inversion and 3-D forward modelling of the data set revealed significant along-strike variation of the electrical conductivity structure. 3-D inversion can recover these features but only if the inversion parameters are tuned in accordance with the particularities of the data set. Based on synthetic 3-D data we explore the model space and test the impacts of a wide range of inversion settings. The tests showed that the recovery of a pronounced regional 2-D structure in inversion of the complete impedance tensor depends on the coordinate system. As interdependencies between data components are not considered in standard 3-D MT inversion codes, 2-D subsurface structures can vanish if data are not aligned with the regional strike direction. A priori models and data weighting, that is, how strongly individual components of the impedance tensor and/or vertical magnetic field transfer functions dominate the solution, are crucial controls for the outcome of 3-D inversion. If deviations from a prior model are heavily penalized, regularization is prone to result in erroneous and misleading 3-D inversion models, particularly in the presence of strong conductivity contrasts. A 'good' overall rms misfit is often meaningless or misleading as a huge range of 3-D inversion results exist, all with similarly 'acceptable' misfits but producing significantly differing images of the conductivity structures. Reliable and meaningful 3-D inversion models can only be recovered if data misfit is assessed systematically in the frequency-space domain. © The Authors 2013 Published by Oxford University Press on behalf of The Royal Astronomical Society. 2013 English 0956540X 10.1093/gji/ggt234 Geophysical Journal International 195 130-147 Helmholtz Centre Potsdam, German Research Centre for Geosciences-GFZ, Potsdam, Germany 1 Continental margins: transforms; Crustal structure; Geomagnetic induction; Inverse theory; Magnetotelluric; North America, Data processing; Electric conductivity; Geomagnetism; Magnetotellurics; Recovery; Strike-slip faults; Structural geology; Tensors, Three dimensional, continental margin; crustal structure; data inversion; electrical conductivity; fault zone; geomagnetic field; magnetotelluric method; three-dimensional modeling, California; San Andreas; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-84885760220&doi=10.1093%2fgji%2fggt234&partnerID=40&md5=6de304a1ceb0b77d5fa8aede272cf1a2 cited By 87 K. Tietze O. Ritter article Lewis2012626 We use cross-correlation of seismic noise recorded at stations in the San Andreas Fault Observatory at Depth (SAFOD) pilot hole to extract P and S waves and measure S-wave anisotropy on the horizontal components. The data are recorded at seven three-component stations at depths from 1857 to 2097 m in the pilot hole. In late September and early 2004 October drilling noise underneath the stations generated propagating waves, which were absent in the rest of October. Estimates of the P- and S-wave velocities from the cross-correlations, on the vertical and horizontal components, respectively, are consistent with velocity measurements taken directly in the borehole. We observe polarization of the S wave, with a fast polarization direction of 120°-130° that is 4 per cent faster than the slow direction. Cross-correlation of the seismic noise can accurately determine S-wave anisotropy. © 2011 The Authors Geophysical Journal International © 2011 RAS. 2012 English 0956540X 10.1111/j.1365-246X.2011.05285.x Geophysical Journal International 188 626-630 Scripps Institution of Oceanography, La Jolla, CA 92093, United States; Earthworks Environment and Resources Ltd, Salisbury, SP2 7NU, United Kingdom 2 Body waves; Cross correlations; Drilling noise; P- and S-waves; Pilot holes; Polarization direction; S-wave velocity; S-waves; San Andreas Fault; Seismic anisotropy; Seismic noise; Shear-wave anisotropy; Three-component, Anisotropy; Interferometry; Polarization; Seismology; Shear waves; Velocity measurement, Shear flow, borehole; correlation; interferometry; P-wave; S-wave; San Andreas Fault; seismic anisotropy; seismic noise; wave propagation; wave velocity, California; Parkfield; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-84855441215&doi=10.1111%2fj.1365-246X.2011.05285.x&partnerID=40&md5=54bbb290a4344058c1fe69e5fc7d2982 cited By 6 M.A. Lewis P. Gerstoft article Becken201265 Fluids residing in interconnected porosity networks have a significant weakening effect on the rheology of rocks and can strongly influence deformation along fault zones. The magnetotelluric (MT) technique is sensitive to interconnected fluid networks and can image these zones on crustal and upper mantle scales. MT images have revealed several prominent electrical conductivity anomalies at the San Andreas Fault which have been attributed to the presence of saline fluids within such networks and which have been associated with tectonic processes. These models suggest that ongoing fluid release in the upper mantle and lower crust is closely related to the mechanical state of the crust. Where fluids are drained into the brittle crust, and where these fluids are kept at high pressures, fault creep is supported. Fluid fluxes from deeper levels, in combination with meteoric and crustal metamorphic fluid inflow, and in response to fault creep, leads to high-conductivity zones developing as fault zone conductors in the brittle portion of crust. In turn, the absence of crustal fluid pathways may be characteristic for mechanically locked segments of the fault. Here, MT models suggest that fluids are trapped at depth and kept at high pressures. We speculate that fluids may infiltrate neighboring rocks and in their wake induce non-volcanic tremor. © 2011 Springer Science+Business Media B.V. 2012 English 01693298 10.1007/s10712-011-9144-0 Surveys in Geophysics 33 Kluwer Academic Publishers 65-105 GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany; Institute of Geophysics, WWU, Corrensstr. 24, 48149 Münster, Germany 1 Creep; Fluids; Magnetotellurics; Strike-slip faults; Volcanic rocks, Electrical conductivity; High-conductivity zones; Interconnected porosity; Magnetotelluric studies; Metamorphic fluids; Non-volcanic tremors; San Andreas fault; Weakening effect, Structural geology, deformation mechanism; electrical conductivity; fault zone; high pressure; magnetotelluric method; porosity; rheology; San Andreas Fault, California; San Andreas; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-82955212963&doi=10.1007%2fs10712-011-9144-0&partnerID=40&md5=298ad4b0fd95956e5af0fad201454c77 cited By 67 M. Becken O. Ritter article Pollitz20121348 A series of near-surface chemical explosions conducted at the San Andreas Fault Observatory at Depth (SAFOD) main hole were recorded by highfrequency downhole receiver arrays in April 2005. These seismic recordings at depths ranging from the surface to 2.3 km constrain the shallow velocity and attenuation structure as well as the first-order characteristics of the source. Forward modeling of the explosions indicates that a source consisting of combined explosion, delayed implosion, and second-order moment-tensor components (corresponding to a distribution of vertical shear dislocations in the rock directly above the explosion) is sufficient to characterize the generated seismic wave fields to first order. Grid searches over source parameters controlling the nonexplosive components allow for the quantification of distributed vertical shear above the source and the estimation of the moment and time delay of the implosive component relative to the explosion. An estimated implosive to explosive moment ratio of 0.34 to 0.43 indicates a net static moment and positive macroscopic volume change. 2012 English 00371106 10.1785/0120110201 Bulletin of the Seismological Society of America 102 1348-1360 U.S. Geological Survey, 345 Middlefield Rd. MS 977, Menlo Park, CA 94025, United States 4 Chemical explosions; Downholes; First-order; Forward modeling; Grid search; High frequency HF; Moment ratio; Near-surface; Receiver array; San Andreas fault; Second orders; Seismic recording; Source characterization; Source parameters; Static moment; Vertical shear; Volume change, Strike-slip faults, Explosions, moment tensor; nuclear explosion; seismic attenuation; seismic velocity; seismic wave; source parameters https://www.scopus.com/inward/record.uri?eid=2-s2.0-84865044400&doi=10.1785%2f0120110201&partnerID=40&md5=5c393e3bb5cd561a752cf045bcb39cd0 cited By 3 F.F. Pollitz J. Rubinstein W. Ellsworth article Ryberg20121341 A seismic reflection/refraction survey across the San Andreas fault near Parkfield, California, has refined our knowledge of the upper crustal structure of the central California Coast Ranges at the San Andreas Fault Observatory at Depth (SAFOD). The survey consisted of a 46-km-long line of seismographs (25-50 m spacing) and 63 explosions (25-200 kg; nominal spacing of 500 m, with some gaps). The traveltimes of refracted P and S waves from the explosions constitute independent data sets of relatively high quality that were inverted to produce P- and S-wave velocity models (V p, V s) along the profile, extending to as much as 5 km depth. The V p and V s models show a prominent lateral drop in velocities a few hundred metres northeast of SAFOD, between the drill hole and the San Andreas fault. The V p model shows particularly well a southwest-dipping velocity inversion beneath SAFOD, the top of which correlates with a fault penetrated by the drill hole that separates granitic rocks above from sedimentary rocks below. In addition to V p and V s models, a V p/V s model was derived. A V p/V s ratio lower than 1.73 is seen only at depth, in a narrow zone beginning at the target earthquakes for SAFOD and extending downward and northeastward into the North America Plate. Clusters in the parameter space spanned by V p/V s ratios and V p can be identified by two different methods, one more intuitive analytical method and one more abstract method based on neural network techniques. These clusters are correlated to different rock types, based on laboratory and in situ data. These clusters are remapped back into x-z plane along the profile. Prominent features mapped this way include Salinian granitic rocks beneath and west of SAFOD, and a body of sedimentary rocks faulted beneath these granitic rocks along what we and others interpret to be a branch of the Buzzard Canyon Fault (BCF) system. These sedimentary rocks extend from this fault to the San Andreas fault system. Unfortunately, our cluster analysis shows no significant discontinuity at the San Andreas fault, owing presumably to the fact that the San Andreas fault is located within sedimentary rocks having similar elastic properties. This paper is an attempt to 'downward' continue a geological map by geophysical means based on elastic properties of rock samples from the region. © 2012 The Authors Geophysical Journal International © 2012 RAS. 2012 English 0956540X 10.1111/j.1365-246X.2012.05585.x Geophysical Journal International 190 1341-1360 Helmholz-Zentrum Potsdam, Deutsches GeoForschungsZentrum GFZ, Telegrafenberg, 14473 Potsdam, Germany; Department of Geosciences, Virginia Tech, 4044 Derring Hall, Blacksburg, VA 24061, United States; U.S. Geological Survey, 345 Middlefield Road MS 977, Menlo Park, CA 94025-3591, United States; Institute of Geosciences, University of Jena, Burgweg 11, D-07749 Jena, Germany 3 Body waves; Continental margin; Controlled source; North America; Seismic tomography, Cluster analysis; Elasticity; Granite; Rock drills; Sedimentary rocks; Seismographs; Seismology; Shear waves; Strike-slip faults; Surveys; Transform faults, Structural geology, body wave; cluster analysis; continental margin; P-wave; S-wave; San Andreas Fault; seismic data; seismic reflection; seismic refraction; seismic source; seismic survey; seismic tomography; seismic velocity; seismograph; seismology; transform fault; upper crust, California; Coast Ranges; San Andreas; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-84865282849&doi=10.1111%2fj.1365-246X.2012.05585.x&partnerID=40&md5=9d718c69b2c15773e96d0f69ccce925e cited By 13 T. Ryberg J.A. Hole G.S. Fuis M.J. Rymer F. Bleibinhaus D. Stromeyer K. Bauer article Popov2012 We present a three-dimensional finite element thermomechanical model idealizing the complex deformation processes associated with evolution of the San Andreas Fault system (SAFS) in northern and central California over the past 20 Myr. More specifically, we investigate the mechanisms responsible for the eastward (landward) migration of the San Andreas plate boundary over time, a process that has largely determined the evolution and present structure of SAFS. Two possible mechanisms had been previously suggested. One mechanism suggests that the Pacific plate first cools and captures uprising mantle in the slab window, subsequently causing accretion of the continental crustal blocks. An alternative scenario attributes accretion to the capture of plate fragments (microplates) stalled in the ceased Farallon-North America subduction zone. Here we test both these scenarios numerically using a recently developed lithospheric-scale code, SLIM3D, that employs free surface, nonlinear temperature- and stress-dependent elastoviscoplastic rheology and allows for self-generation of faults. Modeling suggests that microplate capture is the primary driving mechanism for the eastward migration of the plate boundary, while the slab window cooling mechanism alone is incapable of explaining this phenomenon. We also show that the system evolves to the present day structure of SAFS only if the coefficient of friction at mature faults is low (0.08 for the best fit model). Thus, our model provides an independent constraint supporting the "weak fault in a strong crust" hypothesis for SAFS. © 2012. American Geophysical Union. All Rights Reserved. 2012 English 15252027 10.1029/2012GC004086 Geochemistry, Geophysics, Geosystems 13 Blackwell Publishing Ltd Section of Geodynamic Modeling, GeoForschungsZentrum, DE-14473 Potsdam, Germany; Now at Institute for Geosciences, Johannes Gutenberg University, DE-55099 Mainz, Germany; Schmidt Institute of Physics of the Earth, 123995 Moscow, Russian Federation; Department of Geophysics, Stanford University, Stanford, CA 94305-2215, United States 8 Friction; Geodynamics; Plates (structural components); Strike-slip faults; Tectonics, Best-fit models; California; Coefficient of frictions; Complex deformation; Cooling mechanism; Crustal block; Driving mechanism; Elasto-viscoplastic; Free surfaces; Microplates; Nonlinear temperature; Numeric models; Pacific plates; Plate boundaries; San Andreas fault; Self-generation; Slab windows; Stress-dependent; Subduction zones; Thermomechanical model; Three dimensional finite elements; weak fault in strong crust, Three dimensional, deformation; finite element method; friction; geodynamics; numerical model; Pacific plate; plate boundary; San Andreas Fault; subduction zone; tectonic evolution, California; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-84865788317&doi=10.1029%2f2012GC004086&partnerID=40&md5=ec0ef4838e29cfda147ea7d37e538d59 cited By 23 A.A. Popov S.V. Sobolev M.D. Zoback article Spencer2012280 This study aims to assess whether luminescence emission from fault gouge samples from the San Andreas Fault Observatory at Depth (SAFOD) can be used to determine the age distribution of distinct deformation microstructures. Such age determination could help constrain some of the proposed micromechanical models for shear localization in fault gouge, in addition to providing more accurate time constraint on the seismic cycle itself. The mechanism by which previously trapped charge is reset in minerals in fault gouge is thought to be a combination of frictional heating and mechanical deformation, and these processes may be localized to grain surfaces. An added dating complexity specific to deep samples is the high ambient temperature conditions, which act as a barrier to charge storage in lower energy trapping sites. In this work luminescence experiments are being conducted on minerals from whole-rock samples of intact fault gouge from the SAFOD Phase III core. Initial studies indicate (i) the thermal and radiation history of the mineral lattice can be assessed with TL, (ii) trap resetting is evident in both TL and IRSL data, (iii) a small charge-trapping window between drill hole ambient temperature of ∼112 °C and higher energy lattice excitation via rupture events is evident in TL data from ∼300 to 400 °C, and we tentatively link the source of IRSL to TL within this 300-400 °C region, (iv) IRSL data have low natural intensity but good luminescence characteristics, and (v) SAR IRSL D e data have high over-dispersion but demonstrate ages ranging from decades to centuries may be measured. © 2012 Elsevier B.V.. 2012 English 18711014 10.1016/j.quageo.2012.04.023 Quaternary Geochronology 10 280-284 Department of Geology, Kansas State University, Manhattan, KS 66506, United States; Department of Geography and Geosciences, University of Louisville, Louisville, KY 40292, United States; ISTerre, Université Joseph Fourier, Grenoble, France complexity; deformation; fault gouge; fault zone; luminescence dating; microstructure; resetting; rupture; San Andreas Fault, California; San Andreas; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-84863778952&doi=10.1016%2fj.quageo.2012.04.023&partnerID=40&md5=b65c24b3d7de7f06f76ced200a839785 cited By 12 J.Q.G. Spencer J. Hadizadeh J.-P. Gratier M.-L. Doan article Carpenter2012759 The strength of tectonic faults and the processes that control earthquake rupture remain central questions in fault mechanics and earthquake science. We report on the frictional strength and constitutive properties of intact samples across the main creeping strand of the San Andreas fault (SAF; California, United States) recovered by deep drilling. We find that the fault is extremely weak (friction coefficient, μ = ∼ 0.10), and exhibits both velocity strengthening frictional behavior and anomalously low rates of frictional healing, consistent with aseismic creep. In contrast, wall rock to the northeast shows velocity weakening frictional behavior and positive healing rates, consistent with observed repeating earthquakes on nearby fault strands. We also document a sharp increase in strength to values of μ > ∼0.40 over <1 m distance at the boundary between the fault and adjacent wall rock. The friction values for the SAF are sufficiently low to explain its apparent weakness as inferred from heat flow and stress orientation data. Our results may also indicate that the shear strength of the SAF should remain approximately constant at ∼10 MPa in the upper 5-8 km, rather than increasing linearly with depth, as is commonly assumed. Taken together, our data explain why the main strand of the SAF in central California is weak, extremely localized, and exhibits aseismic creep, while nearby fault strands host repeating earthquakes. © 2012 Geological Society of America. 2012 English 00917613 10.1130/G33007.1 Geology 40 759-762 Department of Geosciences and Energy, Institute Center for Geomechanics, Geofluids, and Geohazards, The Pennsylvania State University, University Park, PA 16802, United States 8 California; Constitutive properties; Deep drilling; Drill core; Earthquake rupture; Fault strands; Friction coefficients; Friction values; Frictional behavior; Frictional properties; Frictional strength; Low rates; Repeating earthquake; San Andreas fault; Sharp increase; Sliding stability; Stress orientations; Tectonic faults, Core drilling; Creep; Earthquakes; Strike-slip faults, Friction, earthquake mechanism; earthquake recurrence; earthquake rupture; fault geometry; friction; rupture; San Andreas Fault; shear strength; slip rate; stability analysis; stress field; tectonic setting; wall rock, California; San Andreas; United States, Calluna vulgaris https://www.scopus.com/inward/record.uri?eid=2-s2.0-84866159192&doi=10.1130%2fG33007.1&partnerID=40&md5=fa0b71405050dbdbf54a37edea2796a9 cited By 82 B.M. Carpenter D.M. Saffer C. Marone article Moore201251 Magnesium-rich clayey gouge similar to that comprising the two actively creeping strands of the San Andreas Fault in drill core from the San Andreas Fault Observatory at Depth (SAFOD) has been identified in a nearby outcrop of serpentinite within the fault zone at Nelson Creek. Each occurrence of the gouge consists of porphyroclasts of serpentinite and sedimentary rocks dispersed in a fine-grained, foliated matrix of Mg-rich smectitic clays. The clay minerals in all three gouges are interpreted to be the product of fluid-assisted, shear-enhanced reactions between quartzofeldspathic wall rocks and serpentinite that was tectonically entrained in the fault from a source in the Coast Range Ophiolite. We infer that the gouge at Nelson Creek connects to one or both of the gouge zones in the SAFOD core, and that similar gouge may occur at depths in between. The special significance of the outcrop is that it preserves the early stages of mineral reactions that are greatly advanced at depth, and it confirms the involvement of serpentinite and the Mg-rich phyllosilicate minerals that replace it in promoting creep along the central San Andreas Fault. © 2011. 2012 English 01918141 10.1016/j.jsg.2011.11.014 Journal of Structural Geology 38 51-60 U.S. Geological Survey, Earthquake Science Center, 345 Middlefield Road, Mail Stop 977, Menlo Park, CA 94025, United States Coast range ophiolites; Metasomatic reactions; SAFOD; San Andreas fault; Serpentinite; Smectite clays, Buildings; Clay minerals; Core drilling; Magnesium; Observatories, Strike-slip faults, correlation; creep; fault gouge; fault zone; metasomatism; ophiolite; outcrop; San Andreas Fault; sedimentary rock; serpentinite; smectite; tectonic setting https://www.scopus.com/inward/record.uri?eid=2-s2.0-84859931557&doi=10.1016%2fj.jsg.2011.11.014&partnerID=40&md5=3a8b494b4880c9fefebb0022c7983086 cited By 44 D.E. Moore M.J. Rymer article Janssen2012118 With optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and synchrotron X-ray diffraction measurements, we analyzed clay microfabrics in ultracataclastic/gouge and cataclastic core samples obtained from the main bore hole of the San Andreas Fault observatory at depth (SAFOD). The analysis reveals a significant contrast between weak clay fabrics observed in the core samples with synchrotron X-ray fabric measurements and strong degree of preferred alignment for clay particles documented with the optical microscope. TEM and SEM observations also show distinct zones of locally aligned and randomly oriented clay minerals. The lack of a strong fabric may be attributed to randomly oriented matrix sheet silicates dominating the fault rocks. The presence of weak fabrics in intensely strained ultracataclasites/fault gouges is attributed to 1) newly formed clay minerals that grew in many orientations, 2) folded and kinked clay minerals, and 3) clay particles that are wrapped around grains. In addition, the locally aligned clay particles may act as barriers to fluid flow, which in turn decrease porosity, expel intergranular pore fluids, and consequently, may increase fluid pressure. © 2012 Elsevier Ltd. 2012 English 01918141 10.1016/j.jsg.2012.07.004 Journal of Structural Geology 43 118-127 GeoForschungsZentrum, Telegrafenberg, Potsdam 14473, Germany; Department of Earth and Planetary Science, University of California, Berkeley, CA 94720, United States Clay fabrics; Clay particles; Fault rock; Fluid pressures; Intergranular pores; Micro fabric; Optical microscopes; Oriented matrix; Phyllosilicate; SAFOD; San Andreas fault; SEM observation; Synchrotron x ray diffraction; Synchrotron x rays; Transmission electron microscopy tem, Clay minerals; Optical microscopy; Scanning electron microscopy; Silicates; Strike-slip faults; Tectonics; Textures; Transmission electron microscopy; X ray diffraction, Core samples, clay mineral; core (planetary); fault gouge; fluid flow; petrofabric; phyllosilicate; porosity; San Andreas Fault; scanning electron microscopy; texture; transmission electron microscopy; X-ray diffraction https://www.scopus.com/inward/record.uri?eid=2-s2.0-84866268607&doi=10.1016%2fj.jsg.2012.07.004&partnerID=40&md5=5aa52acc78f434fdfb3c17c6ead8f344 cited By 22 C. Janssen W. Kanitpanyacharoen H.-R. Wenk R. Wirth L. Morales E. Rybacki M. Kienast G. Dresen article Schleicher2012209 Segments of the modern San Andreas fault experience creep behavior, which is attributed to various factors, including (1) low values of effective normal stress, (2) elevated pore-fluid pressure, and (3) low frictional strength. The San Andreas Fault Observatory at Depth (SAFOD) drill hole in Parkfield, California, provides new insights into frictional properties by recognizing the importance of smectitic clay minerals, as demonstrated by analysis of mudrock and fault gouge samples from zones between 3186 and 3199 m and 3295 and 3313 m measured depths. X-ray diffraction (XRD) results show illite, chlorite, and mixed-layered illite-smectite and chlorite-smectite minerals in the faulted mudrock, whereas serpentine, Mg-rich smectite, and chlorite-smectite minerals are concentrated in the southwest deformation zone and the central deformation zone of the two actively creeping sections in the San Andreas fault. These rocks are abundantly coated by shiny clay mineral layers in some cases, reflecting mineral formation during creep. Secondary- and transmission-electron microscopy (SEM/TEM) and XRD studies of these slip surface coatings reveal thin films of neoformed chlorite-smectite phases, similar to previously described illite-smectite microscale precipitations. The abundance of chlorite-smectite minerals within fault rock of the SAFOD borehole significantly extends the potential role of mineralogic processes to depths up to 10 km, with cataclasis and fluid infiltration creating nucleation sites for neomineralization on displacement surfaces. We propose that localization of illitic to chloritic smectite clay minerals on slip surfaces from near the surface to the brittle-ductile transition promotes creep behavior of faults. © 2012 Geological Society of America. 2012 English 19418264 10.1130/L158.1 Lithosphere 4 Geological Society of America 209-220 Department of Earth and Environmental Sciences, University of Michigan, 1100 North University Avenue, Ann Arbor, MI 48109, United States; Institut Für Geographie Und Geologie, Ernst-Moritz-Arndt Universität, F. Ludwig-Jahn-Strasse 17A, D-17487 Greifswald, Germany 3 Creep; Friction; High resolution transmission electron microscopy; Observatories; Serpentine; Silicate minerals; Strike-slip faults; Structural geology; X ray diffraction, Brittle ductile transitions; Deformation zone; Effective normal stress; Frictional properties; Frictional strength; Mineral formation; Pore fluid pressure; San Andreas fault, Clay minerals, brittle deformation; chlorite; creep; fault displacement; fluid pressure; infiltration; mudstone; precipitation (chemistry); smectite, California; Parkfield; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-84866276240&doi=10.1130%2fL158.1&partnerID=40&md5=92733e71192f0000d8937c8f6836b11a cited By 53 A.M. Schleicher B.A. Pluijm L.N. Warr article Hadizadeh2012246 The San Andreas Fault zone in central California accommodates tectonic strain by stable slip and microseismic activity. We study microstructural controls of strength and deformation in the fault using core samples provided by the San Andreas Fault Observatory at Depth (SAFOD) including gouge corresponding to presently active shearing intervals in the main borehole. The methods of study include high-resolution optical and electron microscopy, X-ray fluorescence mapping, X-ray powder diffraction, energy dispersive X-ray spectroscopy, white light interferometry, and image processing.The fault zone at the SAFOD site consists of a strongly deformed and foliated core zone that includes 2-3 m thick active shear zones, surrounded by less deformed rocks. Results suggest deformation and foliation of the core zone outside the active shear zones by alternating cataclasis and pressure solution mechanisms. The active shear zones, considered zones of large-scale shear localization, appear to be associated with an abundance of weak phases including smectite clays, serpentinite alteration products, and amorphous material. We suggest that deformation along the active shear zones is by a granular-type flow mechanism that involves frictional sliding of microlithons along phyllosilicate-rich Riedel shear surfaces as well as stress-driven diffusive mass transfer. The microstructural data may be interpreted to suggest that deformation in the active shear zones is strongly displacement-weakening. The fault creeps because the velocity strengthening weak gouge in the active shear zones is being sheared without strong restrengthening mechanisms such as cementation or fracture sealing. Possible mechanisms for the observed microseismicity in the creeping segment of the SAF include local high fluid pressure build-ups, hard asperity development by fracture-and-seal cycles, and stress build-up due to slip zone undulations. © 2012 Elsevier Ltd. 2012 English 01918141 10.1016/j.jsg.2012.04.011 Journal of Structural Geology 42 246-260 Department of Geography and Geosciences, University of Louisville, 212 Lutz Hall, Louisville KY 40292, United States; University of Padova, Padova, Italy; University Joseph Fourier and CNRS, ISTerre, BP 53, 3804 Grenoble, France; University of Oslo, PGP, Oslo, Norway; Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy; Georgia State University, Geosciences, Atlanta, GA, United States Cataclasis; Foliated gouge; Pressure solution; SAFOD; San Andreas fault; Shear localizations, Clay minerals; Creep; Electron microscopy; Fracture; Image processing; Strike-slip faults; Structural geology; X ray powder diffraction; X ray spectroscopy, Shear flow, asperity; creep; deformation mechanism; fault gouge; fault zone; foliation; microstructure; San Andreas Fault; seismicity; shear zone; slip, California; San Andreas; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-84865338082&doi=10.1016%2fj.jsg.2012.04.011&partnerID=40&md5=102bc01d3ed4e8190f2fa67056598cc8 cited By 46 J. Hadizadeh S. Mittempergher J.-P. Gratier F. Renard G. Di Toro J. Richard H.A. Babaie article Pollitz20111420 High-frequency and full waveform synthetic seismograms on a 3-D laterally heterogeneous earth model are simulated using the theory of coupled normal modes. The set of coupled integral equations that describe the 3-D response are simplified into a set of uncoupled integral equations by using the Born approximation to calculate scattered wavefields and the pure-path approximation to modulate the phase of incident and scattered wavefields. This depends upon a decomposition of the aspherical structure into smooth and rough components. The uncoupled integral equations are discretized and solved in the frequency domain, and time domain results are obtained by inverse Fourier transform. Examples show the utility of the normal mode approach to synthesize the seismic wavefields resulting from interaction with a combination of rough and smooth structural heterogeneities. This approach is applied to an ~4 Hz shallow crustal wave propagation around the site of the San Andreas Fault Observatory at Depth (SAFOD). © The Author Geophysical Journal International © 2011 RAS. 2011 English 0956540X 10.1111/j.1365-246X.2011.05188.x Geophysical Journal International 187 1420-1442 U.S. Geological Survey, Menlo Park, CA 94025, United States 3 Aspherical; Coupled integral equations; Earth models; Earthquake source; Frequency domains; Full-waveforms; High frequency HF; Inverse Fourier transforms; Normal modes; San Andreas Fault; Seismic wavefields; Structural heterogeneity; Synthetic seismogram; Time domain; Wavefields, Born approximation; Seismology; Three dimensional; Wave propagation, Integral equations, Fourier transform; seismic source; seismic wave; synthetic seismogram; wave field; wave propagation; waveform analysis https://www.scopus.com/inward/record.uri?eid=2-s2.0-81555209057&doi=10.1111%2fj.1365-246X.2011.05188.x&partnerID=40&md5=26194f4901998d8b44167256b36a9cf0 cited By 2 F. Pollitz article Weymer201148 2011 English 18168957 10.5194/sd-11-48-2011 Scientific Drilling Copernicus GmbH 48-50 Integrated Ocean Drilling Program and SAFOD, Texas A and M University, 1000 Discovery Drive, College Station, TX 77845-9547, United States; Department of Geology and Geophysics, TAMU, Center for Tectonophysics and Department of Geology and Geophysics, Texas A and M University, College Station, TX 77843-3115, United States; Geological Survey Earthquake Science Center, 345 Middlefield Road, MS/977Menlo Park, CA 94025, United States 1 https://www.scopus.com/inward/record.uri?eid=2-s2.0-79955129704&doi=10.5194%2fsd-11-48-2011&partnerID=40&md5=68ce30304ca45d0df6257e65dbdbeee8 cited By 0 B. Weymer J. Firth P. Rumford F. Chester J. Chester D. Lockner article Janssen2011179 With transmission electron microscopy (TEM) we observed nanometer-sized pores in four ultracataclastic and fractured core samples recovered from different depths of the main bore hole of the San Andreas Fault Observatory at Depth (SAFOD). Cutting of foils with a focused ion beam technique (FIB) allowed identifying porosity down to the nm scale. Between 40 and 50% of all pores could be identified as in-situ pores without any damage related to sample preparation. The total porosity estimated from TEM micrographs (1-5%) is comparable to the connected fault rock porosity (2.8-6.7%) estimated by pressure-induced injection of mercury. Permeability estimates for cataclastic fault rocks are 10-21-10-19 m2 and 10-17 m2 for the fractured fault rock. Porosity and permeability are independent of sample depth. TEM images reveal that the porosity is intimately linked to fault rock composition and associated with deformation. The TEM-estimated porosity of the samples increases with increasing clay content. The highest porosity was estimated in the vicinity of an active fault trace. The largest pores with an equivalent radius&gt;200nm occur around large quartz and feldspar grains or grain-fragments while the smallest pores (equivalent radius&lt;50nm) are typically observed in the extremely fine-grained matrix (grain size&lt;1&gt;m). Based on pore morphology we distinguish different pore types varying with fault rock fabric and alteration. The pores were probably filled with formation water and/or hydrothermal fluids at elevated pore fluid pressure, preventing pore collapse. The pore geometry derived from TEM observations and BET (Brunauer, Emmett and Teller) gas adsorption/desorption hysteresis curves indicates pore blocking effects in the fine-grained matrix. Observations of isolated pores in TEM micrographs and high pore body to pore throat ratios inferred from mercury injection suggest elevated pore fluid pressure in the low permeability cataclasites, reducing shear strength of the fault. © 2010 Elsevier B.V. 2011 English 0012821X 10.1016/j.epsl.2010.10.040 Earth and Planetary Science Letters 301 179-189 GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany; Department of Earth and Planetary Science, University of California, Berkeley, CA, United States 1-2 Fault rock; Permeability; SAFOD; San Andreas Fault; TEM, Core samples; Fluids; Ion beams; Mercury (metal); Porosity; Quartz; Rocks; Shear strength; Silicate minerals; Tectonics; Transmission electron microscopy, Structural geology, cataclasite; fault zone; feldspar; fluid pressure; hydrothermal fluid; permeability; porosity; quartz; San Andreas Fault; shear strength; transmission electron microscopy https://www.scopus.com/inward/record.uri?eid=2-s2.0-78650257877&doi=10.1016%2fj.epsl.2010.10.040&partnerID=40&md5=62106fb0f8797a5639df8e10800deca4 cited By 53 C. Janssen R. Wirth A. Reinicke E. Rybacki R. Naumann H.-R. Wenk G. Dresen article Wenk201169 Increasing use of diffraction methods to study preferred orientation of minerals has established that quartz in deformed rocks not only displays characteristic c-axis orientation patterns, but that there is also generally a distinct difference in the orientation of positive and negative rhombs. In the trigonal quartz crystal structure positive and negative rhombs are structurally different, and particularly negative rhombs (e.g. {01. 1-1}) are much stiffer than positive rhombs (e.g. {10. 1-1}). Here, we focus on the role of mechanical Dauphiné twinning under stress as a cause of this difference and illustrate with EBSD measurements ubiquitous twinning in quartz-bearing rocks subjected to high stresses. Characteristic twinning is observed in experimentally shocked sandstones and stishovite-bearing quartzites from the Vredefort meteorite impact site in South Africa. Similar twinning is documented for quartz associated with pseudotachylites from the Santa Rosa mylonite zone in Southern California, whereas quartz in underlying ductile mylonites are more or less twin-free. It suggests that twinning was produced by local seismic stresses that caused fracture and frictional melting on fault surfaces. Quartz-bearing breccias from the SAFOD (San Andreas Fault Observatory at Depth) drilling project also show evidence of twinning and suggest high seismic stresses in the currently creeping segment of the San Andreas Fault at Parkfield. From these observations it appears that Dauphiné twin microstructures can be diagnostic of high local and transient stresses. © 2011 Elsevier B.V. 2011 English 00401951 10.1016/j.tecto.2011.06.016 Tectonophysics 510 69-79 Department of Earth and Planetary Science, University of California, Berkeley, CA 94720, United States; GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany; Institut für Geowissenschaften, Geologie, Albert-Ludwigs-Universität, 79085 Freiburg, Germany 1-2 C-axis orientations; Creeping segment; Diffraction methods; Drilling projects; EBSD; Fault breccia; Fault surfaces; Frictional melting; High stress; Mechanical twinning; Meteorite impact; Mylonites; Preferred orientations; Pseudotachylites; San Andreas Fault; Seismic stress; Shock deformation; South Africa; Southern California; Transient stress; Twin-free, Crystal structure; Meteor impacts; Quartz deposits; Rocks; Seismology; Structural geology, Quartz, breccia; crystal structure; deformation mechanism; ductile deformation; experimental study; microstructure; preferred orientation; pseudotachylite; quartz; San Andreas Fault; twinning, South Africa https://www.scopus.com/inward/record.uri?eid=2-s2.0-80052732693&doi=10.1016%2fj.tecto.2011.06.016&partnerID=40&md5=e158acb0dcf8244b34cd65f3c37e4e17 cited By 35 H.-R. Wenk C. Janssen T. Kenkmann G. Dresen article Rybacki2011107 The microstructures of four core samples from the San Andreas Fault Observatory at Depth (SAFOD) were investigated with optical and transmission electron microscopy. These samples, consisting of sandstone, siltstone, and fault gouge from phase III of the drilling campaign (3141-3307m MD), show a complex composition of quartz, feldspar, clays, and amorphous material. Microstructures indicate intense shearing and dissolution-precipitation as main deformation processes. The samples also contain abundant veins filled with calcite. Within the inspected veins the calcite grains exhibit different degrees of deformation with evidence for twinning and crystal plasticity. Dislocation densities (ranging from≈3·1012m-2 to ≈3·1013m-2) and twin line densities (≈22mm-1-165mm-1) are used as paleo-piezometers. The corresponding estimates of differential stresses vary between 33 and 132MPa, deduced from dislocation density and 92-251MPa obtained from twin density, possibly reflecting chronologically different maximum stress states and/or grain scale stress perturbations. Mean values of stress estimates are 68±46MPa and 168±60MPa, respectively, where estimates from dislocation density may represent a lower bound and those from twin density an upper bound. The stress estimates are also compatible with residual lattice strains determined with microfocus Laue diffraction yielding equivalent stresses of 50-300MPa in twinned calcite. The lower stress bound agrees with stress estimates from borehole breakout measurements performed in the pilot hole. From these data and assuming hydrostatic pore pressure and a low intermediate principal stress close to the overburden stress, frictional sliding of the San Andreas Fault at the SAFOD site is constrained to friction coefficients between 0.24 and 0.31. These low friction values may be related to the presence of clays, talc, and amorphous phases found in the fault cores and support the hypothesis of a weak San Andreas Fault. © 2011 Elsevier B.V. 2011 English 00401951 10.1016/j.tecto.2011.05.014 Tectonophysics 509 107-119 GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany; Department of Earth and Planetary Science, University of California, Berkeley, CA, United States 1-2 Amorphous phasis; Calcite grains; Calcite veins; Complex compositions; Crystal plasticity; Deformation process; Differential stress; Dislocation densities; Equivalent stress; Fault core; Fault gouge; Friction coefficients; Frictional sliding; Intermediate principal stress; Lattice strain; Laue diffraction; Line density; Low friction; Low temperature deformations; Lower bounds; Lower stress; Maximum stress; Mean values; Microstructural analysis; Overburden stress; Piezometer; Pilot holes; Residual strains; SAFOD; San Andreas Fault; Scale stress; Siltstones; Upper Bound, Amorphous materials; Calcite; Deformation; Dislocations (crystals); Dissolution; Elasticity; Estimation; Friction; Light transmission; Microstructure; Quartz; Strain; Structural geology; Transmission electron microscopy; Twinning, Carbonate minerals, borehole breakout; calcite; deformation mechanism; dissolution; fault gouge; low temperature; microstructure; precipitation (chemistry); pressure effect; San Andreas Fault; sandstone; siltstone; transmission electron microscopy; twinning, California; San Andreas; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-79961031875&doi=10.1016%2fj.tecto.2011.05.014&partnerID=40&md5=ef8b3a401288ed04c6737632c161e1af cited By 28 E. Rybacki C. Janssen R. Wirth K. Chen H.-R. Wenk D. Stromeyer G. Dresen article Lockner201182 The San Andreas fault accommodates 28-"34-‰mm-‰yr ĝ̂'1 of right lateral motion of the Pacific crustal plate northwestward past the North American plate. In California, the fault is composed of two distinct locked segments that have produced great earthquakes in historical times, separated by a 150-km-long creeping zone. The San Andreas Fault Observatory at Depth (SAFOD) is a scientific borehole located northwest of Parkfield, California, near the southern end of the creeping zone. Core was recovered from across the actively deforming San Andreas fault at a vertical depth of 2.7-‰km (ref. 1). Here we report laboratory strength measurements of these fault core materials at in situ conditions, demonstrating that at this locality and this depth the San Andreas fault is profoundly weak (coefficient of friction, 0.15) owing to the presence of the smectite clay mineral saponite, which is one of the weakest phyllosilicates known. This Mg-rich clay is the low-temperature product of metasomatic reactions between the quartzofeldspathic wall rocks and serpentinite blocks in the fault. These findings provide strong evidence that deformation of the mechanically unusual creeping portions of the San Andreas fault system is controlled by the presence of weak minerals rather than by high fluid pressure or other proposed mechanisms. The combination of these measurements of fault core strength with borehole observations yields a self-consistent picture of the stress state of the San Andreas fault at the SAFOD site, in which the fault is intrinsically weak in an otherwise strong crust. © 2011 Macmillan Publishers Limited. All rights reserved. 2011 English 00280836 10.1038/nature09927 Nature 472 82-86 US Geological Survey, Menlo Park, CA 94025, United States 7341 magnesium; mineral; silicon dioxide, borehole; creep; deformation mechanism; fault gouge; historical record; San Andreas Fault; saponite; serpentine; vertical distribution, article; geography; laboratory; priority journal; strength; United States, California; Parkfield; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-79953738610&doi=10.1038%2fnature09927&partnerID=40&md5=d2bf98ea2248a1701496a7fff041dc35 cited By 291 D.A. Lockner C. Morrow D. Moore S. Hickman article Bradbury2011131 We characterize the lithology and structure of the spot core obtained in 2007 during Phase 3 drilling of the San Andreas Fault Observatory at Depth (SAFOD) in order to determine the composition, structure, and deformation processes of the fault zone at 3. km depth where creep and microseismicity occur. A total of approximately 41. m of spot core was taken from three separate sections of the borehole; the core samples consist of fractured arkosic sandstones and shale west of the SAF zone (Pacific Plate) and sheared fine-grained sedimentary rocks, ultrafine black fault-related rocks, and phyllosilicate-rich fault gouge within the fault zone (North American Plate). The fault zone at SAFOD consists of a broad zone of variably damaged rock containing localized zones of highly concentrated shear that often juxtapose distinct protoliths. Two zones of serpentinite-bearing clay gouge, each meters-thick, occur at the two locations of aseismic creep identified in the borehole on the basis of casing deformation. The gouge primarily is comprised of Mg-rich clays, serpentinite (lizardite ± chrysotile) with notable increases in magnetite, and Ni-Cr-oxides/hydroxides relative to the surrounding host rock. The rocks surrounding the two creeping gouge zones display a range of deformation including fractured protolith, block-in-matrix, and foliated cataclasite structure. The blocks and clasts predominately consist of sandstone and siltstone embedded in a clay-rich matrix that displays a penetrative scaly fabric. Mineral alteration, veins and fracture-surface coatings are present throughout the core, and reflect a long history of syn-deformation, fluid-rock reaction that contributes to the low-strength and creep in the meters-thick gouge zones. © 2011 Elsevier B.V. 2011 English 0012821X 10.1016/j.epsl.2011.07.020 Earth and Planetary Science Letters 310 131-144 Geology Department, Utah State University, Logan, UT 84321-4505, United States; Center for Tectonophysics, Department of Geology and Geophysics, Texas A and M University, College Station, TX 77843, United States; Department of Earth and Atmospheric Sciences, St. Louis University, St. Louis, MO 63108, United States 1-2 Casing deformation; Cataclasite; Clay gouges; Deformation process; During phase; Fault gouge; Fault zone; Fault-related rocks; Fluid-rock interaction; Host rocks; Internal structure; Lizardite; matrix; Microseismicity; North American; Pacific plates; Protoliths; SAFOD; San Andreas Fault; Serpentinite; Siltstones; Ultrafine; Whole-rock geochemistry, Buildings; Coatings; Creep; Iron ores; Kaolinite; Lithology; Minerals; Observatories; Plates (structural components); Rock drilling; Rocks; Sandstone; Sedimentology; Serpentine, Structural geology, borehole; creep; deformation; drilling; fault gouge; fault zone; host rock; lithology; observatory; Pacific plate; San Andreas Fault; sandstone; seismicity; shale https://www.scopus.com/inward/record.uri?eid=2-s2.0-80054076681&doi=10.1016%2fj.epsl.2011.07.020&partnerID=40&md5=d9df595952a680034a6df5ac1be44f94 cited By 61 K.K. Bradbury J.P. Evans J.S. Chester F.M. Chester D.L. Kirschner article Zoback201114 The San Andreas Fault Observatory at Depth (SAFOD) was drilled to study the physical and chemical processes controlling faulting and earthquake generation along an active, plate-bounding fault at depth. SAFOD is located near Parkfield, California and penetrates a section of the fault that is moving due to a combination of repeating microearthquakes and fault creep. Geophysical logs define the San Andreas Fault Zone to be relatively broad (~200 m), containing several discrete zones only 2-3 m wide that exhibit very low P- and S-wave velocities and low resistivity. Two of these zones have progressively deformed the cemented casing at measured depths of 3192 m and 3302 m. Cores from both deforming zones contain a pervasively sheared, cohesionless, foliated fault gouge that coincides with casing deformation and explains the observed extremely low seismic velocities and resistivity. These cores are being now extensively tested in laboratories around the world, and their composition, deformation mechanisms, physical properties, and rheological behavior are studied. Downhole measurements show that within 200 m (maximum) of the active fault trace, the direction of maximum horizontal stress remains at a high angle to the San Andreas Fault, consistent with other measurements. The results from the SAFOD Main Hole, together with the stress state determined in the Pilot Hole, are consistent with a strong crust/weak fault model of the San Andreas. Seismic instrumentation has been deployed to study physics of faulting-earthquake nucleation, propagation, and arrest-in order to test how laboratory-derived concepts scale up to earthquakes occurring in nature. 2011 English 18168957 10.2204/iodp.sd.11.02.2011 Scientific Drilling 14-28 Department of Geophysics, Stanford University, Stanford, CA 94305-2215, United States; U.S. Geological Survery, 345 Middlefield Road, MS 977, Menlo Park, CA 94025-3591, United States 1 Active fault; Bounding faults; California; Casing deformation; Chemical process; Deformation mechanism; Deforming zone; Downhole measurements; Earthquake nucleation; Fault creep; Fault gouge; Fault model; Geophysical logs; Horizontal stress; Low resistivity; Measured depths; Micro-earthquakes; Pilot holes; Rheological behaviors; S-wave velocity; San Andreas Fault; Scale-up; Scientific drilling; Seismic instrumentation; Seismic velocities; Stress state, Deformation; Earthquakes; Parks; Rheology; Structural geology, Tectonics https://www.scopus.com/inward/record.uri?eid=2-s2.0-79955161589&doi=10.2204%2fiodp.sd.11.02.2011&partnerID=40&md5=7b65d928eefa85d49e2ad2bdb555b94c cited By 131 M. Zoback S. Hickman W. Ellsworth article Wang20111047 High pore pressure in the San Andreas fault (California) was hypothesized to explain the prevailing weakness of the fault and may have major implications on the mechanics of earthquakes. However, no evidence of high pore pressure was found in the latest drilling into the San Andreas fault (SAFOD: San Andreas Fault Observatory at Depth) in central California (Zoback et al., 2010). If widely applicable, this result would impact our understanding of earthquake mechanisms on this and other active faults around the world. Here, however, I show that the available evidence from the latest SAFOD drilling may not be sufficient to reject the high pore-pressure hypothesis, and that definite knowledge of pore pressure in the fault zone may require long-term monitoring at the SAFOD site. The inference may also be useful for interpreting results from drilling projects on other active faults. © 2011 Geological Society of America. 2011 English 00917613 10.1130/G32294.1 Geology 39 1047-1050 Department of Earth and Planetary Science, University of California-Berkeley, Berkeley, CA 94720, United States 11 Active fault; California; Drilling projects; Fault zone; Long term monitoring; San Andreas Fault, Earthquakes; Strike-slip faults, Pore pressure, active fault; earthquake mechanism; fault zone; pore pressure; San Andreas Fault, California; San Andreas; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-84863120666&doi=10.1130%2fG32294.1&partnerID=40&md5=440bf76f83593f396fe09eada1b3d784 cited By 10 C.-Y. Wang article Holdsworth2011132 The drilling of a deep borehole across the actively creeping Parkfield segment of the San Andreas Fault Zone (SAFZ), California, and collection of core materials permit direct geological study of fault zone processes at 2-3 km depth. The three drill cores sample both host and fault rocks and pass through two currently active, narrow (1-2 m wide) shear zones enclosed within a broader (ca. 240 m wide) region of inactive foliated gouges. The host rocks preserve primary sedimentary features and are cut by numerous minor faults and small, mainly calcite-filled veins. The development of Fe-enriched smectitic phyllosilicate networks following cataclasis is prevalent in the presently inactive foliated gouges of the main fault zone and in minor faults cutting clay-rich host rocks. Calcite, anhydrite and minor smectitic phyllosilicate veins are interpreted to have formed due to local fluid overpressuring events prior to, synchronous with and after local gouge development. By contrast, the active shear zone gouges lack mineral veins (except as clasts) and contain numerous clasts of serpentinite. Markedly Mg-rich smectitic phyllosilicates are the dominant mineral phases here, suggesting that the fault zone fluids have interacted with the entrained serpentinites. We propose that weakening of the SAFZ down to depths of at least 3 km can be attributed to the pervasive development of interconnected networks of low friction smectitic phyllosilicates and to the operation of stress-induced solution-precipitation creep mechanisms. © 2010 Elsevier Ltd. 2011 English 01918141 10.1016/j.jsg.2010.11.010 Journal of Structural Geology 33 132-144 Reactivation Research Group, Department of Earth Sciences, University of Durham, Durham DH1 3LE, United Kingdom; HPT Laboratory, Utrecht University, Utrecht, Netherlands; Durham GJ Russell Microscopy Facility, Durham University, Durham DH1 3LE, United Kingdom 2 Fault zone; Phyllosilicate; SAFOD; San Andreas Fault; Smectites, Calcite; Carbonate minerals; Clay minerals; Core drilling; Core samples; Fluids; Sedimentary rocks; Silicate minerals, Structural geology, active fault; core analysis; creep; deformation mechanism; drilling; fault zone; foliation; overpressure; phyllosilicate; San Andreas Fault; shear zone; smectite; stress change; stress field, California; Parkfield; San Andreas; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-78751574107&doi=10.1016%2fj.jsg.2010.11.010&partnerID=40&md5=96b51568d32c5906ce6a6ee7d1fc8a68 cited By 142 R.E. Holdsworth E.W.E. Diggelen C.J. Spiers J.H.P. Bresser R.J. Walker L. Bowen article Mittempergher2011 The San Andreas Fault Observatory at Depth (SAFOD) in Parkfield, central California, has been drilled through a fault segment that is actively deforming through creep and microearthquakes. Creeping is accommodated in two fault strands, the Southwest and Central Deforming Zones, embedded within a damaged zone of deformed shale and siltstone. During drilling, no pressurized fluids have been encountered, even though the fault zone acts as a permeability barrier to fluid circulation between the North American and Pacific plates. Microstructural analysis of sheared shales associated with calcite and anhydrite-bearing veins found in SAFOD cores collected at 1.5m from the Southwest Deforming Zone, suggests that transient increases of pore fluid pressure have occurred during the fault activity, causing mode I fracturing of the rocks. Such build-ups in fluid pressure may be related to permeability reduction during fault creep and pressure-solution processes, resulting in localized failure of small fault zone patches and providing a potential mechanism for the initiation of some of the microearthquakes registered in the SAFOD site. Copyright © 2011 by the American Geophysical Union. 2011 English 00948276 10.1029/2010GL046129 Geophysical Research Letters 38 Blackwell Publishing Ltd Dipartimento di Geoscienze, Università di Padova, Via G. Gradenigo 6, I-35131 Padua, Italy; LGIT, Université Joseph Fourier-Grenoble i, BP 53, F-38041 Grenoble Cedex 9, France; Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata, 605, I-00143 Roma, Italy; Department of Geography and Geosciences, University of Louisville, 212 Lutz Hall, Louisville, KY 40292, United States 3 Carbonate minerals; Creep; Drilling fluids; Structural geology, California; Damaged zones; Deforming zone; Fault activity; Fault creep; Fault strands; Fault zone; Fluid circulation; Fluid pressures; Localized failure; Micro-earthquakes; Microstructural analysis; North American; Pacific plates; Permeability barriers; Permeability reduction; Pore fluid pressure; Potential mechanism; San Andreas Fault; Siltstones; Solution process, Shale, creep; fault zone; fluid pressure; microearthquake; microstructure; North American plate; Pacific plate; permeability; San Andreas Fault; shale; siltstone, California; Parkfield; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-79551584054&doi=10.1029%2f2010GL046129&partnerID=40&md5=cb7ea09ae4b9e1f813e8dbafd0e29162 cited By 29 S. Mittempergher G. Di Toro J.P. Gratier J. Hadizadeh S.A.F. Smith R. Spiess article Ellsworth201139 Damage to fault-zone rocks during fault slip results in the formation of a channel of low seismic-wave velocities. Within such channels guided seismic waves, denoted by F g, can propagate. Here we show with core samples, well logs and F g-waves that such a channel is crossed by the SAFOD (San Andreas Fault Observatory at Depth) borehole at a depth of 2.7 km near Parkfield, California, USA. This laterally extensive channel extends downwards to at least half way through the seismogenic crust, more than about 7 km. The channel supports not only the previously recognized Love-type- (F L) and Rayleigh-type- (F R) guided waves, but also a new fault-guided wave, which we name F Φ. As recorded 2.7 km underground, F Φ is normally dispersed, ends in an Airy phase, and arrives between the P- and S-waves. Modelling shows that F Φ travels as a leaky mode within the core of the fault zone. Combined with the drill core samples, well logs and the two other types of guided waves, F Φ at SAFOD reveals a zone of profound, deep, rock damage. Originating from damage accumulated over the recent history of fault movement, we suggest it is maintained either by fracturing near the slip surface of earthquakes, such as the 1857 Fort Tejon M 7.9, or is an unexplained part of the fault-creep process known to be active at this site. © The Geological Society of London 2011. 2011 English 03058719 10.1144/SP359.3 Geological Society Special Publication 359 39-53 US Geological Survey, 345 Middlefield Road, MS-977, Menlo Park, CA 94025, United States; Institute of Earth Science and Engineering, University of Auckland, Auckland 1142, New Zealand 1 earthquake; fault slip; fault zone; P-wave; Rayleigh wave; S-wave; seismic wave, California; San Andreas; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-84055176128&doi=10.1144%2fSP359.3&partnerID=40&md5=d47374a8d84b8ea8505e1f958cb6a7b2 cited By 36 W.L. Ellsworth article Becken201187 The seismicity pattern along the San Andreas fault near Parkfield and Cholame, California, varies distinctly over a length of only fifty kilometres. Within the brittle crust, the presence of frictionally weak minerals, fault-weakening high fluid pressures and chemical weakening are considered possible causes of an anomalously weak fault northwest of Parkfield. Non-volcanic tremor from lower-crustal and upper-mantle depths is most pronounced about thirty kilometres southeast of Parkfield and is thought to be associated with high pore-fluid pressures at depth. Here we present geophysical evidence of fluids migrating into the creeping section of the San Andreas fault that seem to originate in the region of the uppermost mantle that also stimulates tremor, and evidence that along-strike variations in tremor activity and amplitude are related to strength variations in the lower crust and upper mantle. Interconnected fluids can explain a deep zone of anomalously low electrical resistivity that has been imaged by magnetotelluric data southwest of the Parkfield-Cholame segment. Near Cholame, where fluids seem to be trapped below a high-resistivity cap, tremor concentrates adjacent to the inferred fluids within a mechanically strong zone of high resistivity. By contrast, subvertical zones of low resistivity breach the entire crust near the drill hole of the San Andreas Fault Observatory at Depth, northwest of Parkfield, and imply pathways for deep fluids into the eastern fault block, coincident with a mechanically weak crust and the lower tremor amplitudes in the lower crust. Fluid influx to the fault system is consistent with hypotheses of fault-weakening high fluid pressures in the brittle crust. © 2011 Macmillan Publishers Limited. All rights reserved. 2011 English 00280836 10.1038/nature10609 Nature 480 87-90 GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany; University Potsdam, Institute of Geosciences, Karl-Liebknecht-Strasse 24, 14476 Potsdam-Golm, Germany; US Geological Survey, MS 964, Box 25046, Denver, CO 80225, United States; Westfälische Wilhelms Universität Münster, Institute of Geophysics, Corrensstrasse 24, 48149 Münster, Germany 7375 mineral, amplitude; correlation; crust; data set; electrical resistivity; fluid; mantle; pressure effect, amplitude modulation; anisotropy; article; biogeographic region; chemical reaction; earthquake; electrical parameters; enzyme kinetics; friction; liquid; nonhuman; porosity; precipitation; priority journal; rock; salinity; san andreas fault; velocity; volcano, California; San Andreas; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-82555195565&doi=10.1038%2fnature10609&partnerID=40&md5=f8adaa53b017130e6949e5cd60bf4626 cited By 159 M. Becken O. Ritter P.A. Bedrosian U. Weckmann article Haimson201145 An extensive true triaxial testing program was carried out on core samples from three ICDP-sponsored deep scientific boreholes, KTB (Germany), SAFOD (United States), and TCDP (Taiwan). The three rocks differ in the processes that formed them and in many of their mechanical properties. However, all three rocks exhibited similar failure mechanism, in which induced or reopened microcracks are primarily aligned with the σ1-σ2 plane, and the developed fault is steeply inclined in the σ3 direction. Rock strength in all tested rocks increases with σ2 when σ3 is kept constant. Thus, the common Mohr-type criteria, which ignore the effect of σ2, typically underestimate rock strength. Rather a 3D criterion that involves all three principal stresses represents well experimental results. Fracture plane slope for the same σ3 steepens as σ2 rises, contrary to Mohr-type criteria. With respect to deformation, the onset of dilatancy increases with σ2. In conclusion, true triaxial tests conducted on cores from three scientific boreholes, revealed important details of mechanical behavior not otherwise observed in conventional triaxial tests. In addition, they show mechanical behavior similarities as related to σ2 effect regardless of rock type. © 2010 Elsevier B.V. 2011 English 00401951 10.1016/j.tecto.2010.10.011 Tectonophysics 503 45-51 Dept. of Materials Science and Engineering and the Geological Engineering Program, University of Wisconsin, 1509 University Avenue, Madison, WI, 53706, United States 1-2 Dilatancy; Fault angle; Scientific boreholes; True triaxial strength; True triaxial testing, Deformation; Mechanical engineering; Mechanical properties; Rock mechanics; Testing, Rocks, borehole; deformation; dilatancy; failure mechanism; mechanical property; microcrack; rock mechanics; triaxial test, Germany; Taiwan; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-79954642377&doi=10.1016%2fj.tecto.2010.10.011&partnerID=40&md5=15821fb817ddead48ed292c695ef54d2 cited By 21 B. Haimson article Wiersberg2011148 In this contribution we present results from two individual gas monitoring experiments which were conducted during the drilling of the SAFOD (San Andreas Fault Observatory at Depth) boreholes. Gas from circulating drilling mud was monitored during the drilling the SAFOD III side tracks and was later analyzed for δ13C (CH4, C2H6 and C3H8), H/D (CH4) and noble gas isotopes. Furthermore, gas accumulations induced by drill pipe retrieval ("trip gas") from the SAFOD MH and the SAFOD III boreholes were also investigated. The data are interpreted in the context of gas migration processes and the permeability structure of the San Andreas Fault (SAF) around two actively deforming zones at 3194m and 3301m borehole depth. Helium isotope ratios of 0.86 Ra at 3203m and between 0.51 and 0.88 Ra at 3262m (Ra is the atmospheric 3He/4He ratio) indicate an improved flow of mantle volatiles between both fault strands. Much lower values were observed at 3147m (0.26 Ra) and 3312m (0.22 Ra). Hydrocarbon concentrations coincide with the occurrence of shale at ~3150-3200m and below ~3310m depth. The molecular and isotope composition of hydrocarbons and their spatial distributions imply hydrocarbon generation by thermal degradation of organic matter followed by extensive diffusion loss. Carbon isotope data furthermore suggest a thermal maturity of the source rock of approx. 1.4%R0. The concentration of trip gas is generally low in the interval 3100. m-3450. m but exhibits high spatial variability. At 3128. m and 3223. m depth, the trip gas concentrations are as low as in the granite section of the SAFOD Main Hole. Considerable variations of Ra values, trip gas concentrations, and the molecular composition of hydrocarbons when penetrating the active fault strands let us conclude that the permeability of the fault transverse to the fault direction is limited and that the active fault has not been breached over many earthquake cycles such that little or no fluid exchange took place. Diffusion is the dominant mechanism controlling hydrocarbon migration through the fault strands. The elevated Ra values between both fault strands may reflect either episodic or continuous flow of mantle-derived fluids, suggestive of some limited permeability parallel to the fault direction. © 2011 Elsevier B.V. 2011 English 00092541 10.1016/j.chemgeo.2011.02.016 Chemical Geology 284 148-159 Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany 1-2 A-thermal; Active fault; Carbon isotopes; Continuous flows; Deforming zone; Diffusion loss; Dominant mechanism; Drilling mud; Drilling mud gas; Fault direction; Fault strands; Fluid exchange; Gas accumulation; Gas concentration; Gas migration; Gas monitoring; Helium isotopes; Hydrocarbon generation; Hydrocarbon migration; Isotope compositions; Mantle volatiles; Molecular compositions; Noble gas isotopes; Organic matter; Permeability structure; SAFOD; San Andreas Fault; Shale gas; Source rocks; Spatial variability; Thermal degradations, Boreholes; Boring; Buildings; Drill pipe; Helium; Hydrocarbons; Inert gases; Isotopes; Observatories; Oil well drilling; Petroleum geology; Shale; Tectonics, Gas permeability, active fault; borehole; chemical composition; diffusion; drilling; helium isotope; hydrocarbon migration; isotopic composition; noble gas; organic matter; permeability; San Andreas Fault; spatial distribution https://www.scopus.com/inward/record.uri?eid=2-s2.0-79954627078&doi=10.1016%2fj.chemgeo.2011.02.016&partnerID=40&md5=bcbf76b9d5e9779c317bc2a779f242c1 cited By 35 T. Wiersberg J. Erzinger article Ali2011237 4He accumulated in fluids is a well established geochemical tracer used to study crustal fluid dynamics. Direct fluid samples are not always collectable; therefore, a method to extract rare gases from matrix fluids of whole rocks by diffusion has been adapted. Helium was measured on matrix fluids extracted from sandstones and mudstones recovered during the San Andreas Fault Observatory at Depth (SAFOD) drilling in California, USA. Samples were typically collected as subcores or from drillcore fragments. Helium concentration and isotope ratios were measured 4-6 times on each sample, and indicate a bulk 4He diffusion coefficient of 3.5 ± 1.3 × 10-8 cm2 s-1 at 21°C, compared to previously published diffusion coefficients of 1.2 × 10-18 cm2 s-1 (21°C) to 3.0 × 10-15 cm2 s-1 (150°C) in the sands and clays. Correcting the diffusion coefficient of 4Hewater for matrix porosity (~3%) and tortuosity (~6-13) produces effective diffusion coefficients of 1 × 10-8 cm2s-1 (21°C) and 1 × 10-7 (120°C), effectively isolating pore fluid 4He from the 4He contained in the rock matrix. Model calculations indicate that &lt;6% of helium initially dissolved in pore fluids was lost during the sampling process. Complete and quantitative extraction of the pore fluids provide minimum in situ porosity values for sandstones 2.8 ± 0.4% (SD, n = 4) and mudstones 3.1 ± 0.8% (SD, n = 4). © 2010 Springer-Verlag. 2011 English; French; Spanish 14312174 10.1007/s10040-010-0645-6 Hydrogeology Journal 19 237-247 Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964, United States; Department of Earth and Environmental Sciences, Columbia University, 2960 Broadway, New York, NY 10027, United States; Department of Environmental Science, Barnard College, 3009 Broadway, New York, NY 10027, United States; Department of Marine Sciences, University of Connecticut, 1080 Shennecossett Road, Groton, CT 06340, United States; Lawrence-Berkeley National Laboratory, 1 Cyclotron Road, MS70A-4418, Berkeley, CA 94720, United States; National Science Foundation, 4201 Wilson Boulevard, Arlington, VA 22230, United States 1 concentration (composition); diffusion; fluid dynamics; geoaccumulation; helium; hydrogeology; isotopic ratio; porosity; tortuosity; tracer https://www.scopus.com/inward/record.uri?eid=2-s2.0-78751521016&doi=10.1007%2fs10040-010-0645-6&partnerID=40&md5=2e149799966c4357e85d8a5c3e0e0664 cited By 4 S. Ali M. Stute T. Torgersen G. Winckler B.M. Kennedy article Rentsch2010477 We present the principles of a recently developed event location procedure and its application to seismicity recorded in the main borehole of the San Andreas Fault Observatory at Depth (SAFOD). The basic idea of the location approach is the backpropagation of the recorded wavefield along rays using a Gaussian-beam-type weighting factor. In the case of a sufficient number of receivers, the intersection of these rays results in a distinct maximum at the corresponding hypocentre location. We have applied this technique to data recorded with an 80-level three-component (3C) receiver array in the SAFOD main hole and we have located a number of events in the vicinity of the fault system. A modification of the location algorithm also allowed the precise location of one of the so-called target events, which are the subject of recent drilling activities as well as ongoing research at SAFOD. We summarize the principles of the location method and the performed processing steps, provide estimates of the uncertainties for the target event location and test the robustness of the location using six different 3-D velocity models. Furthermore, we discuss the event locations in comparison with borehole logging data and coincident seismic reflection images and we also show identified events with highly correlated waveforms. © 2010 The Authors Journal compilation © 2010 RAS. 2010 English 0956540X 10.1111/j.1365-246X.2010.04638.x Geophysical Journal International 182 477-492 Freie Universität Berlin, FR Geophysik, Malteserstr, 74-100, 12249, Berlin, Germany 1 back propagation; body wave; borehole; San Andreas Fault; seismic migration; seismic reflection; seismicity; seismotectonics; spatial analysis; time series analysis; wave propagation https://www.scopus.com/inward/record.uri?eid=2-s2.0-77954641667&doi=10.1111%2fj.1365-246X.2010.04638.x&partnerID=40&md5=931435e8110016ab1f8838d6604ee1a7 cited By 8 S. Rentsch S. Buske S. Gutjahr J. Kummerow S.A. Shapiro article Reshetnikov2010 We have developed a new passive seismic imaging approach that consists of two steps. First, the hypocenter of the microseismic event is precisely located. Second, this event is treated as a "pseudo-active" seismic source and the reflections within the recorded wavefield are processed by using a directional migration algorithm in order to construct a high-resolution image of the illuminated subsurface region. In this paper we demonstrate the application of our approach to a number of microseismic events recorded by a borehole array in the San Andreas Fault Observatory at Depth main hole. Results obtained were high-resolution 3-D images of different SE-NW-oriented reflectors related to the San Andreas Fault (SAF) system in the close vicinity of the borehole. To support our approach, we compared our findings with other active and passive seismic images and analyzed the correlation with borehole lithology. We revealed a predominantly satisfactory agreement for both juxtapositions. Furthermore, the stacked image of several microearthquakes provides a spatial characterization of the complex internal structure of the SAF, with much higher resolution than can be obtained from surface seismic reflection data. Copyright 2010 by the American Geophysical Union. 2010 English 21699313 10.1029/2009JB007049 Journal of Geophysical Research: Solid Earth 115 Blackwell Publishing Ltd Institute for Geological Sciences, Freie Universitaet Berlin, Bldg. D, Malteserstrasse 74-100, D-12249 Berlin, Germany 12 algorithm; image resolution; imaging method; microearthquake; San Andreas Fault; seismic data; seismic reflection; seismic source; seismology; stacking; wave field https://www.scopus.com/inward/record.uri?eid=2-s2.0-78650949932&doi=10.1029%2f2009JB007049&partnerID=40&md5=4c6c0e251e7cebb83818ad90ee77332f cited By 49 A. Reshetnikov S. Buske S.A. Shapiro article Zoback2010197 This year, the world has faced energetic and destructive earthquakes almost every month. In January, an M = 7.0 event rocked Haiti, killing an estimated 230,000 people. In February, an M = 8.8 earthquake and tsunami claimed over 500 lives and caused billions of dollars of damage in Chile. Fatal earthquakes also occurred in Turkey in March and in China and Mexico in April. 2010 English 00963941 10.1029/2010EO220001 Eos 91 197-199 Department of Geophysics, Stanford University, Stanford, CA, United States; U.S. Geological Survey, Menlo Park, CA, United States 22 earthquake damage; fault zone; hazard assessment; tsunami, California; Chile; China; Haiti; Mexico [North America]; San Andreas; Turkey; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-77954237125&doi=10.1029%2f2010EO220001&partnerID=40&md5=9da6a525ef8692126093a0a23ff1e901 cited By 118 M. Zoback S. Hickman W. Ellsworth article Day-Lewis2010 We analyzed measurements of the direction of maximum horizontal compressive stress as a function of depth in two scientific research wells near the San Andreas Fault in central and southern California. We found that the stress orientations exhibit scale-invariant fluctuations over intervals from tens of cm to several km. Similarity between the scaling of the stress orientation fluctuations and the scaling of earthquake frequency with fault size suggests that these fluctuations are controlled by stress perturbations caused by slip on faults of various sizes in the critically-stressed crust adjacent to the fault. The apparent difference in stress scaling parameters between the two studies wells seem to correspond to differences in the earthquake magnitude-frequency statistics for the creeping versus locked sections of the fault along which these two wells are located. This suggests that stress heterogeneity adjacent to active faults like the San Andreas may reflect variations in stresses and loading conditions along the fault. © 2010 by the American Geophysical Union. 2010 English 00948276 10.1029/2010GL045025 Geophysical Research Letters 37 Blackwell Publishing Ltd Department of Geophysics, Stanford University, Mitchell Building, Stanford, CA 95305-2214, United States; U.S. Geological Survey, MS 977, 345 Middlefield Rd., Menlo Park, CA 94025, United States 24 Compressive stress; Earthquakes; Horizontal wells; Wells, Active fault; Earthquake frequency; Earthquake magnitudes; Loading condition; San Andreas Fault; Scale-invariant; Scaling parameter; Scientific researches; Southern California; Stress orientations; Stress perturbations, Stress analysis, active fault; compression; creep; earthquake magnitude; earthquake recurrence; fault zone; seismicity, California; San Andreas; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-78650496752&doi=10.1029%2f2010GL045025&partnerID=40&md5=388b1e76a33a9242855883e583aaab12 cited By 28 A. Day-Lewis M. Zoback S. Hickman article Wenk2010478 Samples of fault gouge from the San Andreas Fault drill hole (SAFOD), a shale from the North Sea sedimentary basin and schists from metamorphic rocks in the Alps have been analyzed with high energy synchrotron X-rays to determine preferred orientation of mica and clay minerals. The method relies on obtaining 2D diffraction images which are then processed with the crystallographic Rietveld method, implemented in the software MAUD, allowing for deconvolution of phases and extraction of their orientation distributions. It is possible to distinguish between detrital illite/muscovite and authigenic illite/smectite, kaolinite and chlorite, and muscovite and biotite, with strongly overlapping peaks in the diffraction pattern. The results demonstrate that phyllosilicates show large texture variations in various environments, where different mechanisms produce the rock microfabrics: fault gouge fabrics are quite weak and asymmetric with maxima for (001) in the range of 1.5-2.5 multiples of random distribution (m.r.d.). This is attributed to heterogeneous deformation with randomization, as well as dissolution-precipitation reactions. Shale fabrics have maxima ranging from 3 to 9. m.r.d. and this is due to sedimentation and compaction. The strongest fabrics were observed in metamorphic schists (10-14. m.r.d.) and developed by deformation as well as recrystallization in a stress field. In the analyzed samples, fabrics of co-existing quartz are weak. All phyllosilicate textures can be explained by orientation of (001) platelets, with no additional constraints on a-axes. © 2010 Elsevier Ltd. 2010 English 01918141 10.1016/j.jsg.2010.02.003 Journal of Structural Geology 32 478-489 Department of Earth and Planetary Science, University of California, Berkeley, CA 94720, United States 4 Authigenic; Co-existing; Different mechanisms; Diffraction images; Drill hole; Fault gouge; Heterogeneous deformation; High energy synchrotron; Large textures; Micro fabric; North Sea; Orientation distributions; Overlapping peaks; Phyllosilicate; Phyllosilicate fabrics; Precipitation reaction; Preferred orientations; Random distribution; Recrystallizations; San Andreas Fault; Schist; Sedimentary basin; Stress field, Deformation; Dissolution; Holographic interferometry; Kaolinite; Metamorphic rocks; Mica; Oxide minerals; Quartz; Rietveld method; Shale; Silicate minerals; Textures, Fabrics, authigenic mineral; chemical composition; clay mineral; deconvolution; fault gouge; metamorphic rock; phyllosilicate; preferred orientation; recrystallization; Rietveld analysis; schist; sedimentary basin; shale; stress field, Atlantic Ocean; North Sea https://www.scopus.com/inward/record.uri?eid=2-s2.0-77953139643&doi=10.1016%2fj.jsg.2010.02.003&partnerID=40&md5=93e1edbf50eb2aac70074568f8fcb680 cited By 74 H.-R. Wenk W. Kanitpanyacharoen M. Voltolini article Blum20101879 We present observations from a vertical, optical fiber interferometric strainmeter in the San Andreas Fault Observatory at Depth borehole near Parkfield, California. The sensor detects both teleseismic earthquakes and local events, along with coseismic strain steps consistent with theoretical dislocation models. For tele-seismic events, we investigate the possibility of determining local Rayleigh-wave phase velocities beneath the borehole by comparing the ratio of vertical ground acceleration from a nearby seismometer to vertical strain. While similar studies have used horizontal components and rotations, this is the first such attempt utilizing vertical measurements. We show that at periods from around 16-40 seconds, we can recover general dispersion characteristics that are within a few percent of models of realistic local structure. 2010 English 00371106 10.1785/0120090333 Bulletin of the Seismological Society of America 100 1879-1891 Scripps Institution of Oceanography, Dr. University of California-San Diego, 9500 Gilman, La Jolla, CA, 92093, United States; Department of Earth and Environmental Sciences, Ludwig-Maximilians Universität, Theresienstrasse 41, 80333 München, Germany 5 A California; Coseismic deformation; Dislocation models; Dispersion characteristics; Local structure; Rayleigh-wave phase velocity; San Andreas Fault; Seismic event; Strain-meter; Teleseismic earthquakes; Vertical ground accelerations; Vertical strain, Interferometry; Optical fibers; Phase velocity; Seismology, Strain measurement, coseismic process; deformation; interferometer; optical instrument; phase velocity; Rayleigh wave; seismograph; sensor; teleseismic wave; wave velocity, California; Parkfield; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-77957729501&doi=10.1785%2f0120090333&partnerID=40&md5=4c727dcdf02ae0515c3c5c6c398fb901 cited By 11 J. Blum H. Igel M. Zumberge article Schleicher2010667 Mudrock samples were investigated from two fault zones at ~3066 m and ~3296 m measured depth (MD) located outside and within the main damage zone of the San Andreas Fault Observatory at Depth (SAFOD) drillhole at Parkfield, California. All studied fault rocks show features typical of those reported across creep zones with variably spaced and interconnected networks of polished displacement surfaces coated by abundant polished films and occasional striations. Electron microscopy and X-ray diffraction study of the surfaces reveal the occurrence of neocrystallized thin film clay coatings containing illite-smectite (I-S) and chlorite-smectite (C-S) minerals. 40Ar/39Ar dating of the illitic mix-layered coatings demonstrated Miocene to Pliocene crystallization and revealed an older fault strand (8 ± 1.3 Ma) at 3066 m MD, and a probably younger fault strand (4 ± 4.9 Ma) at 3296 m MD. Today, the younger strand is the site of active creep behavior, reflecting a possible (re)activation of these clay-weakened zones. We propose that the majority of slow fault creep is controlled by the high density of thin (&lt;100 nm thick) nanocoatings on fracture surfaces, which are sufficiently smectite-rich and interconnected at low angles to accommodate slip with minimal breakage of stronger matrix clasts. Displacements occur by frictional slip along particle surfaces and hydrated smectitic phases, in combination with intracrystalline deformation of the clay lattice, associated with extensive mineral dissolution, mass transfer, and residual precipitation of expandable layers. The localized concentration of smectite in both I-S and C-S minerals contributes to fault weakening, with fracturing and fluid infiltration creating new nucleation sites for neomineralization on displacement surfaces during continued faulting. The role of newly grown, ultrathin, hydrous clay coatings contrasts with previously proposed scenarios of reworked talc and/or serpentine phases as an explanation for weak fault and creep behavior at these depths. © 2010 Geological Society of America. 2010 English 00917613 10.1130/G31091.1 Geology 38 667-670 Department of Geological Sciences, University of Michigan, 1100 North University Avenue, Ann Arbor, MI 48109, United States; Ernst-Moritz-Arndt Universität Institut für Geographie und Geologie, F. Ludwig-Jahn-Strasse 17A, D-17487 Greifswald, Germany 7 California; Clay coating; Creep behaviors; Damage zones; Drill hole; Fault creep; Fault rock; Fault strands; Fault zone; Fracture surfaces; Frictional slip; High density; Illite-smectite; Interconnected network; Layered coatings; matrix; Measured depths; Mineral dissolution; Miocene; Nano-coatings; Nucleation sites; Older faults; Particle surface; Pliocene; San Andreas Fault; Smectites; Ultra-thin; X-ray diffraction studies, Coatings; Creep; Dissolution; Hydrates; Minerals; Precipitation (chemical); Serpentine; Silicate minerals; Structural geology; X ray diffraction, Clay minerals, argon isotope; clay mineral; creep; deformation; displacement; fault zone; illite; mudstone; San Andreas Fault; smectite, California; Parkfield; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-77955969539&doi=10.1130%2fG31091.1&partnerID=40&md5=89115cb30c4a8f1230ae9b93e8369226 cited By 123 A.M. Schleicher B.A. Pluijm L.N. Warr article Wu2010343 Seismic guided wave dispersion can be used to image fault-zone structure at seismogenic depth. A two-station differential group velocity technique previously used for surface waves was adapted to solve for local fault-zone structure between two stations. This method was extended to solve for fault-zone structure between two earthquakes using differential group arrival times at a single station. The method was tested with finite-difference synthetic data for an inhomogeneous fault, as well as with a pair of shallow earthquakes recorded in the San Andreas Fault Observatory at Depth (SAFOD) borehole station. Results from a pair of deep earthquakes recorded in the SAFOD borehole station indicate that the low-velocity waveguide of the San Andreas Fault extends to >10 km depth. The waveguide at 10-12 km depth is 120-190 m wide and the velocity contrast is >20 per cent, similar to the values in the shallow subsurface. Multiple earthquakes and receivers could be used to map fault zone structure at seismogenic depth as a function of depth and strike. © 2010 The Authors Journal compilation © 2010 RAS. 2010 English 0956540X 10.1111/j.1365-246X.2010.04612.x Geophysical Journal International 182 343-354 Department of Geosciences, Virginia Tech, 4044 Derring Hall, Blacksburg, VA 24061, United States 1 arrival time; fault zone; finite difference method; image analysis; San Andreas Fault; seismic data; seismic velocity; seismic wave; wave dispersion; wave propagation https://www.scopus.com/inward/record.uri?eid=2-s2.0-77954633775&doi=10.1111%2fj.1365-246X.2010.04612.x&partnerID=40&md5=e21a49ba6d90b86cb257c805dc5796cc cited By 25 J. Wu J.A. Hole J.A. Snoke article McGarr20103250 Laboratory stick-slip friction experiments indicate that peak slip rates increase with the stresses loading the fault to cause rupture. If this applies also to earthquake fault zones, then the analysis of rupture processes is simplified inasmuch as the slip rates depend only on the local yield stress and are independent of factors specific to a particular event, including the distribution of slip in space and time. We test this hypothesis by first using it to develop an expression for radiated energy that depends primarily on the seismic moment and the maximum slip rate. From laboratory results, the maximum slip rate for any crustal earthquake, as well as various stress parameters including the yield stress, can be determined based on its seismic moment and the maximum slip within its rupture zone. After finding that our new equation for radiated energy works well for laboratory stick-slip friction experiments, we used it to estimate radiated energies for five earthquakes with magnitudes near 2 that were induced in a deep gold mine, an M 2.1 repeating earthquake near the San Andreas Fault Observatory at Depth (SAFOD) site and seven major earthquakes in California and found good agreement with energies estimated independently from spectra of local and regional ground-motion data. Estimates of yield stress for the earthquakes in our study range from 12 MPa to 122 MPa with a median of 64 MPa. The lowest value was estimated for the 2004 M 6 Parkfield, California, earthquake whereas the nearby M 2.1 repeating earthquake, as recorded in the SAFOD pilot hole, showed a more typical yield stress of 64 MPa. 2010 English 00371106 10.1785/0120100043 Bulletin of the Seismological Society of America 100 3250-3260 U.S. Geological Survey, MS 977, 345 Middlefield Rd, Menlo Park, CA 94025, United States; Department of Earth Sciences, University of New Hampshire, Durham, NH 03824, United States 6 California; Crustal earthquakes; Deep gold mines; Earthquake fault; Earthquake rupture; Ground-motion; Maximum slip; Pilot holes; Radiated energies; Repeating earthquake; Rupture process; Rupture zone; San Andreas Fault; Seismic moment; Slip rates; Space and time; Stick-slip friction; Stress parameter, Earthquakes; Experiments; Friction; Gold mines; Gravitational effects; Laboratories; Slip forming; Tectonics, Yield stress, earthquake magnitude; earthquake rupture; fault zone; ground motion; seismic moment; slip rate https://www.scopus.com/inward/record.uri?eid=2-s2.0-78650125662&doi=10.1785%2f0120100043&partnerID=40&md5=93ba0f7ac73ac8f256ca0092937df571 cited By 13 A. McGarr J.B. Fletcher M. Boettcher N. Beeler J. Boatwright article Jeppson2010 We examine the relationships between borehole geophysical data and physical properties of fault-related rocks within the San Andreas Fault as determined from data from the San Andreas Fault Observatory at Depth borehole. Geophysical logs, cuttings data, and drilling data from the region 3-to 4-km measured depth of the borehole encompass the active part of the San Andreas Fault. The fault zone lies in a sequence of deformed sandstones, siltstone, shale, serpentinite-bearing block-in-matrix rocks, and sheared phyllitic siltstone. The borehole geophysical logs reveal the presence of a low-velocity zone from 3190 to 3410 m measured depth with Vp and Vs values 10-30% lower than the surrounding rocks and a 1-2 m thick zone of active shearing at 3301-3303 m measured depth. Seven low-velocity excursions with increased porosity, decreased density, and mud-gas kick signatures are present in the fault zone. Geologic data on grain-scale deformation and alteration are compared to borehole data and reveal weak correlations and inverse relationships to the geophysical data. In places, Vp and Vs increase with grain-scale deformation and alteration and decrease with porosity in the fault zone. The low-velocity zone is associated with a significant lithologic and structural transition to low-velocity rocks, dominated by phyllosilicates and penetratively foliated, sheared rocks. The zone of active shearing and the regions of low sonic velocity appear to be associated with clay-rich rocks that exhibit fine-scale foliation and higher porosities that may be a consequence of the fault-related shearing of foliated and fine-grained sedimentary rocks. Copyright 2010 by the American Geophysical Union. 2010 English 21699313 10.1029/2010JB007563 Journal of Geophysical Research: Solid Earth 115 Blackwell Publishing Ltd Department of Geology, Utah State University, 4505 Old Main Hill, Logan, UT 84322-4505, United States 12 borehole logging; data inversion; deformation; fault zone; foliation; geophysical method; porosity; rock property; San Andreas Fault; sandstone; shale; siltstone https://www.scopus.com/inward/record.uri?eid=2-s2.0-79251561240&doi=10.1029%2f2010JB007563&partnerID=40&md5=6bcb9dacb91d973b103651e969e6dbc9 cited By 35 T.N. Jeppson K.K. Bradbury J.P. Evans article Tembe2010 2010 English 21699313 10.1029/2009jb000818 Journal of Geophysical Research: Solid Earth 115 Blackwell Publishing Ltd 3 https://www.scopus.com/inward/record.uri?eid=2-s2.0-84904676438&doi=10.1029%2f2009jb000818&partnerID=40&md5=543066649e2677221dfb3976f367b770 cited By 4 S. Tembe D. Lockner T.-F. Wong article Liu2010909 The implementation of EarthScope, the deep exploration program in North American continent has become the focus of attention in the field of Earth Sciences. This paper gives a brief description of the background, scientific problems to be solved, observational approaches adopted,some research progresses of the EarthScope program in the fields such as seismic tomographic imaging, earth stress change, continental structure, topographic survey and so on. 2010 Chinese 10009515 Acta Geologica Sinica (English Edition) 84 909-926 Chinese Academy of Geological Sciences, Beijing, 100037, China 6 exploration; geological survey; imaging method; research program; tomography; topographic mapping, North America https://www.scopus.com/inward/record.uri?eid=2-s2.0-78649812321&partnerID=40&md5=1c58b446fd37cc06a5825e02ca662ee2 cited By 4 G. Liu S. Dong X. Chen Q. Zhou Z. Liu article Powers2010 The distribution of seismicity about strike-slip faults provides measurements of fault roughness and damage zone width. In California, seismicity decays with distance from strike-slip faults according to a power law -(1 + ×2/d2)-g/2. This scaling relation holds out to a fault-normal distance × of 3?6 km and is compatible with a ?rough fault loading? model in which the inner scale d measures the half width of a volumetric damage zone and the roll-off rate g is governed by stress variations due to fault roughness. According to Dieterich and Smith?s 2-D simulations, g approximates the fractal dimension of alongstrike roughness. Near-fault seismicity is more localized on faults in northern California (NoCal, d = 60 ± 20 m, g = 1.65 ± .05) than in southern California (SoCal, d = 220 ± 40 m, g = 1.16 ± .05). The Parkfield region has a damage zone half width (d = 120 ± 30 m) consistent with the SAFOD drilling estimate; its high roll-off rate (g = 2.30 ± .25) indicates a relatively flat roughness spectrum: k-1 versus k-2 for NoCal, k-3 for SoCal. Our damage zone widths (the first direct estimates averaged over the seismogenic layer) can be interpreted in terms of an across-strike ?fault core multiplicity? that is 1 in NoCal, 2 at Parkfield, and 3 in SoCal. The localization of seismicity near individual faults correlates with cumulative offset, seismic productivity, and aseismic slip, consistent with a model in which faults originate as branched networks with broad, multicore damage zones and evolve toward more localized, lineated features with low fault core multiplicity, thinner damage zones, and less seismic coupling. Our results suggest how earthquake triggering statistics might be modified by the presence of faults. Copyright 2010 by the American Geophysical Union. 2010 English 21699313 10.1029/2008JB006234 Journal of Geophysical Research: Solid Earth 115 Blackwell Publishing Ltd Department of Earth Sciences, University of Southern California, ZHS-117, 3651 Trousdale Pkwy, Los Angeles, CA 90089-0742, United States; Southern California Earthquake Center, University of Southern California, ZHS-169, 3651 Trousdale Pkwy., Los Angeles, CA 90089-0742, United States 5 earthquake trigger; normal fault; roughness; seismicity; strike-slip fault; earthquake damage; power law, California; United States; Parkfield https://www.scopus.com/inward/record.uri?eid=2-s2.0-77952280585&doi=10.1029%2f2008JB006234&partnerID=40&md5=8ae84be6c134fc04378638997c869e2d cited By 69 P.M. Powers T.H. Jordan article Janssen2010 Several types of amorphous material in ultracataclastic core samples recovered from 3194 m and 3294 m depth of the main bore hole of the San Andreas Fault Observatory at Depth are identified and described with transmission electron microscopy and scanning electron microscopy. We observed (1) amorphous material on a slickenside surface, (2) glassy bands contained in an ultracataclastic matrix and (3) amorphous rims surrounding quartz or feldspar clasts. Chemical analyses of the amorphous material reveal that silica content is slightly enriched or similar as in the adjacent matrix. We suggest that all amorphous material was formed by comminution of clasts (crush-origin pseudotachylytes) rather than by melting (melt-origin pseudotachylytes). The observed amorphous phases may act as lubricating layers that reduce friction in the San Andreas Fault. Copyright 2010 by the American Geophysical Union. 2010 English 00948276 10.1029/2009GL040993 Geophysical Research Letters 37 Department of Geodynamics and Geomaterials, GFZ German Research Centre for Geosciences, D-14473 Potsdam, Germany; Department of Chemistry and Material Cycles, GFZ German Research Centre for Geosciences, D-14473 Potsdam, Germany; Department of Earth and Planetary Science, University of California, Berkeley, CA 94720-4767, United States 1 Adjacent matrix; Amorphous phasis; matrix; Pseudotachylytes; San Andreas Fault; Silica content, Core samples; Grinding (comminution); Oxide minerals; Quartz; Scanning electron microscopy; Silica; Silicate minerals; Transmission electron microscopy, Amorphous materials, amorphous medium; borehole; chemical analysis; clast; feldspar; melting; San Andreas Fault; silica; slickenside; transmission electron microscopy https://www.scopus.com/inward/record.uri?eid=2-s2.0-75749134827&doi=10.1029%2f2009GL040993&partnerID=40&md5=3f8124d1a90c03153df9830d8d8cf58c cited By 53 C. Janssen R. Wirth E. Rybacki R. Naumann H. Kemnitz H.-R. Wenk G. Dresen article Su2010873 It has long been a dream for mankind to enter the deep Earth to sample and investigate the structures and inner geological progresses. Until now, scientific drilling has been the unique method in our understanding of the processes and structures of the Earth. This paper try to give a brief introduction of the history, the development, the mission, the structure and management, the membership, the project development scheme of International Continental Drilling Program (ICDP). Great advances have been brought about in many fields of earth sciences by continental scientific drilling in recent years. Based on the recent publications and website materials of ICDP, this paper summarize the main developments in Climate Dynamics and Global Environments, in the Study of Impact Craters, in the GeoBiospherc, in Active Volcanic Systems, in Active Faults, in Hotspot Volcanoes, in Convergent Plate Boundaries and Collision Zones, and in Natural Resources. Special introduction on the scientific results of ICDP drilling at Mt. Unzen, Japan and the Hawaii Scientific Drilling Project (HSDP) is introduced in this paper. Fascinating discoveries such as the gouge layer of San Andreas Fault and the finding of talc in cuttings of SAFOD project are also introduced in this paper. As one of the three founding members of ICDP, China has also gained a lot of developments in continental scientific drilling; typical examples are the achievements of Chinese Continental Scientific Drilling (CCSD) and the progress of Lake Qinghai Scientific Drilling Project. The preliminary progresses . of the third approved ICDP project of China -the Chinese Cretaceous Continental Scientific Drilling Project and the development of ICDP-China are also summarized in this paper. 2010 Chinese 10009515 Acta Geologica Sinica (English Edition) 84 873-886 Key Laboratory for Continental Dynamics of MLR, Institute of Geology, Chinese Academy of Geological Sciences, Beijing, 100037, China 6 active fault; climate change; collision zone; crater; deep drilling; hot spot; mantle plume; natural resource; San Andreas Fault; talc, China; Hawaii [United States]; Japan; Kyushu; Nagasaki [Kyushu]; Qinghai; Qinghai Lake; United States; Unzen Volcano https://www.scopus.com/inward/record.uri?eid=2-s2.0-78649844329&partnerID=40&md5=732f4e363917ab0357a84117ef5aab8b cited By 9 D. Su J. Yang article Zhang2009 We apply a three-dimensional (3D) generalized Radon transform (GRT) to scattered P-waves from 575 local earthquakes recorded at 68 temporary network stations for passive-source imaging of (near-vertical) structures close to the San Andreas Fault Observatory at Depth (SAFOD) site. The GRT image profiles through or close by the SAFOD site reveal near-vertical reflectors close to the fault zone as well as in the granite to the southwest and the Franciscan mélange to the northeast of the main fault. Although slightly lower in resolution, these structures are generally similar to features in 2D images produced with steep-dip prestack seismic migration of data from active source seismic reflection and refraction surveys. Our GRT images, however, also reveal several vertical reflectors to the northeast of the SAF that do not appear in the migration images but which are consistent with local geology. These results suggest that in a seismically active area, inverse scattering of earthquake data (for instance with the GRT) can be a viable and, in 3D, economic alternative to an active source survey. Copyright 2009 by the American Geophysical Union. 2009 English 00948276 10.1029/2009GL040372 Geophysical Research Letters 36 Department of Geoscience, University of Wisconsin-Madison, 1215 West Dayton St., Madison, WI 53706, United States; Earth Resources Laboratory, Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139, United States; Department of Earth and Atmospheric Sciences, Saint Louis University, 3507 Laclede Ave., Saint Louis, MO 63108, United States 23 2D images; Active area; California; Earthquake data; Fault zone; Generalized radon transform; Image profile; Inverse scattering; P-waves; Prestack; San Andreas Fault; Seismic migration; Seismic reflections; Seismic waveforms; Source imaging; Temporary networks; Three-dimensional (3D), Earthquakes; Radon; Reflection; Seismic waves; Surveys, Three dimensional, data inversion; earthquake event; fault zone; imaging method; P-wave; prestack migration; Radon transform; seismic migration; seismic reflection; three-dimensional modeling; wave scattering; waveform analysis, California; North America; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-72049121177&doi=10.1029%2f2009GL040372&partnerID=40&md5=0a7771d70165d9f06407f0e6c0dc93ce cited By 20 H. Zhang P. Wang R.D. Van Der Hilst M.N. Toksoz C. Thurber L. Zhu article Zhang2009 We refined the three-dimensional (3-D) Vp, Vs and Vp/Vs models around the San Andreas Fault Observatory at Depth (SAFOD) site using a new double-difference (DD) seismic tomography code (tomoDDPS) that simultaneously solves for earthquake locations and all three velocity models using both absolute and differential P, S, and S-P times. This new method is able to provide a more robust Vp/Vs model than that from the original DD tomography code (tomoDD), obtained simply by dividing Vp by Vs. For the new inversion, waveform cross-correlation times for earthquakes from 2001 to 2002 were also used, in addition to arrival times from earthquakes and explosions in the region. The Vp values extracted from the model along the SAFOD trajectory match well with the borehole log data, providing in situ confirmation of our results. Similar to previous tomographic studies, the 3-D structure around Parkfield is dominated by the velocity contrast across the San Andreas Fault (SAF). In both the Vp and Vs models, there is a clear low-velocity zone as deep as 7 km along the SAF trace, compatible with the findings from fault zone guided waves. There is a high Vp/Vs anomaly zone on the southwest side of the SAF trace that is about 1-2 km wide and extends as deep as 4 km, which is interpreted to be due to fluids and fractures in the package of sedimentary rocks abutting the Salinian basement rock to the southwest. The relocated earthquakes align beneath the northeast edge of this high Vp/Vs zone. We carried out a 2-D correlation analysis for an existing resistivity model and the corresponding profiles through our model, yielding a classification that distinguishes several major lithologies. © 2009 by the American Geophysical Union. 2009 English 15252027 10.1029/2009GC002709 Geochemistry, Geophysics, Geosystems 10 Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, United States; Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706, USA, United States; U.S. Geological Survey, Denver, CO 80225, United States 11 https://www.scopus.com/inward/record.uri?eid=2-s2.0-72049100963&doi=10.1029%2f2009GC002709&partnerID=40&md5=1c1fa8213385ada6e5b5d4618a808a5e cited By 115 H. Zhang C. Thurber P. Bedrosian article Griffith2009 We estimate the static stress drop on small exhumed strike-slip faults in the Lake Edison granodiorite of the central Sierra Nevada (California). The subvertical strike-slip faults were exhumed from 4 to 15 km depth and were chosen because they are exposed in outcrop along their entire tip-to-tip lengths of 8-12 m. Slip nucleated on joints and accumulated by crystal-plastic shearing (forming quartz mylonites from early quartz vein filling in joints) and successive brittle faulting (forming epidote-bearing cataclasites). The occurrence of thin, ≤300 fim wide, pseudotachylytes along some small faults throughout the study area suggests that some, if not all, of the brittle slip on the study area faults may have been seismic. We suggest that the contribution of brittle, cataclastic slip to the total slip along the studied cataclasite-bearing small faults may be estimated by the length of epidote-filled, rhombohedral dilatational jogs (rhombochasms) distributed quasi-periodically along the length of the faults. The interpretation that slip recorded by rhombochasms occurred in single events is based on evidence that (1) epidote crystals are randomly oriented and undeformed within the rhombochasm; (2) cataclasite in principal slip zones does not include clasts of previous cataclasite, and (3) rhombochasm lengths vary systematically along the length of the faults with slip maximum occurring near the fault center, tapering to the fault tips. We thereby constrain both the rupture length and slip. On the basis of these measurements, we calculate stress drops ranging over 90-250 MPa, i.e., one to two orders of magnitude larger than typical seismological estimates for earthquakes, but similar in magnitude to seismological estimates of small (<M2) earthquakes from the San Andreas Fault Observatory at Depth (SAFOD). The slip events described in the present study occurred along small, deep-seated faults, and, given the calculated stress drops and observations that brittle faults exploited joints sealed by quartz-bearing mylonite, we conclude that these were "strong" faults. © 2009 by the American Geophysical Union. 2009 English 21699313 10.1029/2008JB005879 Journal of Geophysical Research: Solid Earth 114 Blackwell Publishing Ltd Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305, United States; Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, I-00143 Roma, Italy; Dipartimento di Geoscienze, Universita di Padova, Via Giotto 1, 1-35137 Padova, Italy 2 earthquake; epidote; faulting; mylonite; quartz; seismology; shear stress; slip; strike-slip fault, California; North America; San Andreas; Sierra Nevada [California]; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-66149164465&doi=10.1029%2f2008JB005879&partnerID=40&md5=b4f6329e642cbee7db5c0a78f56977ef cited By 30 W.A. Griffith G.D. Toro G. Pennacchioni D.D. Pollard S. Nielsen article Schleicher2009173 A detailed mineralogical study is presented of the matrix of mudrocks sampled from spot coring at three key locations along the San Andreas Fault Observatory at depth (SAFOD) drill hole. The characteristics of authigenic illite-smectite (I-S) and chlorite-smectite (C-S) mixed-layer mineral clays indicate a deep diagenetic origin. A randomly ordered I-S mineral with ca. 20-25% smectite layers is one of the dominant authigenic clay species across the San Andreas Fault zone (sampled at 3,066 and 3,436 m measured depths/MD), whereas an authigenic illite with ca. 2-5% smectite layers is the dominant phase beneath the fault (sampled at 3,992 m MD). The most smectite-rich mixed-layered assemblage with the highest water content occurs in the actively deforming creep zone at ca. 3,300-3,353 m (true vertical depth of ca. 2.7 km), with I-S (70:30) and C-S (50:50). The matrix of all mudrock samples show extensive quartz and feldspar (both plagioclase and K-feldspar) dissolution associated with the crystallization of pore-filling clay minerals. However, the effect of rock deformation in the matrix appears only minor, with weak flattening fabrics defined largely by kinked and fractured mica grains. Adopting available kinetic models for the crystallization of I-S in burial sedimentary environments and the current borehole depths and thermal structure, the conditions and timing of I-S growth can be evaluated. Assuming a typical K+ concentration of 100-200 ppm for sedimentary brines, a present-day geothermal gradient of 35°C/km and a borehole temperature of ca. 112°C for the sampled depths, most of the I-S minerals can be predicted to have formed over the last 4-11 Ma and are probably still in equilibrium with circulating fluids. The exception to this simple burial pattern is the occurrence of the mixed layered phases with higher smectite content than predicted by the burial model. These minerals, which characterize the actively creeping section of the fault and local thin film clay coating on polished brittle slip surfaces, can be explained by the influence of either cooler fluids circulating along this segment of the fault or the flow of K+-depleted brines. © Springer-Verlag 2008. 2009 English 00107999 10.1007/s00410-008-0328-7 Contributions to Mineralogy and Petrology 157 173-187 Geozentrum Nordbayern, Universität Erlangen-Nürnberg, Schloßgarten 5, 91054 Erlangen, Germany; Institut für Geographie und Geologie, Ernst-Moritz-Arndt-Universität Greifswald, Friedrich-Ludwig-Jahn-Str. 17a, 17487 Greifswald, Germany; Department of Geological Sciences, University of Michigan, 1100 N. University Ave., Ann Arbor, MI 48109, United States 2 chlorite; crystallization; deformation mechanism; diagenesis; illite; mudstone; San Andreas Fault; smectite; thermal structure, California; North America; Parkfield; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-57849148044&doi=10.1007%2fs00410-008-0328-7&partnerID=40&md5=4605ba50225212c3a552420b6cd39a9a cited By 52 A.M. Schleicher L.N. Warr B.A. Pluijm article McGarr20092815 For one week during September 2007, we deployed a temporary network of field recorders and accelerometers at four sites within two deep, seismically active mines. The ground-motion data, recorded at 200 samples/sec, are well suited to determining source and ground-motion parameters for the mining-induced earthquakes within and adjacent to our network. Four earthquakes with magnitudes close to 2 were recorded with high signal/noise at all four sites. Analysis of seismic moments and peak velocities, in conjunction with the results of laboratory stick-slip friction experiments, were used to estimate source processes that are key to understanding source physics and to assessing underground seismic hazard. The maximum displacements on the rupture surfaces can be estimated from the parameter Rv, where v is the peak ground velocity at a given recording site, and R is the hypocentral distance. For each earthquake, the maximum slip and seismic moment can be combined with results from laboratory friction experiments to estimate the maximum slip rate within the rupture zone. Analysis of the four M 2 earthquakes recorded during our deployment and one of special interest recorded by the in-mine seismic network in 2004 revealed maximum slips ranging from 4 to 27 mm and maximum slip rates from 1.1 to 6:3 m=sec. Applying the same analyses to an M 2.1 earthquake within a cluster of repeating earthquakes near the San Andreas Fault Observatory at Depth site, California, yielded similar results for maximum slip and slip rate, 14 mm and 4:0 m=sec. 2009 English 00371106 10.1785/0120080336 Bulletin of the Seismological Society of America 99 2815-2824 U.S. Geological Survey, MS 977 345 Middlefield Rd. Menlo Park, California 94025, United States; Council for Scientific and Industrial Research Natural Resources and Environmental Unit, P.O. Box 91230, Auckland Park 2006, South Africa 5 Broadband records; California; Deep gold mines; Ground motion parameters; Ground-motion; Hypocentral distance; Maximum displacement; Maximum slip; Peak ground velocity; Peak velocities; Repeating earthquake; Rupture surface; Rupture zone; San Andreas Fault; Seismic hazards; Seismic moment; Seismic networks; Slip rates; Source process; Stick-slip friction; Temporary networks, Experiments; Friction; Gold mines; Mines; Mining; Parameter estimation; Risk assessment; Slip forming; Tectonics, Earthquakes, earthquake catalogue; earthquake hypocenter; earthquake magnitude; gold mine; ground motion; mining-induced seismicity; seismic hazard; seismic moment; slip rate, California; North America; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-70349873690&doi=10.1785%2f0120080336&partnerID=40&md5=eadc13c48dee1062809d2dd7a84c75f7 cited By 16 A. McGarr M. Boettcher J.B. Fletcher R. Sell M.J.S. Johnston R. Durrheim S. Spottiswoode A. Milev article Carpenter2009 We report on frictional properties of rocks within the 3-D crustal volume surrounding the San Andreas Fault Observatory at Depth (SAFOD). Samples include lithologies adjacent to the San Andreas Fault (SAF) in the subsurface, SAFOD borehole rocks, and synthetic fault gouge composed of talc, serpentinite, and quartz. Granodiorite, arkosic sandstone, and siltstone samples from the SAFOD borehole are frictionally strong (μ = 0.56 - 0.66). Sand and clay-rich lithologies from outcrop exhibit friction in the range /x = 0.56 - 0.68. Natural serpentinite thought to abut the SAF at depth exhibits low friction (μ = 0.18 - 0.26). Our results indicate that 1) serpentinite exhibits low strength, but is not weak enough to completely satisfy weak fault models, 2) all other samples are consistent with a strong fault and crust and, 3) if the SAF is weak (μ ≤ 0.2) due to the presence of serpentinite or talc, these minerals would likely need to constitute over 50% by weight of the shear zone. Copyright 2009 by the American Geophysical Union. 2009 English 00948276 10.1029/2008GL036660 Geophysical Research Letters 36 Department of Geosciences, Energy Institute Center for Geomechanics, Geofluids, and Geohazards, Pennsylvania State University, University Park, PA, United States; Department of Geosciences, Energy Institute Center for Geomechanics, Geofluids, and Geohazards, Pennsylvania State University, University Park, PA 16802, United States 5 Fault gouge; Fault model; Frictional behavior; Frictional properties; Granodiorite; Low friction; San Andreas Fault; Serpentinite; Shear zone; Siltstone, Clay minerals; Friction; Oxide minerals; Petrology; Quartz; Silicate minerals; Talc, Three dimensional, borehole; fault gouge; fault zone; friction; granodiorite; lithology; outcrop; quartz; rock mechanics; sandstone; serpentinite; shear zone; siltstone; strength; talc, California; North America; San Andreas; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-65649106899&doi=10.1029%2f2008GL036660&partnerID=40&md5=879e0e1c25af51b6c6c960780e797f2b cited By 77 B.M. Carpenter C. Marone D.M. Saffer article Tembe2009 Analysis of field data has led different investigators to conclude that the San Andreas Fault (SAF) has either anomalously low frictional sliding strength (μ < 0.2) or strength consistent with standard laboratory tests (μ > 0.6). Arguments for the apparent weakness of the SAF generally hinge on conceptual models involving intrinsically weak gouge or elevated pore pressure within the fault zone. Some models assert that weak gouge and/or high pore pressure exist under static conditions while others consider strength loss or fluid pressure increase due to rapid coseismic fault slip. The present paper is composed of three parts. First, we develop generalized equations, based on and consistent with the Rice (1992) fault zone model to relate stress orientation and magnitude to depth-dependent coefficient of friction and pore pressure. Second, we present temperature-and pressure-dependent friction measurements from wet illite-rich fault gouge extracted from San Andreas Fault Observatory at Depth (SAFOD) phase 1 core samples and from weak minerals associated with the San Andreas Fault. Third, we reevaluate the state of stress on the San Andreas Fault in light of new constraints imposed by SAFOD borehole data. Pure talc (μ≈0.1) had the lowest strength considered and was sufficiently weak to satisfy weak fault heat flow and stress orientation constraints with hydrostatic pore pressure. Other fault gouges showed a systematic increase in strength with increasing temperature and pressure. In this case, heat flow and stress orientation constraints would require elevated pore pressure and, in some cases, fault zone pore pressure in excess of vertical stress. Copyright 2009 by the American Geophysical Union. 2009 English 21699313 10.1029/2008JB005883 Journal of Geophysical Research: Solid Earth 114 Blackwell Publishing Ltd Institute for Soil Mechanics and Rock Mechanics, Karlsruhe Institute of Technology, D-76128 Karlsruhe, Germany; Earthquake Hazards Team, U.S. Geological Survey, MS 977, 345 Middlefield Road, Menlo Park, CA 94025, United States; Department of Geosciences, State University of New York at Stony Brook, ESS Building, Stony Brook, NY 11794-2100, United States 11 drilling; fault gouge; fault zone; friction; pore pressure; San Andreas Fault; stress field, Calluna vulgaris https://www.scopus.com/inward/record.uri?eid=2-s2.0-72049090093&doi=10.1029%2f2008JB005883&partnerID=40&md5=a5ae3cda7898196688a8a89157b778ae cited By 44 S. Tembe D. Lockner T.-F. Wong article Schleicher2009 Mineralogical and geochemical changes in mudrock cuttings from two segments of the San Andreas Fault Observatory at Depth (SAFOD) drill hole (3066-3169 and 3292-3368 m measured depth) are analyzed in this study. Bulk rock samples and hand-picked fault-related grains characterized by polished surfaces and slickensides were investigated by X-ray diffraction, electron microscopy and geochemical analysis. The elemental changes in fault-related grains along the sampled San Andreas Fault are attributed to dissolution of detrital grains (particularly feldspar and quartz) and local precipitation of illite-smectite and/or chlorite-smectite mixed layers in fractures and veins. Assuming ZrO 2 and TiO2 to be immobile elements, systematic differences in element concentrations show that most of the elements are depleted in the fault-related grains compared to the wall rock lithology. Calculated mass loss between the bulk rock and picked fault rock ranges from 17 to 58% with a greater mass transport in the shallow trace of the sampled fault that marks the upper limit the fault core. The relatively large amount of element transport at temperatures of ∼110-114°C recorded throughout the core requires extensive fluid circulation during faulting. Whereas dissolution/precipitation may be partly induced by the disequilibrium between fluids and rocks during diagenetic processes, stress-induced dissolution at grain contacts is proposed as the main mechanism for extensive mineral transformation in the fault rocks and localization of neomineralization along grain interface slip surfaces. Copyright 2009 by the American Geophysical Union. 2009 English 21699313 10.1029/2008JB006092 Journal of Geophysical Research: Solid Earth 114 Blackwell Publishing Ltd Geozentrum Nordbayern, Friedrich Alexander Universität Erlangen-Nürnberg, Erlangen, Germany; Department of Geological Sciences, University of Michigan, 1100 N. University Avenue, Ann Arbor, MI 48109-1005, United States; Institut fur Geographie und Geologie, Ernst Moritz Arndt Universität Greifswald, Friedrich-Ludwig-Jahn-Strasse 17a, D-17487 Greifswald, Germany 4 diagenesis; disequilibrium; dissolution; electron microscopy; faulting; fluid-structure interaction; geochemistry; lithology; mass transfer; mineralization; mudstone; observatory; precipitation (chemistry); titanium; X-ray diffraction; zircon, California; North America; San Andreas; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-69249092264&doi=10.1029%2f2008JB006092&partnerID=40&md5=32f6b266705c75fe9efcc8bdac8cd235 cited By 37 A.M. Schleicher S.N. Tourscher L.N. Warr article Springer2009206 The San Andreas Fault Observatory at Depth (SAFOD) drill hole encountered indurated, high-seismic-velocity arkosic sedimentary rocks west of the active trace of the San Andreas fault in central California. The arkosic rocks are juxtaposed against granitic rocks of the Salinian block to the southwest and against fine-grained Great Valley Group and Jurassic Franciscan rocks to the northeast. We identify three distinct lithologic units using cuttings, core petrography, electrical resistivity image logs, zircon fission-track analyses, and borehole-based geophysical logs. The upper arkose occurs from 1920 to 2530 m measured depth (mmd) in the borehole and is composed of five structural blocks defined by bedding orientations, wireline log character, physical properties, and lithologic characteristics. A clay-rich zone between 2530 and 2680 mmd is characterized by low V p and an enlarged borehole. The lower arkose lies between 2680 and 3150 mmd. Fission-track detrital zircon cooling ages are between 64 and 70 Ma, appear to belong to a single population, and indicate a latest Cretaceous to Paleogene maximum depositional age. We interpret these Paleocene-Eocene strata to have been deposited in a proximal submarine fan setting shed from a Salinian source block, and they correlate with units to the southeast, along the western and southern edge of the San Joaquin Basin, and with arkosic conglomerates to the northwest. The arkosic section constitutes a deformed fault-bounded block between the modern strand of the San Andreas fault to the northeast and the Buzzard Canyon fault to the southwest. Significant amounts of slip appear to have been accommodated on both strands of the fault at this latitude. © 2009 Geological Society of America. 2009 English 19418264 10.1130/L13.1 Lithosphere 1 Geological Society of America 206-226 Department of Geology, Utah State University, Logan, UT 84322-4505, United States; Department of Geology, Union College, Schenectady, NY 12308-3107, United States; Earth and Atmospheric Sciences, Saint Louis University, 3507 Laclede Avenue, Saint Louis, MO 63103, United States; Chevron International Exploration and Production, 1500 Louisiana Street, Houston, TX 77002, United States 4 Boreholes; Fission reactions; Observatories; Silicate minerals; Strike-slip faults; Well logging; Zircon, Bedding orientations; Detrital zircon; Geophysical logs; Measured depths; San Andreas fault; Seismic velocities; Submarine fans; Zircon fission tracks, Sedimentary rocks, borehole; electrical resistivity; fission track dating; petrography; San Andreas Fault; sedimentary rock; seismic velocity, California; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-85011533833&doi=10.1130%2fL13.1&partnerID=40&md5=f606ec44cdc55f783f4e53645cd12d26 cited By 18 S.D. Springer J.P. Evans J.I. Garver D. Kirschner S.U. Janecke article Fulton2009 Heat flow data near the San Andreas Fault (SAF) do not reveal a near-fault anomaly as expected from frictional heat generation, an observation interpreted to indicate that the fault slips at a depth-averaged shear stress <20 MPa. The data also contain large unexplained scatter, which has been a separate major issue in the analysis of heat flow within the California Coast Ranges. Here we use numerical models of heat conduction to evaluate the hypothesis that thermal refraction, due to contrasts in thermal conductivity in the subsurface, both produces the observed scatter in heat flow and as a result obscures the thermal signature from frictional heating on a fault that supports large shear stress during slip. Our study focuses on the region around the San Andreas Fault Observatory at Depth (SAFOD) near Parkfield, California. Our results show that surface heat flow is most sensitive to the contrast between Tertiary sediments and basement rocks and to wavelengths of basement topography of ∼10 km. With realistic thermal conductivity contrasts and a reasonable interpretation of this geologic contact, we show that thermal refraction is a plausible explanation for the observed heat flow scatter. However, refraction effects are unable to mask frictional heat generation in a manner consistent with observations. We show that even with large refraction effects, low background heat flow, a regional NW-SE decrease in heat flow, or nonsteady state heat conduction, the data are most consistent with a fault that produces little to no frictional heat. Copyright 2009 by the American Geophysical Union. 2009 English 21699313 10.1029/2008JB005796 Journal of Geophysical Research: Solid Earth 114 Blackwell Publishing Ltd College of Oceanic and Atmospheric Sciences, Oregon State University, 104 COAS Building, Corvallis, OR 97331, United States; Department of Geosciences, Pennsylvania State University, 310 Deike Building, University Park, PA 16802-0000, United States 6 basement rock; coastal zone; data acquisition; fault slip; flow measurement; friction; heat flow; numerical model; shear stress; slip; subsurface flow; thermal conductivity, California; North America; San Andreas; United States, Calluna vulgaris https://www.scopus.com/inward/record.uri?eid=2-s2.0-70349470855&doi=10.1029%2f2008JB005796&partnerID=40&md5=3d6073c8a4af6983f3d3e546761a5051 cited By 14 P.M. Fulton D.M. Saffer article Li2008 Highly damaged rocks within the San Andreas fault zone at Parkfield form a low-velocity waveguide for seismic waves, giving rise to fault-guided waves. Prominent fault-guided waves have been observed at the San Andreas Fault Observatory at Depth (SAFOD) site, including a surface array across the fault zone and a borehole seismograph placed in the SAFOD well at a depth of ∼2.7 km below ground. The resulting observations are modeled here using 3-D finite-difference methods. To fit the amplitude, frequency, and travel-time characteristics of the data, the models require a downward tapering, 30-40-m wide faultcore embedded in a 100-200-m wide jacket. Compared with the intact wall rocks, the core velocities are reduced by ∼40% and jacket velocities by ∼25%. Based on the depths of earthquakes generating guided waves, we estimate that the low-velocity waveguide along the fault at SAFOD extends at least to depths of ∼7 km, more than twice the depth reported in pervious studies. Copyright 2008 by the American Geophysical Union. 2008 English 00948276 10.1029/2007GL032924 Geophysical Research Letters 35 Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089, United States; Institute of Earth Science and Engineering, University of Auckland, Auckland 1142, New Zealand 8 Seismology; Waveguides, American Geophysical Union; Bore hole; Fault zones; Finite difference (FD); Guided waves; Low velocities; Parkfield; San Andreas Fault (SAF); Surface array; Travel time, Guided electromagnetic wave propagation, fault zone; finite difference method; San Andreas Fault; seismic data; seismic wave; seismograph; three-dimensional modeling; wave velocity, California; North America; Parkfield; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-48249130104&doi=10.1029%2f2007GL032924&partnerID=40&md5=cf9b82fdb85e9aca6067ab4146aa0931 cited By 53 Y.-G. Li article Paul2008394 This paper presents a wellbore-stability study of the San Andreas Fault Observatory at Depth (SAFOD) research borehole located near Parkfield, California, USA. In the summer of 2005, the SAFOD borehole was drilled successfully through the active trace of the San Andreas Fault (SAF) in an area characterized by fault creep and frequent microearthquakes. In this study, we report how the analysis of wellbore failures in the upper part of the hole, geophysical logs, and a model for stress gradients in the vicinity of the fault were used to estimate the mud weights required to drill through the fault successfully. Because logging-while-drilling (LWD) acoustic caliper data and real-time hole-volume calculations both showed that relatively little failure occurred while drilling through the SAF, the predicted mud weight was successful in drilling a stable borehole. However, a six-arm caliper log, run after drilling was completed, indicates that there was deterioration of the borehole with time, which appears to be caused by fluid penetration around the borehole. The LWD-resistivity measurements show that essentially no fluid penetration occurred as the hole was being drilled. Because of this, the mud weight used was capable of maintaining a stable wellbore. However, the resistivity data obtained after drilling show appreciable fluid penetration with time, thus negating the effectiveness of the mud weight and leading to time-dependent wellbore failure. Using finite-element modeling (FEM), we show that mud penetration into the fractured medium around the borehole causes failure with time. Copyright © 2008 Society of Petroleum Engineers. 2008 English 10646671 10.2118/102781-PA SPE Drilling and Completion 23 Society of Petroleum Engineers 394-408 Stanford University, Stanford, CA, United States; ConocoPhillips Subsurface Technology Group, Houston, TX, United States 4 Acoustic logging; Boring; Finite element method; Logging while drilling; Strike-slip faults, A-stable; California; Fault creep; Fluid penetration; Logging while drilling; Microearthquakes; Mud weights; San Andreas fault; Wellbore; Wellbore stability studies, Boreholes, borehole geophysics; borehole stability; creep; drilling; drilling fluid; finite element method; microearthquake; numerical model; San Andreas Fault, California; North America; Parkfield; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-58149348346&doi=10.2118%2f102781-PA&partnerID=40&md5=1bb07bdcccb48f9b7b0477270240d3b8 cited By 7 P.K. Paul M. Zoback article Bennington20082934 We present models of the three-dimensional (3D) seismic attenuation structure, both Q p and Q s for a 16 km 2 area centered on the San Andreas Fault Observatory at Depth (SAFOD). The P- and S-wave t*-values used in the inversion were determined from local earthquake data recorded by seismic network and portable array stations within the Parkfield region by inverting arrival spectra for source parameters, t* and site response. Two techniques for determining the site response, the joint and alternating methods, were compared and it was found that the alternating method significantly underestimated site response variations. The t*-values were inverted to obtain 3D frequency-independent Q p and Q s models using 3D V p and V s models and associated event locations. A shallow low-Q area (Q p and Q s about 50-75) on the southwest edge of both models is attributed to the low-velocity Cenozoic sedimentary rocks that overlie the Salinian basement rock. A high-Q feature (Q p and Q s about 250 to 300) abuts this area and is interpreted as the Salinian basement. Adjacent to the San Andreas fault (SAF) trace, on its southwest side, there is a low-Q feature (Q p and Q s about 50-80) attributed to a wedge of sedimentary rocks; uniformly low Q p- and Q s-values suggest that the wedge is fluid rich. A low-Q basin feature (Q p and Q's about 50-75) on the northeast side of the SAF is interpreted as a fluid rich zone. Beneath this area there is a high-Q feature (Q p and Q s about 220-300), which may be caused by crack closure due to increased pressure with depth in the rocks of the Franciscan formation. Given these high Q-values, it seems unlikely that this area acts as a fluid pathway for fluids entering the fault zone from the east into the seismogenic zone of the SAF. 2008 English 00371106 10.1785/0120080175 Bulletin of the Seismological Society of America 98 2934-2947 Department of Geology and Geophysics, University of Wisconsin-Madison, Madison, WI, United States; Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, United States 6 Alternating methods; California; Cenozoic; Earthquake datum; Event locations; Fault zones; Fluid pathways; Low velocities; Portable arrays; Q-values; Salinian basements; San andreas faults; Seismic attenuations; Seismic networks; Seismogenic zones; Site response; Source parameters; Three-dimensional (3D), Buildings; Crack closure; Earthquakes; Fluids; Sedimentology; Tectonics; Three dimensional, Sedimentary rocks, data inversion; fault zone; P-wave; S-wave; San Andreas Fault; seismic attenuation; seismic data; seismic source; three-dimensional modeling, California; North America; Parkfield; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-63049099031&doi=10.1785%2f0120080175&partnerID=40&md5=78f1a2ceaab0bed46b3c8b3cdee718e4 cited By 31 N. Bennington C. Thurber S. Roecker article Niu2008204 Measuring stress changes within seismically active fault zones has been a long-sought goal of seismology. One approach is to exploit the stress dependence of seismic wave velocity, and we have investigated this in an active source cross-well experiment at the San Andreas Fault Observatory at Depth (SAFOD) drill site. Here we show that stress changes are indeed measurable using this technique. Over a two-month period, we observed an excellent anti-correlation between changes in the time required for a shear wave to travel through the rock along a fixed pathway (a few microseconds) and variations in barometric pressure. We also observed two large excursions in the travel-time data that are coincident with two earthquakes that are among those predicted to produce the largest coseismic stress changes at SAFOD. The two excursions started approximately 10 and 2 hours before the events, respectively, suggesting that they may be related to pre-rupture stress induced changes in crack properties, as observed in early laboratory studies. ©2008 Macmillan Publishers Limited. All rights reserved. 2008 English 00280836 10.1038/nature07111 Nature 454 Nature Publishing Group 204-208 Department of Earth Science, MS-126, Rice University, 6100 Main Street, Houston, TX 77005, United States; Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, NW, Washington, DC 20015, United States; Earth Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, United States 7201 crack; earthquake; fault zone; measurement method; monitoring; rupture; seismic velocity; seismic wave, article; earthquake; environmental monitoring; measurement; pressure; priority journal; rock; stress, California; North America; San Andreas; San Andreas Fault Zone; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-47049108874&doi=10.1038%2fnature07111&partnerID=40&md5=d7b380346e97976c0cb0492ce90e8e88 cited By 219 F. Niu P.G. Silver T.M. Daley X. Cheng E.L. Majer article Parés2008 We present a protocol for using paleomagnetic analysis to determine the absolute orientation of core recovered from the SAFOD borehole. Our approach is based on determining the direction of the primary remanent magnetization of a spot core recovered from the Great Valley Sequence during SAFOD Phase 2 and comparing its direction to the expected reference field direction for the Late Cretaceous in North America. Both thermal and alternating field demagnetization provide equally resolved magnetization, possibly residing in magnetite, that allow reorientation. Because compositionally similar siltstones and fine-grained sandstones were encountered in the San Andreas Fault Zone during Stage 2 rotary drilling, we expect that paleomagnetic reorientation will yield reliable core orientations for continuous core acquired from directly within and adjacent to the San Andreas Fault during SAFOD Phase 3, which will be key to interpretation of spatial properties of these rocks. Copyright 2008 by the American Geophysical Union. 2008 English 00948276 10.1029/2007GL030921 Geophysical Research Letters 35 Department of Geological Sciences, University of Michigan, 2534 C. C. Little Building, Ann Arbor, MI 48109-1063, United States; Geology Department, University of Erlangen-Nuernberg, Schloßgarten 5, D-91054 Erlangen, Germany; U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, United States 2 Boreholes; Demagnetization; Magnetic fields; Magnetization; Sandstone; Tectonics, Fine-grained sandstones; Paleomagnetic analysis; Paleomagnetic reorientation, Geomagnetism, borehole; Cretaceous; demagnetization; paleomagnetism; remanent magnetization; San Andreas Fault, California; Central Valley [California]; North America; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-40849114815&doi=10.1029%2f2007GL030921&partnerID=40&md5=69f8f375b001572985f15c80978b6042 cited By 6 J.M. Parés A.M. Schleicher B.A. Pluijm S. Hickman article Wiersberg20081675 Data are presented on the molecular composition of drill-mud gas from the lower sedimentary section (1800-3987 m) of the SAFOD (San Andreas Fault Observatory at Depth) Main Hole measured on-line during drilling, as well as C and H isotope data from off-line mud gas samples. Hydrocarbons, H2 and CO2 are the most abundant non-atmospheric gases in drill-mud when drilling seismogenic zones. Gas influx into the well at depth is related to the lithology and permeability of the drilled strata: larger formation gas influx was detected when drilling through organic-rich shales and permeable sandstones. The SAF (San Andreas Fault), encountered between approximately 3100 m and 3450 m borehole depth, is generally low in gas, but is encompassed by two gas-rich zones (2700-2900 m and below 3550 m) at the fault margins with enhanced 222Rn activities and distinct gas compositions. Within the fault, two interstratified gas-rich lenses (3150-3200 m and 3310-3340 m) consist of CO2 and hydrocarbons (upper zone), but almost exclusively of hydrocarbons (lower zone). The isotopic composition indicates an organic source of hydrocarbons and CO2 in the entire sedimentary section of the well. Hydrocarbons in sedimentary strata are partly of microbial origin down to ∼2500 m borehole depth. The contribution of thermogenic gas increases between ∼2500 m and 3200 m. Below ∼3200 m, hydrocarbons fully derive from thermal degradation of organic matter. The lack of H2 in the center of the fault and the high concentration of H2 in the fractured zones at the fault margins are consistent with H2 formation by interaction of water with fresh silica mineral surfaces generated by tectonic activities, however, this needs to be verified by laboratory experiments. Based on these studies, it is concluded that the fault zone margins consist of strata with enhanced permeability, separated by a low-permeability fault center. © 2008 Elsevier Ltd. All rights reserved. 2008 English 08832927 10.1016/j.apgeochem.2008.01.012 Applied Geochemistry 23 1675-1690 GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany 6 Boreholes; Drilling; Gas fuel analysis; Hydrocarbons; Sedimentation, Gas compositions; Seismogenic zones; Spatial distribution, Seismographs, analytical method; chemical composition; concentration (composition); gas; hydrocarbon; isotopic composition; measurement method; permeability; sandstone; seismic zone; spatial distribution, California; North America; San Andreas Fault Zone; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-44149122181&doi=10.1016%2fj.apgeochem.2008.01.012&partnerID=40&md5=226dc21ae4a0b197f92f5597bf9c2eb1 cited By 64 T. Wiersberg J. Erzinger article Vasconcelos2008 Deconvolution interferometry successfully recovers the impulse response between two receivers without the need for an independent estimate of the source function. Here we extend the method of interferometry by deconvolution to multicomponent data in elastic media. As in the acoustic case, elastic deconvolution interferometry retrieves only causal scattered waves that propagate between two receivers as if one acts as a pseudosource of the point-force type. Interferometry by deconvolution in elastic media also generates artifacts because of a clamped-point boundary condition imposed by the deconvolution process. In seismic-while-drilling (SWD) practice, the goal is to determine the subsurface impulse response from drill-bit noise records. Most SWD technologies rely on pilot sensors and/or models to predict the drill-bit source function, whose imprint is then removed from the data. Interferometry by deconvolution is of most use to SWD applications in which pilot records are absent or provide unreliable estimates of bit excitation. With a numerical SWD subsalt example, we show that deconvolution interferometry provides an image of the subsurface that cannot be obtained by correlations without an estimate of the source autocorrelation. Finally, we test the use of deconvolution interferometry in processing SWD field data acquired at the San Andreas Fault Observatory at Depth (SAFOD). Because no pilot records were available for these data, deconvolution outperforms correlation in obtaining an interferometric image of the San Andreas fault zone at depth. © 2008 Society of Exploration Geophysicists. All rights reserved. 2008 English 00168033 10.1190/1.2904985 Geophysics 73 Society of Exploration Geophysicists S129-S141 Colorado School of Mines, Department of Geophysics, Center for Wave Phenomena, Egham, United Kingdom; ION Geophysical, GXT Imaging Solutions, Egham, United Kingdom; Colorado School of Mines, Department of Geophysics, Center for Wave Phenomena, Golden, CO, United States 3 Autocorrelation; Deconvolution; Drilling; Elastic waves; Imaging techniques; Impulse response; Interferometry; Seismic waves, Drill-bit seismic imaging; Elastic deconvolution interferometry; Multicomponent data; Seismic-while-drilling (SWD), Seismology, autocorrelation; boundary condition; deconvolution; drill bit; elastic wave; imaging method; interferometry; San Andreas Fault; seismic data; wave propagation; wave scattering https://www.scopus.com/inward/record.uri?eid=2-s2.0-43549085037&doi=10.1190%2f1.2904985&partnerID=40&md5=dd282820fd9e84472203dc767a76d183 cited By 118 I. Vasconcelos R. Snieder article Vasconcelos2008349 2008 English 00963941 10.1029/2008EO380001 Eos 89 Blackwell Publishing Ltd 349 Colorado School of Mines, Golden, United States; Earth and Ocean Sciences Division, Duke University Durham, NC, United States; Paulsson Geophysical Services, Inc., Brea, CA, United States 38 drill bit; fault; noise; San Andreas Fault, California; North America; San Andreas; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-67649497579&doi=10.1029%2f2008EO380001&partnerID=40&md5=144f0f9cc5508ec703d2a83786010b40 cited By 6 I. Vasconcelos R. Snieder P. Sava T. Taylor P. Maun A. Chavarria article Becken2008718 Magnetotelluric (MT) data from 66 sites along a 45-km-long profile across the San Andreas Fault (SAF) were inverted to obtain the 2-D electrical resistivity structure of the crust near the San Andreas Fault Observatory at Depth (SAFOD). The most intriguing feature of the resistivity model is a steeply dipping upper crustal high-conductivity zone flanking the seismically defined SAF to the NE, that widens into the lower crust and appears to be connected to a broad conductivity anomaly in the upper mantle. Hypothesis tests of the inversion model suggest that upper and lower crustal and upper-mantle anomalies may be interconnected. We speculate that the high conductivities are caused by fluids and may represent a deep-rooted channel for crustal and/or mantle fluid ascent. Based on the chemical analysis of well waters, it was previously suggested that fluids can enter the brittle regime of the SAF system from the lower crust and mantle. At high pressures, these fluids can contribute to fault-weakening at seismogenic depths. These geochemical studies predicted the existence of a deep fluid source and a permeable pathway through the crust. Our resistivity model images a conductive pathway, which penetrates the entire crust, in agreement with the geochemical interpretation. However, the resistivity model also shows that the upper crustal branch of the high-conductivity zone is located NE of the seismically defined SAF, suggesting that the SAF does not itself act as a major fluid pathway. This interpretation is supported by both, the location of the upper crustal high-conductivity zone and recent studies within the SAFOD main hole, which indicate that pore pressures within the core of the SAF zone are not anomalously high, that mantle-derived fluids are minor constituents to the fault-zone fluid composition and that both the volume of mantle fluids and the fluid pressure increase to the NE of the SAF. We further infer from the MT model that the resistive Salinian block basement to the SW of the SAFOD represents an isolated body, being 5-8km wide and reaching to depths >7km, in agreement with aeromagnetic data. This body is separated from a massive block of Salinian crust farther to the SW. The NE terminus of resistive Salinian crust has a spatial relationship with a near-vertical zone of increased seismic reflectivity ∼15km SW of the SAF and likely represents a deep-reaching fault zone. © 2008 The Authors Journal compilation © 2008 RAS. 2008 English 0956540X 10.1111/j.1365-246X.2008.03754.x Geophysical Journal International 173 718-732 GeoForschungsZentrum Potsdam, Geophysical Deep Sounding, Telegrafenberg, 14473 Potsdam, Germany; University of California Riverside, Department of Earth Sciences, 2207 Geology, Riverside, CA 92521, United States; US Geological Survey, MS 964, Box 25046, Denver, CO 80225, United States; University Potsdam, Department of Geosciences, Karl-Liebknecht-Strasse 24, 14476 Potsdam, Germany 2 aeromagnetic survey; crustal structure; data interpretation; data inversion; electrical conductivity; electrical resistivity; lower crust; magnetotelluric method; San Andreas Fault; seismic reflection; transform fault; two-dimensional modeling, California; North America; Parkfield; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-42349098421&doi=10.1111%2fj.1365-246X.2008.03754.x&partnerID=40&md5=bf67f788fee28c56e3fd52836b243ef3 cited By 74 M. Becken O. Ritter S.K. Park P.A. Bedrosian U. Weckmann M. Weber article Solum200764 2007 English 18168957 10.2204/iodp.sd.s01.34.2007 Scientific Drilling 64-67 U.S. Geological Survey, Earthquake Hazards Team, 345 Middlefield Road, Mail Stop 977, Menlo Park, Calif., United States; Department of Geosciences, Stony Brook University, 255 Earth and Space Sciences Building (ESS), Stony Brook N.Y. 11794-2100, United States; Department of Geology, Utah State University, Logan, Utah 84322-4505, United States; Department of Earth and Atmospheric Sciences, Saint Louis University, Verhaegen Hall, 3634 Lindell Boulevard, Suite 117, St. Louis, Mo. 63103, United States; Department of Geology and Geophysics, Texas A and M University, College Station, Texas 77843-3115, United States; Department of Geology and Geophysics, M.T. Halbouty Building, Texas A and M University, College Station, Texas 77843-3115, United States; Department of Geological Sciences, University of Michigan, 4534B C.C. Little Building, 1100 North University Avenue, Ann Arbor, Mich. 48109-1005, United States; Department of Geological Sciences, University of Michigan, 2534 C.C. Little Building, 1100 North University Avenue, Ann Arbor, Mich. 48109-1005, United States; Department of Geology, Utah State University, Logan, Utah 84322-4505, United States 1 SUPPL. 1 https://www.scopus.com/inward/record.uri?eid=2-s2.0-72049094408&doi=10.2204%2fiodp.sd.s01.34.2007&partnerID=40&md5=229ee14ef0408006486dcec0541fe7f7 cited By 11 J.G. Solum S. Hickman D.A. Lockner S. Tembe J.P. Evans S.D. Draper D.C. Barton D.L. Kirschner J.S. Chester F.M. Chester B.A. Pluijm A.M. Schleicher D.E. Moore C. Morrow K. Bradbury W.M. Calvin T.-F. Wong article Morrow2007 A technique is presented for estimating frictional strength of narrow shear zones based on hand selection of drillhole cuttings separates. Tests were conducted on cuttings from the SAFOD scientific drillhole near Parkfield, California. Since cuttings are mixed with adjacent material as they travel up the drillhole, these fault-derived separates give a better representation of the frictional properties of narrow features than measurements from the bulk material alone. Cuttings from two shear zones (one an active trace of the San Andreas fault) contain a significant weight percent of clay-rich grains that exhibit deformation-induced slickensides. In addition, cuttings from the active SAF trace contain around 1% serpentine. Coefficients of friction for clay-rich and serpentine grains were 0.3-0.5 and 0.4-0.45, respectively. These values are around 0.12 lower than the friction coefficient of the corresponding bulk cuttings, providing an improved estimate of the frictional strength of the San Andreas fault. Copyright 2007 by the American Geophysical Union. 2007 English 00948276 10.1029/2007GL029665 Geophysical Research Letters 34 U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, United States; State University of New York at Stony Brook, Stony Brook, NY 11794, United States; San Houston State University, Huntsville, TX 77340, United States 11 Drilling; Friction; Shear strength, Friction coefficient; Frictional strength; Shear zone, Tectonics, deformation; estimation method; San Andreas Fault; serpentine; shear zone; strength, California; North America; Parkfield; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-34547972644&doi=10.1029%2f2007GL029665&partnerID=40&md5=47dcbe8f1a215c19b5d28ba5b9e4f68d cited By 30 C. Morrow J. Solum S. Tembe D. Lockner T.-F. Wong article Zhang2007 We developed a three-dimensional (3D) shear-wave splitting tomography method to image the spatial anisotropy distribution by back projecting shear wave splitting delay times along ray paths derived from a 3D shear velocity model, assuming the delay times are accumulated along the ray paths. The local strength of the anisotropy is indicated by a parameter of anisotropy percentage, K. Using the shearwave splitting delay times for 575 earthquakes measured at PASO and HRSN stations, we imaged a detailed 3D anisotropy percentage model around the San Andreas Fault Observatory at Depth (SAFOD). The anisotropy percentage model shows strong heterogeneities, consistent with the strong spatial variations in both measured delay times and fast polarization directions. The San Andreas Fault (SAF) zone is highly anisotropic down to a depth of ∼4 km and then becomes less anisotropic at greater depths. Outside the fault zone, the highly anisotropic zone extends as deep as ∼7 km, consistent with the systematic depth dependence of the average time delays. To the southwest of the SAF, the Salinian granitic block shows relatively strong anisotropic: anomalies that are presumably caused by aligned microcracks consistent with the direction of the regional maximum compressive horizontal stress. To the northeast of the fault zone, a strong anisotropic anomaly between depths ∼2 and ∼4 km corresponds to a serpentinite body sandwiched between Franciscan rocks. Copyright 2007 by the American Geophysical Union. 2007 English 00948276 10.1029/2007GL031951 Geophysical Research Letters 34 Department of Geology and Geophysics, University of Wisconsin-Madison, 1215 W. Dayton Street, Madison, WI 53706, United States; Department of Earth Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, United States; Department of Earth and Environment Sciences, Rensselaer Polytechnic Institute, Troy, NY 12180, United States 24 Anisotropy; Compressive stress; Mathematical models; Microcracks; Shear waves; Three dimensional; Tomography; Velocity, Shear wave splitting delay; Shear-wave splitting tomography; Spatial anisotropy distribution, Seismic waves, anisotropy; fault zone; imaging method; microcrack; polarization; ray tracing; S-wave; San Andreas Fault; seismic tomography; spatial variation; wave splitting, California; North America; Parkfield; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-39549117811&doi=10.1029%2f2007GL031951&partnerID=40&md5=cd3a14bb37b5ffe0e27a4e8bc93e4d3c cited By 39 H. Zhang Y. Liu C. Thurber S. Roecker article Moore2007795 The section of the San Andreas fault located between Cholame Valley and San Juan Bautista in central California creeps at a rate as high as 28 mm yr -1 (ref. 1), and it is also the segment that yields the best evidence for being a weak fault embedded in a strong crust. Serpentinized ultramafic rocks have been associated with creeping faults in central and northern California, and serpentinite is commonly invoked as the cause of the creep and the low strength of this section of the San Andreas fault. However, the frictional strengths of serpentine minerals are too high to satisfy the limitations on fault strength, and these minerals also have the potential for unstable slip under some conditions. Here we report the discovery of talc in cuttings of serpentinite collected from the probable active trace of the San Andreas fault that was intersected during drilling of the San Andreas Fault Observatory at Depth (SAFOD) main hole in 2005. We infer that the talc is forming as a result of the reaction of serpentine minerals with silica-saturated hydrothermal fluids that migrate up the fault zone, and the talc commonly occurs in sheared serpentinite. This discovery is significant, as the frictional strength of talc at elevated temperatures is sufficiently low to meet the constraints on the shear strength of the fault, and its inherently stable sliding behaviour is consistent with fault creep. Talc may therefore provide the connection between serpentinite and creep in the San Andreas fault, if shear at depth can become localized along a talc-rich principal-slip surface within serpentinite entrained in the fault zone. ©2007 Nature Publishing Group. 2007 English 00280836 10.1038/nature06064 Nature 448 Nature Publishing Group 795-797 US Geological Survey, Mail Stop 977, 345 Middlefield Road, Menlo Park, CA 94025, United States 7155 mineral; serpentine; talc, creep; fault slip; fault zone; hydrothermal fluid; San Andreas Fault; serpentinite; shear strength; sliding; talc; ultramafic rock, article; chemical reaction; friction; gravity; priority journal; rock; temperature, California; Cholame Valley; North America; San Juan Bautista; San Luis Obispo County; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-34547957163&doi=10.1038%2fnature06064&partnerID=40&md5=767bc8f06bee7c3c3f410fd0cf989c1c cited By 325 D.E. Moore M.J. Rymer article Bleibinhaus2007 A seismic reflection and refraction survey across the San Andreas Fault (SAF) near Parkfield provides a detailed characterization of crustal structure across the location of the San Andreas Fault Observatory at Depth (SAFOD). Steep-dip prestack migration and frequency domain acoustic waveform tomography were applied to obtain highly resolved images of the upper 5 km of the crust for 15 km on either side of the SAF. The resulting velocity model constrains the top of the Salinian granite with great detail. Steep-dip reflection seismic images show several strong-amplitude vertical reflectors in the uppermost crust near SAFOD that define an ∼2-km-wide zone comprising the main SAF and two or more local faults. Another prominent subvertical reflector at 2-4 km depth ∼9 km to the northeast of the SAF marks the boundary between the Franciscan terrane and the Great Valley Sequence. A deep seismic section of low resolution shows several reflectors in the Salinian crust west of the SAF. Two horizontal reflectors around 10 km depth correlate with strains of seismicity observed along-strike of the SAF. They represent midcrustal shear zones partially decoupling the ductile lower crust from the brittle upper crust. The deepest reflections from ∼25 km depth are interpreted as crust-mantle boundary. Copyright 2007 by the American Geophysical Union. 2007 English 21699313 10.1029/2006JB004611 Journal of Geophysical Research: Solid Earth 112 Blackwell Publishing Ltd Department of Geosciences, Virginia Tech., 4044 Derring Hall, Blacksburg, VA 24061, United States; GeoForschungsZentrum, Telegrafenberg E322, Potsdam 14471, Germany; U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025, United States; Earth Resources Laboratory, Massachusetts Institute of Technology, Cambridge, MA, United States 6 crust-mantle boundary; crustal structure; prestack migration; San Andreas Fault; seismic reflection; seismic refraction; seismic tomography; seismic wave; waveform analysis, California; North America; Parkfield; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-34548380833&doi=10.1029%2f2006JB004611&partnerID=40&md5=dff59c55e1ec756dd8c9e8df12847552 cited By 106 F. Bleibinhaus J.A. Hole T. Ryberg G.S. Fuis article Hickman200729 2007 English 18168957 10.2204/iodp.sd.s01.39.2007 Scientific Drilling 29-32 U.S. GeologicalSurvey, 345 Middlefield Road, Mail Stop 977, Menlo Park, Calif., 94025, United States; Stanford University, Mitchell Building, Stanford, Calif., 94305-2215, United States; Chevron Energy Technology Company, 6001 Bollinger Canyon Road, San Ramon, Calif., 94583, United States; Duke University, 109A Old Chem, Box 90227, Durham, N.C., 27708, United States; Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Troy, N.Y., 12180, United States; Department of Geology and Geophysics, University of Wisconsin, 1215 West Dayton Street, Madison, Wis., 53706, United States 1 SUPPL. 1 https://www.scopus.com/inward/record.uri?eid=2-s2.0-51449106268&doi=10.2204%2fiodp.sd.s01.39.2007&partnerID=40&md5=6b9e15e33208e84fa895397387ed61e6 cited By 31 S. Hickman M. Zoback W. Ellsworth N. Boness P. Malin S. Roecker C. Thurber article Ellsworth200784 2007 English 18168957 10.2204/iodp.sd.s01.04.2007 Scientific Drilling 84-87 U.S. Geological Survey (USGS), 3A-109, 345 Middlefield Road Mail Stop 977, Menlo Park, Calif. 94025, United States; Old Chemistry, Duke University, Box 90227, Durham, N.C. 27708, United States; Geological Survey of Japan, Institute of Geology and Geoinformation (AIST), Tsukuba Central 7, 1-1-1 Higashi, Japan; Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Science Center 1S06, Troy, N.Y. 12180, United States; Berkeley Seismological Lab, University of California Berkeley, 207 McCone Hall, Calif. 94720-4760, United States; Norwegian Seismic Array (NORSAR), P.O. Box 53, N-2027 Kjeller, Norway; Department of Geology and Geophysics, University of Wisconsin, 1215 West Dayton Street, Madison, Wis. 53706, United States; LDEO-Seismology Geology and Tectonophysics, 210 Seismology, 61 Route 9W - P.O. Box 1000, Palisades, N.Y. 10964, United States; Energy Technology Centers (ETC), San Ramon, Calif. 94583, United States; USGS, 345 Middlefield Road, Menlo Park, Calif. 94025, United States; Department of Geophysics, Stanford University, Mitchell Bldg., Stanford, Calif. 94305, United States 1 SUPPL. 1 https://www.scopus.com/inward/record.uri?eid=2-s2.0-72049098074&doi=10.2204%2fiodp.sd.s01.04.2007&partnerID=40&md5=9a2477b3c6dc316811d75fa8f600f1ad cited By 13 W.L. Ellsworth K. Imanishi R. Nadeau V. Oye F. Waldhauser N.L. Boness S.H. Hickman M.D. Zoback article Tobin20075 2007 English 18168957 10.2204/iodp.sd.s01.80.2007 Scientific Drilling 5-16 Department of Geology and Geophysics, University of Wisconsin-Madison, 1215 West Dayton Street, Madison, Wis. 53706, United States; Center for Deep Earth Exploration (CDEX), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 3173-25 Showa-machi, Kanazawa-ku, Yokohama, Kanagawa, 236-0001, Japan; Marine Geodynamics, IFM-GEOMAR, Wischhofstr. 1-3, 24148, Kiel, Germany; U.S. Geological Survey, MS977, 345 Middlefield Road, Menlo Park, Calif., 94025, United States; Department of Earth and Planetary Sciences, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan 1 SUPPL. 1 https://www.scopus.com/inward/record.uri?eid=2-s2.0-77954510216&doi=10.2204%2fiodp.sd.s01.80.2007&partnerID=40&md5=4ae2928358e732e747c19e3ebbec6a3a cited By 6 H. Tobin H. Ito J. Behrmann S. Hickman G. Kimura article Dreger2007 We investigate the rupture process of a sequence of repeating Mw 2.1 earthquakes on the San Andreas Fault in Parkfield spanning the occurrence of the September 28, 2004 mainshock by inverting seismic moment rate functions obtained from empirical Green's function deconvolution. The results show that these events have extremely concentrated slip patches with radii on the order of 10-20 m, with peak slip between 8.4 and 11.4 cm. The rupture speed and rise time are consistent with values of larger earthquakes. The spatial distribution of stress drop for the events shows low average values 2.5-5.6 MPa and very large peak values of 66.7-93.9 MPa. The results show that strong asperities can exist at small scales on an otherwise weak fault, and helps reconcile differences between traditional spectra-based and tectonic loading methods for determining the stress drop of small repeating earthquakes. Copyright 2007 by the American Geophysical Union. 2007 English 00948276 10.1029/2007GL031353 Geophysical Research Letters 34 Berkeley Seismological Laboratory, University of California, Berkeley, CA 94720, United States 23 Mathematical models; Seismic response; Seismographs; Seismology; Tectonics, Rupture process; Rupture speed; Seismic moment rate; Tectonic loading, Earthquakes, deconvolution; earthquake magnitude; earthquake rupture; Green function; San Andreas Fault; seismic moment; spatial distribution https://www.scopus.com/inward/record.uri?eid=2-s2.0-39049102445&doi=10.1029%2f2007GL031353&partnerID=40&md5=e740b15fa23706c9209baaeff19bc38d cited By 83 D. Dreger R.M. Nadeau A. Chung article dAlessio2007 A suite of new techniques in thermochronometry allow analysis of the thermal history of a sample over a broad range of temperature sensitivities. New analysis tools must be developed that fully and formally integrate these techniques, allowing a single geologic interpretation of the rate and timing of exhumation and burial events consistent with all data. We integrate a thermal model of burial and exhumation, (U-Th)/He age modeling, and fission track age and length modeling. We then use a genetic algorithm to efficiently explore possible time-exhumation histories of a vertical sample profile (such as a borehole), simultaneously solving for exhumation and burial rates as well as changes in background heat flow. We formally combine all data in a rigorous statistical fashion. By parameterizing the model in terms of exhumation rather than time-temperature paths (as traditionally done in fission track modeling), we can ensure that exhumation histories result in a sedimentary basin whose thickness is consistent with the observed basin, a physically based constraint that eliminates otherwise acceptable thermal histories. We apply the technique to heat flow and thermochronometry data from the 2.1 -km-deep San Andreas Fault Observatory at Depth pilot hole near the San Andreas fault, California. We find that the site experienced <1 km of exhumation or burial since the onset of San Andreas fault activity ∼30 Ma. 2007 English 21699313 10.1029/2006JB004725 Journal of Geophysical Research: Solid Earth 112 Blackwell Publishing Ltd U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, United States; Earthquake Research Institute, University of Tokyo, Tokyo, Japan 8 burial (geology); exhumation; genetic algorithm; heat flow; numerical model; parameterization; thermochronology, California; North America; San Andreas; United States, Calluna vulgaris https://www.scopus.com/inward/record.uri?eid=2-s2.0-35348873786&doi=10.1029%2f2006JB004725&partnerID=40&md5=0ac9b107aef43b7f6e76ab5db1a1074e cited By 8 M.A. d'Alessio C.F. Williams article Bradbury2007299 We examine drill cuttings from the San Andreas Fault Observatory at Depth (SAFOD) boreholes to determine the lithology and deformational textures in the fault zones and host rocks. Cutting samples represent the lithologies from 1.7-km map distance and 3.2-km vertical depth adjacent to the San Andreas Fault. We analyzed two hundred and sixty-six grain-mount thin-sections at an average of 30-m-cuttings sample spacing from the vertical 2.2-km-deep Pilot Hole and the 3.99-km-long Main Hole. We identify Quaternary and Tertiary(?) sedimentary rocks in the upper 700 m of the holes; granitic rocks from 760-1920 m measured depth; arkosic and lithic arenites, interbedded with siltstone sequences, from 1920 to ~3150 m measured depth; and interbedded siltstones, mudstones, and shales from 3150 m to 3987 m measured depth. We also infer the presence of at least five fault zones, which include regions of damage zone and fault core on the basis of percent of cataclasite abundances, presence of deformed grains, and presence of alteration phases at 1050, 1600-2000, 2200-2500, 2700-3000, 3050-3350, and 3500 m measured depth in the Main Hole. These zones are correlated with borehole geophysical signatures that are consistent with the presence of faults. If the deeper zones of cataclasite and alteration intensity connect to the surface trace of the San Andreas Fault, then this fault zone dips 80-85° southwest, and consists of multiple slip surfaces in a damage zone ~250-300 m thick. This interpretation is supported by borehole geophysical studies, which show this area is a region of low seismic velocities, reduced resistivity, and variable porosity. 2007 English 1553040X 10.1130/GES00076.1 Geosphere 3 299-318 Utah State University, Department of Geology, 4505 Old Main Hill, Logan, UT 84322-4505, United States; U.S. Geological Survey, Earthquake Hazards Team, 345 Middlefield Road, MS 977, Menlo Park, CA 94025, United States; Baker Hughes Inteq, 2001 Rankin Road, Houston, TX 77267-0968, United States; Chevron International Exploration and Production, 1500 Louisiana Street, Houston, TX 77002, United States; Shell International Exploration and Production, Inc., Bellaire Technology Center, 3737 Bellaire Blvd, Houston, TX 77025, United States 5 borehole; cutting; deformation mechanism; drilling; fault zone; geophysical method; granite; mineralogy; numerical model; sedimentary rock; seismic velocity; slip; texture, California; San Andreas; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-57849149673&doi=10.1130%2fGES00076.1&partnerID=40&md5=101adf48b66598a4e7034d858b7fe32b cited By 54 K.K. Bradbury D.C. Barton J.G. Solum S.D. Draper J.P. Evans article Li200773 2007 English 18168957 10.2204/iodp.sd.s01.09.2007 Scientific Drilling 73-77 Department of Earth Sciences, University of Southern California, 3651 Trousdale Parkway, Los Angeles, Calif. 90089, United States; Division of Earth and Ocean Sciences, Duke University, 109A Old Chemistry Box 90227, Durham, N.C., 27708, United States; Department of Earth and Space Sciences, University of California, 595 Charles Young Drive East, Box 951567, Los Angeles, Calif., 90095-1567, United States 1 SUPPL. 1 https://www.scopus.com/inward/record.uri?eid=2-s2.0-72749125134&doi=10.2204%2fiodp.sd.s01.09.2007&partnerID=40&md5=dae4719a0410c9327776ee16441779e1 cited By 4 Y.-G. Li J.E. Vidal article Schleicher200768 2007 English 18168957 10.2204/iodp.sd.s01.33.2007 Scientific Drilling 68-70 Universität Würzburg, Geologisches Institut, Pleicherwall 1, 97070 Würzburg, Germany; University of Michigan, Department of Geological Sciences, 1100 University Avenue, C.C.Little Building, Ann Arbor, Mich. 48109, United States; Centre de Géochimie de la Surface (CNRS-ULP), 1 rue Blessig, 67084 Strasbourg, France; U.S. Geological Survey, Earthquake Hazards Team, 345 Middlefield Road, MS 977 Menlo Park, Calif. 94025, United States 1 SUPPL. 1 https://www.scopus.com/inward/record.uri?eid=2-s2.0-78651579355&doi=10.2204%2fiodp.sd.s01.33.2007&partnerID=40&md5=57e75aaf81c4a0d33226278cf47ea6dc cited By 2 A.M. Schleicher B.A. Pluijm L.N. Warr J.G. Solum article Vasconcelos20072175 The San Andreas Fault Observatory at Depth provides the most comprehensive set of data on the structure and dynamics of the San Andreas fault. We use two independent experiments recorded by the seismometer arrays of the SAFOD Pilot and Main Holes to resolve the localized structure of the San Andreas fault zone and of an intermediate fault zone at depth. From Pilot Hole recordings of the drilling noise coming from the Main Hole, we reconstruct the waves that propagate between the Pilot Hole sensors and use them to image the fault zone structure. The use of correlated drilling noise leads to a high-resolution image of a major transform fault zone. Another independent image is generated from the detonation of a surface explosive charge recorded at a large 178-sensor array placed in the Main Hole. The images reveal the San Andreas fault as well as an active blind fault zone that could potentially rupture. This is confirmed by two independent methods. The structure and the activity of the imaged faults is of critical importance in understanding the current stress state and activity of the San Andreas fault system. © 2007 Society of Exploration Geophysicists. 2007 English 10523812 10.1190/1.2792918 SEG Technical Program Expanded Abstracts 26 Society of Exploration Geophysicists 2175-2179 Center for Wave Phenomena, Colorado School of Mines, Golden, CO 80401, United States; Earth and Ocean Sciences Division, Duke University, Durham, NC 27708, United States; Paulsson Geophysical Services, Inc., Brea, CA 92821, United States 1 https://www.scopus.com/inward/record.uri?eid=2-s2.0-37549020744&doi=10.1190%2f1.2792918&partnerID=40&md5=d1037bc652a9bcf1ed611207c07c278b cited By 3 I. Vasconcelos S.T. Taylor R. Snieder J.A. Chavarria P. Sava P. Malin article Wiersberg2007 We have analyzed noble gas isotopes in 19 mud gas samples from 116-3943 m borehole depth of the San Andreas Fault Observatory at Depth (SAFOD) Main Hole in the context of origin and spatial variability of fluids occurring at seismogenic depths. The measured 3He/4He ratios range between 0.40 and 1.02 Ra (Ra is the atmospheric 3He/4He ratio of 1.39 × 10-6), with 4He/20Ne values between 0.33 and 4.92, revealing a mixture of three components to the total helium: (1) atmospheric helium, (2) helium with a crustal signature, and (3) mantle-derived helium. The air-corrected 3He/4He ratios fall between 0.2 Ra and 0.9 Ra. Samples from the 2117-3196 m depth show a relatively constant helium isotope composition (0.35-0.46 Ra), indicating that ∼5% of the helium in this section the Pacific Plate is derived from the mantle. The contribution of mantle-derived helium increases slightly in the transition from the Pacific Plate to the North American Plate and reaches maximal values of ∼12% on the North American Plate (below ∼3500 m borehole depth). On the basis of our observations, we suggest that the San Andreas Fault plays a role for fluid flux from greater depths, but higher amounts of mantle-derived fluids rise up through other, more permeable faults, situated on the North American Plate of the San Andreas Fault Zone (SAFZ). Lateral fluid dispersion at shallow depths through permeable country rock of the North American Plate may explain the observed increase in 3He/ 4He ratios with increasing distance to the SAF. Copyright 2007 by the American Geophysical Union. 2007 English 15252027 10.1029/2006GC001388 Geochemistry, Geophysics, Geosystems 8 GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany 1 https://www.scopus.com/inward/record.uri?eid=2-s2.0-42349097184&doi=10.1029%2f2006GC001388&partnerID=40&md5=65fd6214b021945c13944c12aba6ed89 cited By 46 T. Wiersberg J. Erzinger article Schleicher2006 The clay mineralogy and texture of rock fragments from the SAFOD borehole at 3067 m and 3436 m measured depth (MD) was investigated by electron microscopy (SEM, TEM) and X-ray-diffraction (XRD). The washed and ultrasonically cleaned samples show slickenfiber striations and thin films of Ca-K bearing smectite that are formed on polished fault surfaces, along freshly opened fractures and within adjacent mineralized veins. The cation composition and hydration behavior of these films differ from the Namontmorillonite of the fresh bentonite drilling mud, although there is more similarity with circulated mud recovered from 3479 m MD. We propose that these thin film smectite precipitates formed by natural nucleation and crystal growth during fault creep, probably associated with the shallow circulation of low temperature aqueous fluids along this shallow portion of the San Andreas Fault. Copyright 2006 by the American Geophysical Union. 2006 English 00948276 10.1029/2006GL026505 Geophysical Research Letters 33 American Geophysical Union Department of Geological Sciences, University of Michigan, Ann Arbor, MI, United States; Universität Würzburg, Geologisches Institut, Würzburg, Germany; Earthquake Hazards Team, U.S. Geological Survey, Menlo Park, CA, United States; Centre de Géochimie de la Surface, Université Louis Pasteur, CNRS, Strasbourg, France; Department of Geological Sciences, University of Michigan, 4534b C. C. Little Building, 1100 N. University Ave., Ann Arbor, MI 48109-1005, United States; Earthquake Hazards Team, U.S. Geological Survey, MS 977, 345 Middlefield Road, Menlo Park, CA 94025, United States; Centre de Géochimie de la Surface, Université Louis Pasteur, CNRS, 1 rue Blessig, F-67084 Strasbourg, France 16 Clay; Crystal growth; Rocks; Scanning electron microscopy; Textures; Thin films; Transmission electron microscopy; X ray diffraction analysis, Aqueous fluids; Drilling mud; Fault creep; Slickenfiber striations, Boreholes, bentonite; borehole; calcium; cation; clay; crystal structure; hydration; mudstone; potassium; rock property; sampling; San Andreas Fault; scanning electron microscopy; smectite; texture; transmission electron microscopy; X-ray diffraction https://www.scopus.com/inward/record.uri?eid=2-s2.0-33845669469&doi=10.1029%2f2006GL026505&partnerID=40&md5=eaab60d782d571cc151be4548cea5a09 cited By 67 A.M. Schleicher J.G. Solum L.N. Warr article Hole2006 Refraction traveltimes from a 46-km long seismic survey across the San Andreas Fault were inverted to obtain two-dimensional velocity structure of the upper crust near the SAFOD drilling project. The model contains strong vertical and lateral velocity variations from <2 km/s to ∼6 km/s. The Salinian terrane west of the San Andreas Fault has much higher velocity than the Franciscan terrane east of the fault. Salinian basement deepens from 0.8 km subsurface at SAFOD to ∼2.5 km subsurface 20 km to the southwest. A strong reflection and subtle velocity contrast suggest a steeply dipping fault separating the Franciscan terrane from the Great Valley Sequence. A low-velocity wedge of Cenozoic sedimentary rocks lies immediately southwest of the San Andreas Fault. This body is bounded by a steep fault just northeast of SAFOD and approaches the depth of the shallowest earthquakes. Multiple active and inactive fault strands complicate structure near SAFOD. Copyright 2006 by the American Geophysical Union. 2006 English 00948276 10.1029/2005GL025194 Geophysical Research Letters 33 Department of Geosciences, Virginia Polytechnic Institute and State University, 4044 Derring Hall, Blacksburg, VA 24061, United States; GeoForschungsZentrum, Telegrafenberg E322, D-14471 Potsdam, Germany; U. S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025, United States; Department of Earth and Environmental Sciences, University of Munich, Theresienstr. 41, D-80333 Munich, Germany 7 Acoustic wave refraction; Earthquakes; Sedimentary rocks; Seismic prospecting; Seismic waves; Two dimensional; Velocity, San Andreas fault zone; Seismic refraction, Seismology, fault zone; San Andreas Fault; seismic refraction; seismic survey; travel time; upper crust https://www.scopus.com/inward/record.uri?eid=2-s2.0-33646833994&doi=10.1029%2f2005GL025194&partnerID=40&md5=034d6a648cf52cf2f4bd38269408e102 cited By 61 J.A. Hole T. Ryberg G.S. Fuis F. Bleibinhaus A.K. Sharma article Malin2006 Fault-guided waves reveal a low-velocity fault segment a few hundred meters southwest of the main strand of the San Andreas Fault (SAf) system. In 2004, the San Andreas Fault Observatory at Depth (SAFOD) Main Hole was drilled 2.5 km underground and 0.7 km west of the SAF surface trace. A 3-component, 4.5-Hz seismograph was installed near the bottom of this hole. This instrument recorded fault zone guided (Fg) waves originating from earthquakes along the main SAF ∼2 km north and 3 km south of the SAFOD site. This ∼5 km length corresponds to a distinctive low-velocity structure imaged in 2003 using microearthquakes recorded on the Pilot Hole array. Because this structure transmits Fg-waves from the main fault, it is probably connected to the main SAF and is most likely a major, unmapped fault. Copyright 2006 by the American Geophysical Union. 2006 English 00948276 10.1029/2006GL025973 Geophysical Research Letters 33 Nicholas School of the Environment and Earth Sciences, Division of Earth and Ocean Sciences, Duke University, Durham, NC, United States; Division of Earth and Ocean Sciences, Nicholas School of the Environment and Earth Sciences, Duke University, Durham, NC 27708-0235, United States 13 Conformal mapping; Drilling; Seismic waves; Seismographs; Tomography; Velocity measurement, Fault-guided wave mapping; Low-velocity fault; Surface trace, Structural geology, earthquake; fault; fault zone; P-wave; San Andreas Fault; seismograph; tomography, California; North America; San Andreas; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-33750074815&doi=10.1029%2f2006GL025973&partnerID=40&md5=1644d238d9d8e1a7746d1a5cfe075c3d cited By 19 P. Malin E. Shalev H. Balven C. Lewis-Kenedi article Imanishi200681 We estimate the source parameters of 34 microearthquakes at Parkfield, CA, ranging in size from M -0.2 to M 2.1, by analyzing seismograms recorded by the 32-level, 3-component seismic array installed in the SAFOD Pilot Hole. We succeeded in obtaining stable spectral ratios by stacking the ratios calculated from the moving windows taken along the record following the direct waves. These spectral ratios were modeled to determine seismic moments and corner frequencies assuming an omega-squared model. Static stress drops and apparent stresses of microearthquakes at Parkfield display moment-independent scaling in agreement with scaling laws reported for moderate and large earthquakes. It is likely that the dynamics of microearthquakes at Parkfield is macroscopically similar to that of larger tectonic earthquakes. © 2006 by the American Geophysical Union. 2006 English 9781118666272; 9780875904351 00658448 10.1029/170GM10 Geophysical Monograph Series 170 Blackwell Publishing Ltd McGarr A., Abercrombie R., Di Toro G., Kanamori H. 81-90 Geological Survey of Japan, AIST, Tsukuba, Japan; U. S. Geological Survey, Menlo Park, CA, United States Energy dissipation; Seismic waves; Seismology, Corner frequency; Large earthquakes; Micro-earthquakes; Moment independents; Scaling relationships; Source parameters; Spectral ratios; Tectonic earthquakes, Earthquakes https://www.scopus.com/inward/record.uri?eid=2-s2.0-85040180537&doi=10.1029%2f170GM10&partnerID=40&md5=5426edadeda0660d3b07654862e6920f cited By 69 K. Imanishi W.L. Ellsworth article Wilson200639 2006 English 00092347 10.1021/cen-v084n004.p039 Chemical and Engineering News 84 American Chemical Society 39-41 C and EN West Coast News Bureau, New Zealand 4 Flow of fluids; Pressure effects; Real time systems; Rocks; Tectonics, Chemical clue; Frictional change; San Andreas Fault; San Andreas Fault Observatory at Depth (SAFOD),, Earthquakes https://www.scopus.com/inward/record.uri?eid=2-s2.0-33746610246&doi=10.1021%2fcen-v084n004.p039&partnerID=40&md5=73a4f3de52cbc0f221733193f3904fa1 cited By 0 E.K. Wilson article Roecker2006189 The Parkfield Area Seismic Observatory (PASO) was a dense, telemetered seismic array that operated for nearly 2 years in a 15 km aperture centered on the San Andreas Fault Observatory at Depth (SAFOD) drill site. The main objective of this deployment was to refine the locations of earthquakes that will serve as potential targets for SAFOD drilling and in the process develop a high (for passive seismological techniques) resolution image of the fault zone structure. A challenging aspect of the analysis of this data set was the known existence of large (20-25%) contrasts in seismic wavespeed across the San Andreas Fault. The resultant distortion of raypaths could challenge the applicability of approximate ray tracing techniques. In order to test the sensitivity of our hypocenter locations and tomographic image to the particular ray tracing and inversion technique employed, we compare an initial determination of locations and structure developed using a coarse grid and an approximate ray tracer [Thurber, C., Roecker, S., Roberts, K., Gold, M., Powell, M.L., and Rittger, K., 2003. Earthquake locations and three-dimensional fault zone structure along the creeping section of the San Andreas fault near Parkfield, CA: Preparing for SAFOD, Geophys. Res. Lett., 30 3, 10.1029/2002GL016004.] with one derived from a relatively fine grid and an application of a finite difference algorithm [Hole, J.A., and Zelt, B.C., 1995. 3-D finite-difference reflection traveltimes, Geophys. J. Int., 121, 2, 427-434.]. In both cases, we inverted arrival-time data from about 686 local earthquakes and 23 shots simultaneously for earthquake locations and three-dimensional Vp and Vp/Vs structure. Included are data from an active source seismic experiment around the SAFOD site as well as from a vertical array of geophones installed in the 2-km-deep SAFOD pilot hole, drilled in summer 2002. Our results show that the main features of the original analysis are robust: hypocenters are located beneath the trace of the fault in the vicinity of the drill site and the positions of major contrasts in wavespeed are largely the same. At the same time, we determine that shear wave speeds in the upper 2 km of the fault zone are significantly lower than previously estimated, and our estimate of the depth of the main part of the seismogenic zone decreases in places by ∼ 100 m. Tests using "virtual earthquakes" (borehole receiver gathers of picks for surface shots) indicate that our event locations near the borehole currently are accurate to about a few tens of meters horizontally and vertically. © 2006 Elsevier B.V. All rights reserved. 2006 English 00401951 10.1016/j.tecto.2006.02.026 Tectonophysics 426 189-205 Rensselaer Polytechnic Institute, Troy, NY, United States; University of Wisconsin-Madison, Madison, WI, United States 1-2 data inversion; fault zone; ray tracing; seismic tomography; seismic zone; travel time, California; North America; Parkfield; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-33749523010&doi=10.1016%2fj.tecto.2006.02.026&partnerID=40&md5=96e8bd6570a6b0fb6ce570bbf699ad24 cited By 65 S. Roecker C. Thurber K. Roberts L. Powell article Mark:2006:1816-8957:32 SAFOD, the San Andreas Fault Observatory at Depth (Fig. 1), completed an important milestone in July 2005 by drilling through the San Andreas Fault at seismogenic depth. SAFOD is one of three major components of EarthScope, a U.S. National Science Foundation (NSF) initiative being conducted in collaboration with the U.S. Geological Survey (USGS). The International Continental Scientific DrillingProgram (ICDP) provides engineering and technical support for the project as well as online access to project data and information (https://www.icdp-online.org/sites/sanandreas/news/news1.html). In 2002, the ICDP, the NSF, and the USGS provided funding for a pilot hole project at the SAFOD site. Twenty scientifi c papers summarizing the results of the pilot hole project as well as pre-SAFOD site characterization studies were published in Geophysical Research Letters (Vol.31, Nos. 12 and 15, 2004). 2006 1816-8957 doi:10.2204/iodp.sd.2.07.2006 Scientific Drilling 2006 32-33 2 https://www.ingentaconnect.com/content/doaj/18168957/2006/00002006/00000002/art00007 Mark D. Zoback article Solum2006 Washed cuttings provide a continuous record of the rocks encountered during drilling of the main hole of the San Andreas Fault Observatory at Depth (SAFOD). Both protolith and fault rocks exhibit a wide variety of mineral assemblages that reflect variations in some combination of lithology, P-T conditions, deformation mechanisms, and fluid composition and abundance. Regions of distinct neomineralization bounded by faults may record alteration associated with fluid reservoirs confined by faults. In addition, both smectites occurring as mixed-layer phases and serpentine minerals are found in association with active strands of the San Andreas Fault that were intersected during drilling, although their rheological influence is not yet fully known. Faults containing these mineralogical phases are prime candidates for continuous coring during Phase 3 of SAFOD drilling in the summer of 2007. Copyright 2006 by the American Geophysical Union. 2006 English 00948276 10.1029/2006GL027285 Geophysical Research Letters 33 Earthquake Hazards Team, U.S. Geological Survey, Menlo Park, CA, United States; Department of Geological Sciences, University of Michigan, Ann Arbor, MI, United States; Department of Geology, Utah State University, Logan, UT, United States; Department of Geology, Utah State University, 4505 Old Main Hill, Logan, UT 84322-4505, United States; Earthquake Hazards Team, U.S. Geological Survey, MS 977, 345 Middlefield Road, Menlo Park, CA 94025, United States; Department of Geological Sciences, University of Michigan, Little Bldg., 1100 N. University Ave., 2534 C.C, Ann Arbor, MI 48109, United States 21 Deformation; Reservoirs (water); Rheology; Rock drilling; Rocks; Serpentine, Fluid reservoirs; Mineralogical phases; Smectites; Washed cuttings, Geophysics, drilling; fault; lithology; mineralogy; protolith; rock; summer, California; North America; San Andreas; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-34248522543&doi=10.1029%2f2006GL027285&partnerID=40&md5=f63de7ab4297b514d845f0aca9f87f76 cited By 97 J.G. Solum S.H. Hickman D.A. Lockner D.E. Moore A.M. Schleicher J.P. Evans article Tembe2006 We investigated the frictional properties of drill cuttings and core obtained from 1.85-3.1 km true vertical depth in the SAFOD scientific borehole in central California. Triaxial frictional sliding experiments were conducted on samples from primary lithologic traits and significant shear zones, including the inferred active trace of the San Andreas fault. The samples were deformed at room temperature under constant effective normal stresses of 10, 40, and 80 MPa with axial shortening rates of 0.01-1.0 μm s-1. The weakest samples were from shale, claystone, and siltstone units with friction coefficient μ = 0.4-0.55. Stronger samples were from quartzo-feldspathic rocks with μ ≥ 0.6. Materials tested from two shear, zones at 2560 and 3067 m measured depth had μ = 0.4-0.55 and velocity strengthening behavior consistent with fault creep at depths &lt;4 km. The coefficient of friction for bulk samples from the inferred trace of the San Andreas fault was ∼0.6. Copyright 2006 by the American Geophysical Union. 2006 English 00948276 10.1029/2006GL027626 Geophysical Research Letters 33 Department of Geosciences, State University of New York, Stony Brook, NY 11794-2100, United States; U.S. Geological Survey, 345 Middlefield Road MS/977, Menlo Park, CA 94025, United States; Department of Geography and Geosciences, Sam Houston State University, Huntsville, TX 77341, United States 23 Boreholes; Core samples; Friction; Lithology; Tectonics, Drill cuttings; Triaxial frictional sliding, Core analysis, borehole geophysics; creep; cutting; effective stress; fault; friction; San Andreas Fault; shear zone; sliding, California; North America; San Andreas; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-34547965515&doi=10.1029%2f2006GL027626&partnerID=40&md5=4e37484196b6515fd7a22a30a43333b7 cited By 45 S. Tembe D.A. Lockner J.G. Solum C.A. Morrow T.-F. Wong D.E. Moore article dAlessio2006 Heat flow measurements along much of the San Andreas fault (SAF) constrain the apparent coefficient of friction (μapp) of the fault to &lt;0.2, much lower than laboratory-derived friction values for most geologic materials. However, heat flow data are sparse near the creeping section of the SAF, a frictional "asperity" where the fault slips almost exclusively by aseismic creep. We test the hypothesis that the creeping section has a substantially higher or lower μ app than adjacent sections of the SAF. We use numerical models to explore the effects of faults with spatially and temporally heterogeneous frictional strength on the spatial distribution of surface heat flow. Heat flow from finite length asperities is uniformly lower than predicted by assuming an infinitely long fault. Over geologic time, lateral offset from strike-slip faulting produces heat flow patterns that are asymmetric across the fault and along strike. We explore a range of asperity sizes, slip rates, and displacement histories for comparing predicted spatial patterns of heat flow with existing measurements. Models with μapp ∼ 0.1 fit the data best. For most scenarios, heat flow anomalies from a frictional asperity with μapp &gt; 0.2 should be detectable even with the sparse existing observations, implying that μapp for the creeping section is as low as the surrounding SAF. Because the creeping section does not slip in large earthquakes, the mechanism controlling its weakness is not related to dynamic processes resulting from high slip rate earthquake ruptures. Copyright 2006 by the American Geophysical Union. 2006 English 21699313 10.1029/2005JB003780 Journal of Geophysical Research: Solid Earth 111 Blackwell Publishing Ltd U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, United States; Department of Earth and Planetary Science, University of California, Berkeley, CA 94720-4767, United States 5 creep; earthquake mechanism; earthquake rupture; flow pattern; friction; heat flow; San Andreas Fault https://www.scopus.com/inward/record.uri?eid=2-s2.0-33745739891&doi=10.1029%2f2005JB003780&partnerID=40&md5=bd8bbb5c807da52ab577a7ed3eed265c cited By 28 M.A. d'Alessio C.F. Williams R. Bürgmann article Cochran2006 Local seismic arrays were deployed at two locations along the San Andreas fault (SAF) near Parkfield, California, before and after the 2004 M 6.0 Parkfield earthquake. Using local earthquakes we determine the anisotropic field within 12 km of the main trace of the SAF at the two array locations separated by 12 km. The initial array, near the SAFOD site, was deployed for six weeks in October and November 2003, and the second array, located near the town of Parkfield, was deployed for 3 months following the 28 September 2004 M 6.0 Parkfield earthquake. We find the fast shear-wave polarization direction nearly fault-parallel (N40°W) for stations on the main fault trace and within 100 m to the southwest of the SAP at both array locations. These fault-parallel measurements span the 100- to 150-m-wide zone of pervasive cracking and damage interpreted from fault-zone-trapped waves associated with the main fault core (Li et al., 2004, 2006). Outside of this zone, the fast orientations are scattered with some preference for orientations near N10°E, roughly parallel to the regional maximum horizontal compressive stress direction (σh). In addition, fast directions are preferentially oriented parallel to a northern branch of the SAF recorded on stations in the 2004 Parkfield deployment. The measured anisotropy is likely due to a combination of stress-aligned microcracks away from the fault and shear fabric within the highly evolved fault core. The majority of our measurements are taken outside of the main fault core, and we estimate the density of microcracks from the measured delay times. Apparent crack densities are approximately 3%, with large scatter. The data suggest weak depth dependence to the measured delay times for source depths between 2 and 7 km. Below 7-km source depth, the delay times do not correlate with depth suggesting higher confining pressure is forcing the microcracks to close. No coseismic variation in the anisotropic parameters is observed, suggesting little to no influence on measured splitting due to the 2004 M 6.0 Parkfield earthquake. However, the premainshock and postmainshock data presented here are from arrays separated by 12 km, limiting our sensitivity to small temporal changes in anisotropy. 2006 English 00371106 10.1785/0120050804 Bulletin of the Seismological Society of America 96 S364-S375 Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California at San Diego, San Diego, CA 92093-0225, United States; Earth and Space Sciences Department, Institute of Geophysics and Space Physics, University of California, Los Angeles, CA 90095-1567, United States; Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740, United States 4 B Anisotropic parameters; Postmainshock data; Shallow crust; Shear-wave polarization, Anisotropy; Earthquake effects; Microcracks; Polarization; Shock waves; Compressive stress; Earthquakes; Seismic waves; Shear waves; Tectonics, Earthquakes; Seismology, anisotropy; crust; earthquake event; fault zone; microcrack; Parkfield earthquake 2004; S-wave; San Andreas Fault; trapped wave; coseismic process; earthquake; seismic anisotropy, California; North America; Parkfield; United States, Fast shear-wave polarization; Fault-parallel measurements; Fault-zone-rapped waves; San Andreas fault (SAF); Shallow crust https://www.scopus.com/inward/record.uri?eid=2-s2.0-33845787072&doi=10.1785%2f0120050804&partnerID=40&md5=bcf2e01418fd7f9833cb2fa9cf27c8f0 cited By 68 E.S. Cochran Y.-G. Li J.E. Vidale article Boness2006 We present an analysis of shear velocity anisotropy using data in and near the San Andreas Fault Observatory at Depth (SAFOD) to investigate the physical mechanisms controlling velocity anisotropy and the effects of frequency and scale. We analyze data from borehole dipole sonic logs and present the results from a shear-wave-splitting analysis performed on waveforms from microearthquakes recorded on a downhole seismic array. We show how seismic anisotropy is linked either to structures such as sedimentary bedding planes or to the state of stress, depending on the physical properties of the formation. For an arbitrarily oriented wellbore, we model the apparent fast direction that is measured with dipole sonic logs if the shear waves are polarized by arbitrarily dipping transversely isotropic (TI) structural planes (bedding/ fractures). Our results indicate that the contemporary state of stress is the dominant mechanism governing shear velocity anisotropy in both highly fractured granitic rocks and well-bedded arkosic sandstones. In contrast, within the finely laminated shales, anisotropy is a result of the structural alignment of clays along the sedimentary bedding planes. By analyzing shear velocity anisotropy at sonic wavelengths over scales of meters and at seismic frequencies over scales of several kilometers, we show that the polarization of the shear waves and the amount of anisotropy recorded are strongly dependent on the frequency and scale of investigation. The shear anisotropy data provide constraints on the orientation of the maximum horizontal compressive stress SHmax and suggest that, at a distance of only 200 m from the San Andreas fault (SAF), SHmax is at an angle of approximately 70° to the strike of the fault. This observation is consistent with the hypothesis that the SAF is a weak fault slipping at low levels of shear stress. © 2006 Society of Exploration Geophysicists. 2006 English 00168033 10.1190/1.2231107 Geophysics 71 Society of Exploration Geophysicists F131-F146 Stanford University, Stanford, CA 94305, United States; Chevron Energy Technology Company, 6001 Bollinger Canyon Road, San Ramon, CA 94583, United States; Stanford University, Department of Geophysics, Mitchell Building, Stanford, CA 94305, United States 5 Compressive stress; Earthquakes; Granite; Sandstone; Seismic waves; Shale; Shear waves, Seismic array; Seismic frequencies; Shear velocity anisotropy; Shear-wave-splitting analysis; Sonic wavelengths, Seismology, earthquake mechanism; faulting; microearthquake; S-wave; seismic anisotropy; seismic velocity; seismology; waveform analysis https://www.scopus.com/inward/record.uri?eid=2-s2.0-33748850138&doi=10.1190%2f1.2231107&partnerID=40&md5=782aa915abad4d994151296ae14d011a cited By 113 N.L. Boness M.D. Zoback article Rymer2006237 This field trip is along the central section of the San Andreas fault and consists of eight stops that illustrate surface evidence of faulting, in general, and features associated with active fault creep, in particular. Fault creep is slippage along a fault that occurs either in association with small-magnitude earthquakes or without any associated large-magnitude earthquakes. Another aspect of the trip is to highlight where there are multiple fault traces along this section of the San Andreas fault zone in order to gain a better understanding of plateboundary processes. The first stop is along the Calaveras fault, part of the San Andreas fault system, at a location where evidence of active fault creep is abundant and readily accessible. The stops that follow are along the San Andreas fault and at convenient locations to present and discuss rock types juxtaposed across the fault that have been transported tens to hundreds of kilometers by right-lateral motion along the San Andreas fault. Stops 6 and 7 are examples of recent studies of different aspects of the fault: drilling into the fault at the depth of repeating magnitude (M) 2 earthquakes with the San Andreas Fault Observatory at Depth (SAFOD) and the geological, geophysical, and seismological study of M 6 earthquakes near the town of Parkfield. Along with the eight official stops on this field trip are 12 "rolling stops" - sites of geologic interest that add to the understanding of features and processes in the creeping section of the fault. Many of the rolling stops are located where stopping is difficult to dangerous; some of these sites are not appropriate for large vehicles (buses) or groups; some sites are not appropriate for people at all. We include photographs of or from many of these sites to add to the reader's experience without adding too many stops or hazards to the trip. An extensive set of literature is available for those interested in the San Andreas fault or in the creeping section, in particular. For more scientifically oriented overviews of the fault, see Wallace (1990) and Irwin (1990); for a more generalized overview with abundant, colorful illustrations, see Collier (1999). Although the presence of small sections of the San Andreas fault was known before the great 1906 San Francisco earthquake, it was only after that event and subsequent geologic investigations reported in Lawson (1908) that showed the fault as a long structure, extending all the way from east of Los Angeles into northern California. Prentice (1999) described the importance of the 1908 "Lawson report" and how it pivotally influenced the understanding of the San Andreas. Hill (1981) presented a wonderful introduction to the evolution of thought on the San Andreas. Geologic maps and maps of the most recently active fault trace in the creeping section, or large parts of it, include those by Brown (1970), Dibblee (1971, 1980), and Wagner et al. (2002); detailed geologic maps are discussed at various stops in this guide. Various aspects of the creeping section of the San Andreas fault have been the focus of many geologic field trips in the past few decades. Guidebooks for some of those trips include those by Gribi (1963a, 1963b), Brabb et al. (1966), Rogers (1969), Bucknam and Haller (1989), Harden et al. (2001), and Stoffer (2005). The creeping section of the San Andreas fault zone lies between areas that experienced large-displacement surface breakage during great earthquakes in 1857 and 1906 (Fig. 1, inset). Burford and Harsh (1980) divided the creeping section into three segments: (1) a northwest section where the creep rate increases to the southeast in step-like increments, (2) a central section where the creep rate is relatively constant at a maximum value of ∼30 mm/yr (∼1.2 in/yr), and (3) a southeast section where the creep rate decreases to the southeast (Fig. 2). The rate of slip along the creeping section of the fault zone has been measured using creepmeters, alignment arrays, and laser distance-measuring devices. The aperture of measurements over which these measurements are made ranges from 10 m (∼33 ft) (creepmeters) to 100 m (∼330 ft) (alignment arrays) to kilometers and tens of kilometers (laser measuring devices). Creepmeter and alignment-array measurements are here termed "near- fault" measurements; laser measurements over distances of 1-2 km (∼0.6-1.2 mi) are termed "intermediate-scale" measurements; laser measurements over tens of kilometers (miles) are termed "broadscale" measurements. Comparisons among near-fault, intermediate-scale, and broadscale measurements and geologic maps show that the northwest part of the creeping section of the fault is composed of two narrow zones of active deformation, one along the San Andreas fault and one along the Calaveras-Paicines fault, whereas the central and southeast sections are both composed of a single relatively narrow zone of deformation. The southeast section is transitional to a locked zone southeast of Cholame; a locked fault is one that slips only in association with a moderate to large earthquake. Throughout the creeping section of the San Andreas fault zone, broadscale measurements generally indicate more deformation than near-fault and intermediate-scale measurements, which are in reasonably close agreement except at Monarch Peak (Mustang Ridge), near the center of the creeping section and our Stop 5 (Figs. 1 and 2). Features that we see on this trip include offset street curbs, closed depressions (sag ponds), fault scarps (steep slopes formed by movement along a fault), a split and displaced tree, offset fence lines, fresh fractures, and offset road lines (Fig. 3 is a sketch showing some of the landforms that represent deformation by an active fault). We also see evidence of long-term maturity of the San Andreas fault, as indicated by fault features and displaced rock types (Fig. 4). Finally, we will visit sites of ongoing research into the processes associated with earthquakes and their effects. Discussions include drilling into the San Andreas fault at the SAFOD drill site and the 2004 Parkfield earthquake and its effects and implications. © 2006 Geological Society of America. All rights reserved. 2006 English 10.1130/2006.1906SF(16) GSA Field Guides 7 Geological Society of America 237-272 U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, United States Creep; Curbs; Earthquakes; Infill drilling; Locks (fasteners); Observatories; Strike-slip faults, Creeping section; Fault structure; Hollister; Parkfield; SAFOD; San Andreas fault, Structural geology https://www.scopus.com/inward/record.uri?eid=2-s2.0-84946417267&doi=10.1130%2f2006.1906SF%2816%29&partnerID=40&md5=85a7796ca33da1b8ff433a7313ab6982 cited By 6 M.J. Rymer S.H. Hickman P.W. Stoffer article Boness2006825 We present shear velocity anisotropy data from crustal earthquakes in California and demonstrate that it is often possible to discriminate structural anisotropy (polarization of the shear waves along the fabric of major active faults) from stress-induced anisotropy (polarization parallel to the maximum horizontal compressive stress). Stress directions from seismic stations located near (but not on) the San Andreas fault indicate that the maximum horizontal compressive stress is at a high angle to the strike of the fault. In contrast, seismic stations located directly on one of the major faults indicate that shear deformation has significantly altered the elastic properties of the crust, inducing shearwave polarizations parallel to the fault plane. © 2006 Geological Society of America. 2006 English 00917613 10.1130/G22309.1 Geology 34 825-828 Department of Geophysics, Stanford University, Stanford, CA 94305, United States; Chevron, 6001 Bollinger Canyon Road, San Ramon, CA 94583, United States 10 Crustal stress; San Andreas fault; Seismic anisotropy, Anisotropy; Compressive stress; Earthquakes; Mapping; Seismic waves; Shear stress; Tectonics, Structural geology, active fault; deformation mechanism; earthquake; elastic property; fault plane; polarization; S-wave; San Andreas Fault; seismic anisotropy, California; North America; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-33750085990&doi=10.1130%2fG22309.1&partnerID=40&md5=4a28fcd66966b78cbc933c6db7ae10d3 cited By 144 N.L. Boness M.D. Zoback article Cohen200542 The San Andreas Fault Observatory at Depth (SAFOD) project to reveal various mysteries associated with an earthquake is discussed. By drilling directly into the fault, the researchers will be able to understand the causes of earthquakes. The results from SAFOD can help to refine the way scientists model earthquakes, and could even help determine the effective ways of predicting a quake's location, timing and size. All the activity is aimed at helping geophysicists understand the setting in which earthquakes develop and the factors that control them. 2005 English 02624079 New Scientist 185 42-45 2485 Cracks; Data acquisition; Global positioning system; Imaging techniques; Methane; Observatories; Oil well drilling; Project management; Seismic waves; Seismology; Tectonics, San Andreas Fault Observatory at Depth (SAFOD) project; Stanford University, California; US Geological Survey; US National Resource Council, Earthquakes https://www.scopus.com/inward/record.uri?eid=2-s2.0-13444271805&partnerID=40&md5=b8250b97a60e4530a64fd0f94ff5bce2 cited By 0 P. Cohen article Wilson2005749 Grain size reduction and gouge formation are found to be ubiquitous in brittle faults at all scales, and most slip along mature faults is observed to have been localized within gouge zones. This fine-grain gouge is thought to control earthquake instability, and thus understanding its properties is central to an understanding of the earthquake process. Here we show that gouge from the San Andreas fault, California, with ∼160 km slip, and the rupture zone of a recent earthquake in a South African mine with only ∼0.4 m slip, display similar characteristics, in that ultrafine grains approach the nanometre scale, gouge surface areas approach 80 m2g-1, and grain size distribution is nonfractal. These observations challenge the common perception that gouge texture is fractal and that gouge surface energy is a negligible contributor to the earthquake energy budget. We propose that the observed fine-grain gouge is not related to quasi-static cumulative slip, but is instead formed by dynamic rock pulverization during the propagation of a single earthquake. 2005 English 00280836 10.1038/nature03433 Nature 434 749-752 School of Geology and Geophysics, University of Oklahoma, Norman, OK 73019, United States; Department of Geological Sciences, University of Nevada, Reno, NV 89557, United States 7034 Comminution; Earthquakes; Fractals; Grain size and shape; Rocks, Earthquake instability; Energetics; Fine-grain gouge; Gouge formation, Particle size analysis, earthquake; particle size, article; earthquake; energy consumption; energy transfer; mining; particle size; priority journal; rock; surface property https://www.scopus.com/inward/record.uri?eid=2-s2.0-17644372729&doi=10.1038%2fnature03433&partnerID=40&md5=c40ab10403bf9473b969399d26366d2c cited By 234 B. Wilson T. Dewers Z. Reches J. Brune article Oye2005751 To identify and constrain the target zone for the planned SAFOD Main Hole through the San Andreas Fault (SAF) near Parkfield, California, a 32-level three-component (3C) geophone string was installed in the Pilot Hole (PH) to monitor and improve the locations of nearby earthquakes. The orientation of the 3C geophones is essential for this purpose, because ray directions from sources may be determined directly from the 3D particle motion for both P and S waves. Due to the complex local velocity structure, rays traced from explosions and earthquakes to the PH show strong ray bending. Observed azimuths are obtained from P-wave polarization analysis, and ray tracing provides theoretical estimates of the incoming wave field. The differences between the theoretical and the observed angles define the calibration azimuths. To investigate the process of orientation with respect to the assumed velocity model, we compare calibration azimuths derived from both a homogeneous and 3D velocity model. Uncertainties in the relative orientation between the geophone levels were also estimated for a cluster of 36 earthquakes that was not used in the orientation process. The comparison between the homogeneous and the 3D velocity model shows that there are only minor changes in these relative orientations. In contrast, the absolute orientations, with respect to global North, were significantly improved by application of the 3D model. The average data residual decreased from 13° to 7°, supporting the importance of an accurate velocity model. We explain the remaining residuals by methodological uncertainties and noise and with errors in the velocity model. 2005 English 00371106 10.1785/0120040130 Bulletin of the Seismological Society of America 95 751-758 NORSAR, 2027 Kjeller, Norway; U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA, United States 2 Calibration; Data reduction; Earthquakes; Installation, California, USA; Pilot Hole (PH); San Andreas Fault (SAF); Three-component (3C) geophones, Seismology, geophone; orientation; seismometry, California; North America; Parkfield; United States; Western Hemisphere; World https://www.scopus.com/inward/record.uri?eid=2-s2.0-20444395420&doi=10.1785%2f0120040130&partnerID=40&md5=29d5d82f99cf07746f5936f60db81585 cited By 14 V. Oye W.L. Ellsworth article Schorlemmer20051086 2005 English 00280836 10.1038/4341086a Nature 434 1086 Swiss Seismological Service, ETH Zürich, 8093 Zürich, Switzerland 7037 earthquake rupture, article; diagnostic accuracy; earthquake; forecasting; mathematical analysis; prediction; priority journal; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-20844433060&doi=10.1038%2f4341086a&partnerID=40&md5=3990fd47115302e70e8a80387b4145e6 cited By 154 D. Schorlemmer S. Wiemer article Taylor2005 Scientists recently used the drillbit seismic technique to image fractures and shear zones associated with the San Andreas fault near Parkfield, Calif. Low-level energy produced by the drillbit served as a downhole seismic source, and the wave response was measured by an array of geophones at the surface. In the complex structural environment underlying Parkfield, numerous faults that comprise a flower structure are interpreted to cut the Cenozoic sedimentary cover and the Salinian block at the San Andreas Fault Observatory at Depth (SAFOD) project drillsite. Based on updated surface geologic maps by Rymer, these fractures and faults appear to trend subparallel to the San Andreas fault (SAF) and have been mapped up to 3 km to the southwest of the main trace of the SAF. Our study included the interpretation of a surface seismic profile and the drillbit seismic data. These data indicate that faulting is more abundant and shearing more pervasive as one approaches the main trace of the San Andreas fault. In addition, our interpretation of the locations and dips of faults in the PSINE profile correlates very well with locations and dips of several linear features imaged in the migrated drillbit seismic data. 2005 English 00301388 Oil and Gas Journal 103 42-44+46-50 Duke University, Durham, NC, United States; WesternGeco, Houston, TX, United States; Schlumberger-Doll Research, Ridgefield, CT, United States 41 Fracture; Sedimentary rocks; Seismic waves; Seismology; Structural geology; Tectonics, Drillbit seismic technique; Geophones; Surface seismic profile, Seismic prospecting, fracture network; imaging method; San Andreas Fault; seismic method https://www.scopus.com/inward/record.uri?eid=2-s2.0-28144444601&partnerID=40&md5=5432d750490440a598764e7c32bdea94 cited By 0 S.T. Taylor C. Stolte J.B.U. Haldorsen R. Coates P. Malin E. Shalev article Rubinstein20051 We use the two target repeating earthquake sequences of SAFOD to identify time varying properties of the shallow crust in the Parkfield area at the surface and in shallow boreholes. At the surface, we find that the 2004 Parkfield earthquake caused direct S wave delays exceeding 7 ms, and S coda delays exceeding 15 ms. We attribute these delays to cracks formed or opened during the strong shaking of the Parkfield earthquake. Observations at depth show that the direct S wave arrival time was much less affected by the Parkfield earthquake. This provides evidence that damage caused by strong shaking (nonlinear strong ground motion), is limited to the very near surface (<100 m). Copyright 2005 by the American Geophysical Union. 2005 English 00948276 10.1029/2005GL023189 Geophysical Research Letters 32 1-5 Department of Geophysics, Stanford University, 397 Panama Mall, Stanford, CA 94305-22215, United States 14 Boreholes; Cracks; Earthquakes; Geologic models; Rock mechanics; Seismic waves; Seismology; Structural geology, Depth constraints; Earthquake sequences; Ground motion; Shallow crust, Tectonics, arrival time; damage; earthquake; ground motion; S-wave, California; North America; Parkfield; United States; Western Hemisphere; World https://www.scopus.com/inward/record.uri?eid=2-s2.0-25444513487&doi=10.1029%2f2005GL023189&partnerID=40&md5=2b8738fba293ed9b66342303a5982585 cited By 90 J.L. Rubinstein G.C. Beroza article Roecker2004 Taking advantage of large datasets of both gravity and elastic wave arrival time observations available for the Parkfield, California region, we generated an image consistent with both types of data. Among a variety of strategies, the best result was obtained from a simultaneous inversion with a stability requirement that encouraged the perturbed model to remain close to a starting model consisting of a best fit to the arrival time data. The preferred model looks essentially the same as the best-fit arrival time model in areas where ray coverage is dense, with differences being greatest at shallow depths and near the edges of the model where ray paths are few. Earthquake locations change by no more than about 100 m, the general effect being migration of the seismic zone to the northeast, closer to the surface trace of the San Andreas Fault. Copyright 2004 by the American Geophysical Union. 2004 English 00948276 10.1029/2003GL019396 Geophysical Research Letters 31 L12S041-4 Department of Earth/Environ. Sci., Rensselaer Polytechnic Institute, 110 8th Street, Troy, NY 12180-3590, United States; Department of Geology and Geophysics, University of Wisconsin-Madison, 1215 West Dayton St., Madison, WI 53706, United States; U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025, United States 12 Drilling; Earthquakes; Gravitational effects; Mathematical models; Perturbation techniques; Seismology; Site selection; Earthquakes; Strike-slip faults, Best-fit arrival time model; Datasets; Seismic zones, Elastic waves; Elastic waves, earthquake hypocenter; elastic wave; gravity survey; modeling; San Andreas Fault; seismic hazard, California; North America; Parkfield; United States, Arrival-time data; Earthquake location; Hypocenter location; Joint inversion; Large datasets; San Andreas fault; Shallow depths; Surface traces https://www.scopus.com/inward/record.uri?eid=2-s2.0-6044257846&doi=10.1029%2f2003GL019396&partnerID=40&md5=6901919967e545c52d4f6415199643d5 cited By 69 S. Roecker C. Thurber D. McPhee article Chavarria2004 In July 2002 we installed a vertical array of seismometers in the San Andreas Fault Observatory at Depth (SAFOD) Pilot Hole (PH). The bottom of this 32 level, 1240 m long array of 3- components is located at a depth of ∼2100 m below ground. Surface-explosion and microearthquake seismograms recorded by the array give valuable insights into the structure of the SAFOD site. The ratios of P- and S-wave velocities.(Vp/Vs) along the array suggest the presence of two faults intersecting the PH. The Vp/Vs ratios also depend on source-location, with high values to the NW, and lower ones to the SE, correlating with high and low creep rates along the SAF, respectively. Since higher ratios can be produced by increasing fluid saturation, we suggest that this effect might account for both our observations and their correlation with the creep distribution. Copyright 2004 by the American Geophysical Union. 2004 English 00948276 10.1029/2003GL019382 Geophysical Research Letters 31 L12S07 1-5 Division of Earth and Ocean Sciences, Duke University, Durham, NC 27708, United States 12 Correlation methods; Creep; Earthquakes; Geophysics; Observatories; Site selection; Wave propagation; Creep; Shear waves; Strike-slip faults; Wave propagation, Fluid saturation; Pilot hole (PH); Seismic arrays; Seismograms, Seismology; Seismology, earthquake; San Andreas Fault; seismic wave; seismometry; wave propagation, Creep distribution; Fluid saturations; P- and S-wave velocities; Propagation effect; San Andreas fault; Source location; Surface explosions; Vertical arrays https://www.scopus.com/inward/record.uri?eid=2-s2.0-6044266087&doi=10.1029%2f2003GL019382&partnerID=40&md5=8821e24c660fa234ccfd6ddc30b1a920 cited By 11 J.A. Chavarria E. Shalev article Boness2004 A comprehensive suite of geophysical logs was collected in the SAFOD Pilot Hole from a depth of 775 m to 2150 m in highly fractured Salinian granite. The Pilot Hole intersected numerous macroscopic fractures and faults with extremely varied orientations. Despite the highly variable orientation of the fractures and faults, the fast polarization direction of the shear waves is very consistent with the direction of maximum horizontal compression determined from wellbore breakouts and drilling induced tensile fractures. At least three major shear zones were intersected by the borehole that are characterized by anomalously low velocity and resistivity, anomalously high shear velocity anisotropy and an absence of stress-induced wellbore breakouts (which suggests anomalously low differential stress). We argue that the physical mechanism responsible for the seismic velocity anisotropy observed in the Pilot Hole is the preferential closure of fractures in response to an anisotropic stress state. Copyright 2004 by the American Geophysical Union. 2004 English 00948276 10.1029/2003GL019020 Geophysical Research Letters 31 L15S17 1-4 Department of Geophysics, Stanford University, Stanford, CA, United States 15 Drilling; Electromagnetic wave polarization; Fracture; Granite; Magnetic anisotropy; Seismic prospecting, Pilot holes; Shear velocity; Wellbores, Boreholes, borehole geophysics; in situ stress; physical property; San Andreas Fault; seismic anisotropy; seismic velocity, California; North America; Parkfield; United States; Western Hemisphere; World https://www.scopus.com/inward/record.uri?eid=2-s2.0-5144225952&doi=10.1029%2f2003GL019020&partnerID=40&md5=445ece89cf56d552cf799565ddc6a7d1 cited By 125 N.L. Boness M.D. Zoback article Hickman2004 Borehole breakouts and drilling-induced tensile fractures in the 2.2-km-deep SAFOD pilot hole at Parkfield, CA, indicate significant local variations in the direction of the maximum horizontal compressive stress, SHmax, but show a generalized increase in the angle between SHmax and the San Andreas Fault with depth. This angle ranges from a minimum of 25 ± 10° at 1000-1150 m to a maximum of 69 ± 14° at 2050-2200 m. The simultaneous occurrence of tensile fractures and borehole breakouts indicates a transitional strike-slip to reverse faulting stress regime with high horizontal differential stress, although there is considerable uncertainty in our estimates of horizontal stress magnitudes. If stress observations near the bottom of the pilot hole are representative of stresses acting at greater depth, then they are consistent with regional stress field indicators and an anomalously weak San Andreas Fault in an otherwise strong crust. Copyright 2004 by the American Geophysical Union. 2004 English 00948276 10.1029/2004GL020043 Geophysical Research Letters 31 L15S12 1-4 U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, United States; Department of Geophysics, Stanford University, Stanford, CA, United States 15 Compressive stress; Drilling; Fracture; Geophysical prospecting; Stress analysis, Borehole breakouts; Pilot holes; Stress fields; Stress orientations, Boreholes, borehole geophysics; in situ stress; orientation; San Andreas Fault; stress measurement, California; North America; Parkfield; United States; Western Hemisphere; World https://www.scopus.com/inward/record.uri?eid=2-s2.0-7044264976&doi=10.1029%2f2004GL020043&partnerID=40&md5=a9c00d9e3b3d52667207dd81f11084ab cited By 204 S. Hickman M. Zoback article Townend2004 Throughout central and southern California, a uniform NNE-SSW direction of maximum horizontal compressive stress is observed that is remarkably consistent with the superposition of stresses arising from lateral variations in lithospheric buoyancy in the western United States, and farfield Pacific-North America plate interaction. In central California, the axis of maximum horizontal compressive stress lies at a high angle to the San Andreas fault (SAF). Despite relatively few observations near (±10 km) the fault, observations in the greater San Francisco Bay area indicate an angle of as much as 85°, implying extremely low fault strength. In southern California, observations of stress orientations near the SAF are rotated slightly counter-clockwise with respect to the regional field. Nevertheless, we observe an approximately constant angle between the SAF and the maximum horizontal stress direction of 68 ± 7° along ∼400 km of the fault, indicating that the SAF has moderately low frictional strength in southern California. Copyright 2004 by the American Geophysical Union. 2004 English 00948276 10.1029/2003GL018918 Geophysical Research Letters 31 L15S11 1-5 School of Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand; Department of Geophysics, Stanford University, Stanford, CA 94305-2215, United States 15 Buoyancy; Compressive stress; Geophysics; Lithography; Stress analysis, Lithospheric buoyancy; Stress orientations; Tectonic stress, Tectonics, in situ stress; San Andreas Fault; stress field; tectonic setting https://www.scopus.com/inward/record.uri?eid=2-s2.0-7044280211&doi=10.1029%2f2003GL018918&partnerID=40&md5=ae3a7689f308a81f0a7799066a2840fd cited By 211 M.D. Zoback article Erzinger2004 The real-time mud gas monitoring during drilling of the 2.2 km deep SAFOD Pilot Hole has proved successful. A nearly complete depth profile was obtained for methane, radon, helium, and, with limitations, for carbon dioxide. Within the sedimentary section (0-768 m) mud gas yielded the highest CH4, CO2, and 222Rn concentrations, whereas He was comparably low. Four major gas-rich zones were identified. Methane is a mixture of microbial and thermogenic origin. Mud gas concentrations of CH4 and Rn are much lower in the underlying granitic rocks and the average background level of He is higher. In addition to several gas-rich zones, two shear zones inferred from geophysical measurements are also indicated by increased mud gas concentrations. Gas results permit speculation that one major shear zone is hydraulically connected to the sedimentary section above. Up to 9% of the helium is of mantle origin, which is probably a lower limit. Copyright 2004 by the American Geophysical Union. 2004 English 00948276 10.1029/2003GL019395 Geophysical Research Letters 31 L15S18 1-4 GeoForschungsZentrum Potsdam, Potsdam D-14473, Germany 15 Carbon dioxide; Drilling; Geophysical prospecting; Granite; Helium; Methane; Mud logging; Radon; Sedimentology, Geophysical measurements; Mantle; Mud gas concentrations; Pilot holes, Boreholes, borehole geophysics; borehole logging; gas; San Andreas Fault; source rock, California; North America; Parkfield; United States; Western Hemisphere; World https://www.scopus.com/inward/record.uri?eid=2-s2.0-7044235222&doi=10.1029%2f2003GL019395&partnerID=40&md5=99626d15fa4304c7a63b6072fd1cb9cc cited By 25 J. Erzinger T. Wiersberg E. Dahms article Fulton2004 Improved interpretations of the strength of the San Andreas Fault near Parkfield, CA based on thermal data require quantification of processes causing significant scatter and uncertainty in existing heat flow data. These effects include topographic refraction, heat advection by topographically-driven groundwater flow, and uncertainty in thermal conductivity. Here, we re-evaluate the heat flow data in this area by correcting for full 3-D terrain effects. We then investigate the potential role of groundwater flow in redistributing fault-generated heat, using numerical models of coupled heat and fluid flow for a wide range of hydrologic scenarios. We find that a large degree of the scatter in the data can be accounted for by 3-D terrain effects, and that for plausible groundwater flow scenarios frictional heat generated along a strong fault is unlikely to be redistributed by topographically-driven groundwater flow in a manner consistent with the 3-D corrected data. Copyright 2004 by the American Geophysical Union. 2004 English 00948276 10.1029/2003GL019378 Geophysical Research Letters 31 L15S15 1-4 Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82071, United States; Department of Geology and Geophysics, University of Utah, Salt Lake City, UT 84112, United States; U.S. Geological Survey, Menlo Park, CA, United States 15 Chemical analysis; Data acquisition; Groundwater; Hydrology; Refraction; Topology, Parkfield; Thermal data, Heat transfer, borehole geophysics; heat flow; San Andreas Fault; strength; weak rock, California; North America; Parkfield; United States; Western Hemisphere; World https://www.scopus.com/inward/record.uri?eid=2-s2.0-7244234028&doi=10.1029%2f2003GL019378&partnerID=40&md5=03d14cf20aafb193e0bdce7c57efa3df cited By 31 P.M. Fulton D.M. Saffer R.N. Harris B.A. Bekins article Solum2004 This paper establishes a reference phyllosilicate data set from the SAFOD Pilot Hole for future SAFOD drilling and presents an application of these data for studies of exhumed fault segments. The chlorite assemblages in cuttings from two intervals of the SAFOD Pilot Hole are separated into two populations based on X-ray diffraction characteristics. The first population is found in granite in both the deeper and shallower interval, whereas the second population occurs only in clastic sedimentary rocks in the shallower interval. The characteristics of the first population match those for protolith and cataclasite of the exhumed Punchbowl Fault, whereas samples from intensely deformed ultracataclasite are most similar to the second. This supports previous findings that the mineral assemblages in the ultracataclasite formed after the cessation of motion along the fault, above a depth of ∼2 km, and that mineral assemblages in these exhumed fault rocks have been overprinted by post-faulting alteration. Copyright 2004 by the American Geophysical Union. 2004 English 00948276 10.1029/2004GL019909 Geophysical Research Letters 31 L15S19 1-4 Department of Geological Sciences, University of Michigan, 425 E. University Ave., Ann Arbor, MI 48109-1063, United States 15 Boreholes; Drilling; Geophysical prospecting; Granite; Sedimentary rocks; X ray diffraction analysis, Fault rocks; Mineral assemblages; Pilot holes, Silicate minerals, borehole geophysics; cataclasite; fault slip; mineralogy; phyllosilicate; San Andreas Fault https://www.scopus.com/inward/record.uri?eid=2-s2.0-7044249760&doi=10.1029%2f2004GL019909&partnerID=40&md5=97a0d5a5fd4f43d07b7790061f0bbe76 cited By 21 J.G. Solum B.A. Pluijm article Li2004 We used dense linear seismic arrays across and along the San Andreas Fault (SAF) at Parkfield, California to record fault zone trapped waves generated by explosions and microearthquakes in 2002. Prominent trapped waves appeared at stations close to the SAF main fault trace while some energy was trapped in the north strand at the array site. Observations and 3-D finite-difference simulations of trapped waves at 2-5 Hz show evidence of a damaged core zone on the main SAF. The zone from the surface to seismogenic depths is marked by a low-velocity waveguide ∼150 m wide, in which Q is 10-50 and shear velocities are reduced by 30-40% from wall-rock velocities, with the greatest velocity reduction at shallow depth. We interpret that this distinct low-velocity zone on the main SAF is a remanent of damage due to past large earthquakes on the principal fault plane at Parkfield. A less-developed low-velocity zone may be evident on the north strand that experienced minor breaks in the 1966 M6 event. Copyright 2004 by the American Geophysical Union. 2004 English 00948276 10.1029/2003GL019044 Geophysical Research Letters 31 L12S06 1-5 Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089, United States; Institute of Geophys./Planetary Phys, University of California, Los Angeles, CA 90095, United States 12 Computer simulation; Earthquakes; Finite difference method; Structural analysis; Velocity; Waveguides; Strike-slip faults; Velocity; Water waves, Fault zones; Seismic arrays; Trapped waves; Velocity reduction, Seismology; Shear flow, low velocity zone; San Andreas Fault; seismic velocity; trapped wave, California; North America; Parkfield; United States, Damaged structures; Fault-zone trapped waves; Finite difference simulations; Large earthquakes; Linear seismic array; Low velocity zones; Micro-earthquakes; San Andreas fault https://www.scopus.com/inward/record.uri?eid=2-s2.0-6044248858&doi=10.1029%2f2003GL019044&partnerID=40&md5=79f2cc7bbb511d07eb6333915e93de89 cited By 100 Y.-G. Li J.E. Vidale E.S. Cochran article Thurber2004 We present results from the tomographic analysis of seismic data from the Parkfield area using three different inversion codes. The models provide a consistent view of the complex velocity structure in the vicinity of the San Andreas, including a sharp velocity contrast across the fault. We use the inversion results to assess our confidence in the absolute location accuracy of a potential target earthquake. We derive two types of accuracy estimates, one based on a consideration of the location differences from the three inversion methods, and the other based on the absolute location accuracy of "virtual earthquakes." Location differences are on the order of 100-200 m horizontally and up to 500 m vertically. Bounds on the absolute location errors based on the "virtual earthquake" relocations are ∼ 50 m horizontally and vertically. The average of our locations places the target event epicenter within about 100 m of the SAF surface trace. Copyright 2004 by the American Geophysical Union. 2004 English 00948276 10.1029/2004GL019398 Geophysical Research Letters 31 L12S02 1-4 Department of Geology and Geophysics, University of Wisconsin-Madison, 1215 W. Dayton St., Madison, WI 53706, United States; Department of Earth/Environ. Sci., Rensselaer Polytechnic Institute, Troy, NY 12180, United States; U.S. Geological Survey, 345 Middefield Rd., Menlo Park, CA 94035, United States 12 Data acquisition; Error analysis; Geophysics; Mathematical models; Seismology; Tomography; Velocity measurement; Earthquakes; Faulting; Geophysics; Seismology; Strike-slip faults, Epicenter; Inversion codes; Tomographic analysis, Earthquakes; Location, earthquake; microstructure; San Andreas Fault; seismic hazard; seismic tomography; velocity structure, California; North America; United States, Absolute location errors; Fine-scale structures; Inversion methods; Inversion results; Potential targets; San Andreas fault; Velocity contrasts; Velocity structure https://www.scopus.com/inward/record.uri?eid=2-s2.0-6044257847&doi=10.1029%2f2004GL019398&partnerID=40&md5=98a07d9e7444a741f178c4db525ba8e8 cited By 136 C. Thurber S. Roecker H. Zhang S. Baher W. Ellsworth article Hickman2004 2004 English 00948276 10.1029/2004GL020688 Geophysical Research Letters 31 L12S01 1-4 U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, United States; Department of Geophysics, Stanford University, Stanford, CA 94305, United States 12 earthquake; San Andreas Fault; seismic hazard, California; North America; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-6044266089&doi=10.1029%2f2004GL020688&partnerID=40&md5=54c4d886fe8465f8ee1477152692dbe8 cited By 110 S. Hickman M. Zoback W. Ellsworth article Nadeau2004 Large numbers of small earthquakes recorded over 2 decades and analyzed with advanced techniques are used to characterize the detailed kinematics, structure and recurrence interval scaling properties of micro-seismicity in a 4 × 4 km lateral and 6 km deep crustal volume encompassing the region of the SAFOD deep drilling experiment. The characterization reveals that the seismically active San Andreas fault in the vicinity of SAFOD's repeating magnitude 2 target earthquakes is composed of two sub-parallel fault strands that are creeping at comparable rates and that one of the strands lies between the SAFOD drilling platform and SAFOD's target events. In the region, ∼55% of the earthquakes are members of 52 characteristically repeating earthquake sequences. The recurrence intervals of the repeating target events are consistent with the interval scaling of the other sequences. However this. scaling is contrary to that expected from standard constant stress-drop theory. Copyright 2004 by the American Geophysical Union. 2004 English 00948276 10.1029/2003GL019409 Geophysical Research Letters 31 L12S08 1-4 Berkeley Seismological Laboratory, University of California, Berkeley, CA 94720, United States; Ist. Naz di Geofisica Vulcanologia, Rome, Italy 12 Creep; Geophysics; Kinematics; Seismology; Stresses; Structural analysis; Theory; Drilling platforms; Faulting; Geophysics; Kinematics; Strike-slip faults; Tectonics, Crustal volume; Micro-seismicity; Scaling properties; Stress-drop theory, Earthquakes; Earthquakes, deep drilling; earthquake recurrence; kinematics; microtremor; San Andreas Fault; seismicity, California; North America; United States, Constant stress; Fault strands; Recurrence intervals; Repeating earthquake; San Andreas fault; Scaling properties; Small earthquakes; Target regions https://www.scopus.com/inward/record.uri?eid=2-s2.0-6044269252&doi=10.1029%2f2003GL019409&partnerID=40&md5=f382c4ea9603d48e2488701cb8fb7190 cited By 50 R.M. Nadeau A. Michelini R.A. Uhrhammer D. Dolenc T.V. McEvilly article Chéry2004 Stress measurements made in the SAFOD pilot hole provide an opportunity to study the relation between crustal stress outside the fault zone and the stress state within it using an integrated mechanical model of a transform fault loaded in transpression. The results of this modeling indicate that only a fault model in which the effective friction is very low (<0.1) through the seismogenic thickness of the crust is capable of matching stress measurements made in both the far field and in the SAFOD pilot hole. The stress rotation measured with depth in the SAFOD pilot hole (∼28°) appears to be a typical feature of a weak fault embedded in a strong crust and a weak upper mantle with laterally variable heat flow, although our best model predicts less rotation (15°) than observed. Stress magnitudes predicted by our model within the fault zone indicate low shear stress on planes parallel to the fault but a very anomalous mean stress, approximately twice the lithostatic stress. Copyright 2004 by the American Geophysical Union. 2004 English 00948276 10.1029/2004GL019521 Geophysical Research Letters 31 L15S13 1-5 Lab. Dynamique de la Lithosphere, Université de Montpellier 2, Montpellier F-34095, France; Department of Geophysics, Stanford University, Stanford, CA 94305, United States; U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, United States 15 Friction; Geophysical prospecting; Mathematical models; Seismology; Shear stress; Stress analysis, Fault zones; Pilot holes; Stress measurements; Transpression, Boreholes, borehole geophysics; in situ stress; model; San Andreas Fault; stress measurement, California; North America; Parkfield; United States; Western Hemisphere; World https://www.scopus.com/inward/record.uri?eid=2-s2.0-7044237470&doi=10.1029%2f2004GL019521&partnerID=40&md5=e59f288a51dba9e0d734db61f03675cb cited By 53 J. Chéry M.D. Zoback S. Hickman article Langbein2004 A network of 13 continuous GPS stations near Parkfield, California has been converted from 30 second to 1 second sampling with positions of the stations estimated in real-time relative to a master station. Most stations are near the trace of the San Andreas fault, which exhibits creep. The noise spectra of the instantaneous 1 Hz positions show flicker noise at high frequencies and change to frequency independence at low frequencies; the change in character occurs between 6 to 8 hours. Our analysis indicates that 1-second sampled GPS can estimate horizontal displacements of order 6 mm at the 99% confidence level from a few seconds to a few hours. High frequency GPS can augment existing measurements in capturing large creep events and postseismic slip that would exceed the range of existing creepmeters, and can detect large seismic displacements. Copyright 2004 by the American Geophysical Union. 2004 English 00948276 10.1029/2003GL019408 Geophysical Research Letters 31 L15S20 1-4 U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, United States; Cecil Ida Gn Inst Geophys/Plnt Phys, Scripps Institution of Oceanography, University of California, 9500 Gilman Drive, La Jolla, CA 92093-0225, United States 15 Acoustic noise; Atmospheric spectra; Global positioning system; Natural frequencies; Neural networks, Master station; Real time relative; San andreas fault, Real time systems, borehole geophysics; fault slip; GPS; monitoring system; San Andreas Fault; seismicity, California; North America; Parkfield; United States; Western Hemisphere; World https://www.scopus.com/inward/record.uri?eid=2-s2.0-7244234479&doi=10.1029%2f2003GL019408&partnerID=40&md5=e1316025edce2192fa2010d2a9add2f2 cited By 105 J. Langbein Y. Bock article McPhee2004 We present newly compiled magnetic, gravity, and geologic datasets from the Parkfield region around the San Andreas Fault Observatory at Depth (SAFOD) pilot hole in order to help define the structure and geophysical setting of the San Andreas Fault (SAF). A 2-D cross section of the SAF zone at SAFOD, based on new, tightly spaced magnetic and gravity observations and surface geology, shows that as drilling proceeds NE toward the SAF, it is likely that at least 2 fault bounded magnetic slivers, possibly consisting of magnetic granitic rock, serpentinite, or unusually magnetic sandstone, will be encountered. The upper 2 km of the model is constrained by an order of magnitude increase in magnetic susceptibility at 1400 m depth observed in pilot hole measurements. NE of the SAF, a flat lying, tabular body of serpentinite at 2 km depth separates two masses of Franciscan rock and truncates against the SAF. 2004 English 00948276 10.1029/2004GL019363 Geophysical Research Letters 31 L12S03 1-4 U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, United States 12 Data acquisition; Granite; Gravitational effects; Magnetic materials; Magnetic susceptibility; Mathematical models; Rock drilling; Rocks; Sandstone; Serpentine; Magnetic susceptibility; Magnetism; Rock drilling; Strike-slip faults, Crustal structure; Magnetic granitic rock; San Andreas Fault (SAF); Surface geology, Geology; Structural geology, crustal structure; geological mapping; gravity survey; magnetic survey; San Andreas Fault; serpentinite, Crustal structure; Granitic rocks; Pilot holes; Potential field; San Andreas fault; Serpentinite https://www.scopus.com/inward/record.uri?eid=2-s2.0-6044248859&doi=10.1029%2f2004GL019363&partnerID=40&md5=d295a82a0c6ab7778f2f244c68e980a2 cited By 60 D.K. McPhee R.C. Jachens C.M. Wentworth article Blythe2004 The San Andreas Fault Observatory at Depth (SAFOD) pilot hole traverses the upper 2 km of a site 1.8 km west of the San Andreas fault (SAF) near Parkfield, California. In order to evaluate the burial and exhumation history of the site and its relationship to the kinematics and mechanics of the SAF, we use 15 apatite fission-track (FT) and 5 (U-Th)/He analyses from pilot hole samples to document their thermal history. Sample ages decrease with depth: FT and (U-Th)/He ages range from ∼60 and ∼31 Ma, respectively, in the upper 800 m of the hole to ∼3 and 1 Ma at the base of the hole (2.2 km depth, 93°C). Thermal modeling of the distribution of FT lengths indicates three events in the last 80 Ma: 1) cooling and exhumation of >60°C that culminated at ∼30 Ma; 2) reheating of ∼50°C from ∼30 to 8-4 Ma, probably as the result of basin subsidence and burial by 1-1.5 km of sediments; and 3) cooling of ∼30°C and estimated Coast Range exhumation of ∼1 km since 8-4 Ma. Copyright 2004 by the American Geophysical Union. 2004 English 00948276 10.1029/2003GL019407 Geophysical Research Letters 31 L15S16 1-4 Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740, United States; Department of Earth/Planetary Sci., University of California, Berkeley, CA 94720-4767, United States 15 Cooling; Geophysical prospecting; Heating; Kinematics; Mathematical models; Sediments; Thermal effects; Thermoanalysis, Fission-track (FT); Pilot holes; San Andreas Fault (SAF); Thermochronometry, Boreholes, borehole geophysics; burial (geology); exhumation; fission track dating; San Andreas Fault; thermal evolution; thermochronology, California; North America; Parkfield; United States; Western Hemisphere; World https://www.scopus.com/inward/record.uri?eid=2-s2.0-7044246180&doi=10.1029%2f2003GL019407&partnerID=40&md5=9b67824577f645379d695170784ca903 cited By 15 A.E. Blythe M.A. d'Alessio R. Bürgmann article Oye2004 In August 2002, an array of 32 three-component geophones was installed in the San Andreas Fault Observatory at Depth (SAFOD) Pilot Hole (PH) at Parkfield, CA. As an independent test of surface-observation-based microearthquake locations, we have located such events using only data recorded on the PH array. We then compared these locations with locations from a combined set of PH and Parkfield High Resolution Seismic Network (HRSN) observations. We determined the uncertainties in the locations as they relate to errors in the travel time picks and the velocity model by the bootstrap method. Based on the PH and combined locations, we find that the "C2" cluster to the northeast of the PH has the smallest location uncertainties. Events in this cluster also have the most similar waveforms and largest magnitudes. This confirms earlier suggestions that the C2 cluster is a promising target for the SAFOD Main Hole. Copyright 2004 by the American Geophysical Union. 2004 English 00948276 10.1029/2003GL019403 Geophysical Research Letters 31 L12S10 1-5 NORSAR, Instituttveien 25, Kjeller N-2007, Norway; Division of Earth and Ocean Sciences, Duke University, Durham, NC 27708, United States 12 Data acquisition; Earthquakes; Error analysis; Mathematical models; Observatories; Waveform analysis; Seismology; Strike-slip faults, Bootstrap method; High resolution seismic network (HRSN); Microearthquakes; Pilot hole (PH), Geophysics; Location, earthquake hypocenter; microtremor; San Andreas Fault; seismic method, Bootstrap method; High resolution seismic; Location uncertainty; San Andreas fault; Seismic arrays; Surface observation; Three component; Velocity model https://www.scopus.com/inward/record.uri?eid=2-s2.0-6044250185&doi=10.1029%2f2003GL019403&partnerID=40&md5=995a01754f523a4e29311afca7cdf301 cited By 17 V. Oye J.A. Chavarria article Imanishi2004 We estimate the source parameters of #3 microearthquakes by jointly analyzing seismograms recorded by the 32-level, 3-component seismic array installed in the SAFOD Pilot Hole. We applied an inversion procedure to estimate spectral parameters for the omega-square model (spectral level and corner frequency) and Q to displacement amplitude spectra. Because we expect spectral parameters and Q to vary slowly with depth in the well, we impose a smoothness constraint on those parameters as a function of depth using a linear first-differenfee operator. This method correctly resolves corner frequency and Q, which leads to a more accurate estimation of source parameters than can be obtained from single sensors. The stress drop of one example of the SAFOD target repeating earthquake falls in the range of typical tectonic earthquakes. Copyright 2004 by the American Geophysical Union. 2004 English 00948276 10.1029/2004GL019420 Geophysical Research Letters 31 L12S09 1-5 Geological Survey of Japan, Natl. Inst. Adv. Indust. Sci/Technol, 1-1 Higashi 1-Chrome, Tsukuba, Ibaraki 305-8567, Japan; U.S. Geological Survey, Menlo Park, CA 94025, United States; USGS Alaska Volcano Observatory, 4200 University Ave., Anchorage, AK 99508, United States 12 Frequencies; Function evaluation; Mathematical models; Mathematical operators; Parameter estimation; Seismology; Sensors; Tectonics; Frequency estimation; Geophysics; Seismology, Displacement amplitude spectra; Seismic arrays; Seisomograms, Earthquakes; Earthquakes, deep drilling; earthquake mechanism; San Andreas Fault; source parameters, Accurate estimation; Difference operators; Displacement amplitudes; Earthquake source parameters; Repeating earthquake; Smoothness constraints; Spectral parameters; Tectonic earthquakes https://www.scopus.com/inward/record.uri?eid=2-s2.0-6044242059&doi=10.1029%2f2004GL019420&partnerID=40&md5=e7668fbbbcb6503ab407bdfaa0c28211 cited By 53 K. Imanishi W.L. Ellsworth S.G. Prejean article Unsworth2004 The magnetotelluric dataset collected on the San Andreas Fault at Parkfield has been re-analyzed using superior inversion algorithms that have been developed in recent years. A combination of constrained inversion, forward modeling and synthetic inversion studies are used, and show that at the SAFOD site, the low resistivity fault zone extends to a depth of 2-3 km. An extended zone of low resistivity cast of the San Andreas Fault may be connected to the SAF at seismogenic depths. The connection increases along the SAF to the northwest and may be related to the transition from locked to creeping seismic behavior. Copyright 2004 by the American Geophysical Union. 2004 English 00948276 10.1029/2003GL019405 Geophysical Research Letters 31 L12S05 1-4 Institute for Geophysical Research, Department of Physics, University of Alberta, Edmonton, Alta. T6G 2J1, Canada; GeoForschungsZentrum, Telegrafenberg, Potsdam D-14473, Germany 12 Algorithms; Data reduction; Geophysics; Magnetoelectric effects; Mathematical models; Seismology; Strike-slip faults, Datasets; Fault zone; Magnetotelluric exploration, Electric conductivity; Magnetotellurics, electrical resistivity; magnetotelluric method; San Andreas Fault, Constrained inversions; Electrical resistivity structures; Forward modeling; Inversion algorithm; Low resistivity; Magnetotelluric exploration; San Andreas fault; Seismic behavior https://www.scopus.com/inward/record.uri?eid=2-s2.0-6044272858&doi=10.1029%2f2003GL019405&partnerID=40&md5=cc4ecccb71b7ba7a6b5f4c25764c9e54 cited By 98 M. Unsworth P.A. Bedrosian article Williams2004 Detailed thermal measurements have been acquired in the 2.2-km-deep SAFOD pilot hole, located 1.8 km west of the SAF near Parkfield, California. Heat flow from the basement section of the borehole (770 to 2160 m) is 91 mW m-2, higher than the published 74 mW m -2 average for the Parkfield area. Within the resolution of the measurements, heat flow is constant across faults that intersect the borehole, suggesting that fluid flow does not alter the conductive thermal regime. Reanalysis of regional heat flow reveals an increase in heat flow along the SAF northwest of Parkfield. This transition corresponds to a shallowing base of seismicity and a change in fault behavior near the northern terminus of the M6 1966 Parkfield earthquake rupture. The persistence of elevated heat flow in the Coast Ranges to the west appears to rule out frictional heating on the SAF as the source of the SAFOD value. 2004 English 00948276 10.1029/2003GL019352 Geophysical Research Letters 31 L15S14 1-4 U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, United States; U.S. Geological Survey, Sacramento, CA 95819, United States 15 Earthquakes; Flow of fluids; Friction; Geophysical prospecting; Heating; Seismology; Thermal effects; Thermoanalysis, Heat flow; San Andreas Fault; Seismicity, Boreholes, borehole geophysics; heat flow; San Andreas Fault; strength, California; North America; Parkfield; United States; Western Hemisphere; World https://www.scopus.com/inward/record.uri?eid=2-s2.0-7044274675&doi=10.1029%2f2003GL019352&partnerID=40&md5=b8430912f3697dd5ee69d4ad7c7bf45e cited By 67 C.F. Williams F.V. Grubb S.P. Galanis Jr. article Chavarria20031746 The San Andreas Fault Observatory at Depth pilot hole is located on the southwestern side of the Parkfield San Andreas fault. This observatory includes a vertical seismic profiling (VSP) array. VSP seismograms from nearby micro-earthquakes contain signals between the P and S waves. These signals may be P and S waves scattered by the local geologic structure. The collected scattering points form planar surfaces that we interpret as the San Andreas fault and four other secondary faults. The scattering process includes conversions between P and S waves, the strengths of which suggest large contrasts in material properties, possibly indicating the presence of cracks or fluids. 2003 English 00368075 10.1126/science.1090711 Science 302 1746-1748 Division of Earth and Ocean Sciences, Nicholas Sch. Environ./Earth Sci., Duke University, Durham, NC 27708, United States; U.S. Geological Survey, MS977, 345 Middlefield Road, Menlo Park, CA 94025, United States 5651 Cracks; Earthquakes; Fluids; Scattering, Vertical seismic profiling (VSP), Seismology, geological structure; microearthquake; P-wave; S-wave; San Andreas Fault; vertical seismic profile, article; earthquake; electric resistance; geology; material state; priority journal; rock; sediment; United States, California; North America; Parkfield; United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-0344305771&doi=10.1126%2fscience.1090711&partnerID=40&md5=deee153cf6ff5ae7bd84013df7f3c4f6 cited By 57 J.A. Chavarria P. Malin R.D. Catchings E. Shalev article Thurber200312 Arrival-time data from 453 local earthquakes and 6 explosions at Parkfield, CA, are inverted for earthquake locations and three-dimensional Vp and Vp/Vs structure. The structure is dominated by the velocity contrast across the SAF, with the southwest side about 20-25% faster, consistent with previous studies. Nearly all the earthquakes occur almost directly beneath the fault trace. We find high-Vp/Vs anomalies that correlate with low-resistivity features in a magnetotelluric model that are interpreted to represent fluids. We locate a magnitude 2 earthquake that is a potential target event for the final stage of SAFOD drilling, yielding a depth of 3.1 km below surface and an epicenter 100 m southwest of the fault trace. Nonlinear analyses indicate 95%-confidence relative and absolute uncertainties on the order of 500-700 m vertically and 200-300 m horizontally for this target earthquake. 2003 English 00948276 Geophysical Research Letters 30 12-1 University of Wisconsin-Madison, 1215 W. Dayton St., Madison, WI 53706, United States; Dept. of Earth/Environmental Sci., Rensselaer Polytechnic Institute, Troy, NY 12180, United States; Dept. of Geological Science, Brown University, Providence, RI 02912, United States 3 Data reduction; Drilling; Explosions; Geophysics, Arrival-time data, Earthquakes, earthquake epicenter; San Andreas Fault; seismicity; velocity structure, United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-0037998579&partnerID=40&md5=cd9f0b7f0f7a4d5a859dfc9b2bdf593f cited By 91 C. Thurber S. Roecker K. Roberts M. Gold L. Powell K. Rittger article Lee200321 The project to determine the fault zone by San Andreas Fault Observatory at Depth (SAFOD) is discussed. The aim of the project is to drill straight into the heart of San Andreas Fault Zones and place sensors that will predict earthquake accurately. Engineers will be using SAFOD collected data to design roads and buildings to withstand the sort of earthquakes which they are going to expose. 2003 English 00137758 Engineer 292 21-24 7638 Earthquake faults, Drilling; Earthquakes; Investments; Project management; Seismology; Strategic planning, Seismic prospecting https://www.scopus.com/inward/record.uri?eid=2-s2.0-0242492783&partnerID=40&md5=6ddce663b4200b1e0d551ec1aeaf4628 cited By 0 A. Lee article Thurber2003717 A pair of papers in 1976 lead-authored by Kei Aki heralded the beginning of the field of seismic tomography of the lithosphere. The 1976 paper by Aki, Christofferson, and Husebye introduced a simple and approximate yet elegant technique for using body-wave arrival times from teleseismic earthquakes to infer the three-dimensional (3-D) seismic velocity heterogeneities beneath a seismic array or network (teleseismic tomography). Similarly, a 1976 paper by Aki and Lee presented a method for inferring 3-D structure beneath a seismic network using body-wave arrival times from local earthquakes (local earthquake tomography). Following these landmark papers, many dozens of papers and numerous books have been published presenting exciting applications of and/or innovative improvements to the methods of teleseismic and local earthquake tomography, many by Aki's students. This paper presents a brief review of these two types of tomography methods, discussing some of the underlying assumptions and limitations. Thereafter some of the significant methodological developments are traced over the past two and a half decades, and some of the applications of tomography that have reaped the benefits of these developments are highlighted. One focus is on the steady improvement in structural resolution and inference power brought about by the increased number and quality of seismic stations, and in particular the value of utilizing shear waves. The paper concludes by discussing exciting new scientific projects in which seismic tomography will play a major role - the San Andreas Fault Observatory at Depth (SAFOD) and USArray, the initial components of Earthscope. 2003 English 00334553 10.1007/PL00012555 Pure and Applied Geophysics 160 Birkhauser Verlag AG 717-737 Dept. of Geology/Geophysics, Univ. of Wisconsin-Madison, 1215 W. Dayton St., Madison, WI 53706, United States 3-4 body wave; earthquake; lithospheric structure; seismic tomography; seismic velocity; teleseismic wave https://www.scopus.com/inward/record.uri?eid=2-s2.0-0037343015&doi=10.1007%2fPL00012555&partnerID=40&md5=a90082100f3995377520c91bf0303d0d cited By 15 article Catchings20022493 A 5-km-long, high-resolution seismic imaging survey across the San Andreas fault (SAF) zone and the proposed San Andreas Fault Observatory at Depth (SAFOD) drill site near Parkfield, California, shows that velocities vary both laterally and vertically. Velocities range from <1.0 km/sec near the surface to as much as 4.8 km/sec at 750-m depth. The lowest velocities (<1.0 to ∼3.0 km/sec) correspond to unconsolidated sediment, mudstone, and sandstone in the near surface, and the higher velocities (>4.0 km/sec) probably correspond to granitic rock of the Salinian block, which is exposed a few kilometers southwest of the SAF. The depth to the top of probable granitic rock varies laterally along the seismic profile but is about 600 m below the surface at the proposed SAFOD site. We observe a prominent, lateral low-velocity zone (LVZ) beneath and southwest of the surface trace of the SAF. The LVZ is about 1.5 km wide at 300-m depth but tapers to about 600 m wide at 750-m depth. At the maximum depth of the velocity model (750 m), the LVZ is centered approximately 400 m southwest of the surface trace of the SAF. Similar velocities and velocity gradients are observed at comparable depths on both sides of the LVZ, suggesting that the LVZ is anomalous relative to rocks on either side of it. Velocities within the LVZ are lower than those of San Andreas fault gouge, and the LVZ is also anomalous with respect to gravity, magnetic, and resistivity measurements. Because of its proximity to the surface trace of the SAF, it is tempting to suggest that the LVZ represents a zone of fractured crystalline rocks at depth. However, the LVZ instead probably represents a tectonic sliver of sedimentary rock that now rests adjacent to or encompasses the SAF. Such a sliver of sedimentary rock implies fault strands on both sides and possibly within the sliver, suggesting a zone of fault strands at least 1.5 km wide at a depth of 300 m, tapering to about 600 m wide at 750-m depth. Fluids within the sedimentary sliver are probably responsible for observed low-resistivity values. 2002 English 00371106 10.1785/0120010263 Bulletin of the Seismological Society of America 92 2493-2503 U.S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94587, United States; U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94587, United States; U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94587, United States; Department of Geological Sciences, Virginia Tech, 4044 Derring Hall, Blacksburg, VA 24061-0420, United States; Geometrics Inc., 2190 Fortune Drive, San Jose, CA 95131, United States; Geometrics Inc., 2190 Fortune Drive, San Jose, CA 95131, United States 6 Granite; Imaging techniques; Landforms; Sandstone; Sediments; Surveying; Velocity measurement, Seismic imaging surveys, Seismology, crustal structure; fault zone; San Andreas Fault; seismic velocity; seismicity, United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-0036704542&doi=10.1785%2f0120010263&partnerID=40&md5=df6820e3515c778a01445f34d6b34a17 cited By 61 R.D. Catchings M.J. Rymer M.R. Goldman J.A. Hole R. Huggins C. Lippus article Harris200234 Researchers from universities around the country and from the United States Geological Survey (USGS), headquartered in Reston, Virginia, are planning to dig a pilot hole inside California's San Andreas Fault with hopes of eventually developing a larger, adjacent hole that will serve as an earthquake observatory. The observatory, to be called the San Andreas Fault Observatory at Depth (SAFOD), will be designed to determine the physical and chemical processes at work in an active fault zone. 2002 English 08857024 Civil Engineering 72 34-35 7 Earthquakes; Geological surveys; Observatories, Earthquake observatory, Civil engineering, active fault; borehole; earthquake; observatory, United States https://www.scopus.com/inward/record.uri?eid=2-s2.0-0036647072&partnerID=40&md5=d97f323dfdf13ac5ebd790097132438f cited By 0 C.A. Harris article Roeloffs20001226 The US Geological Survey, in cooperation with other institutions, continues to monitor the San Andreas Fault (SAF) near Parkfield, California, hoping to capture high resolution records of continuous deformation before, during and after a magnitude 6 earthquake, as well as the details of its rupture initiation and strong ground motion. Despite the failure of the prediction that the next M 6 Parkfield earthquake would occur before 1993, Parkfield still has a higher known probability (1 to 10% per year) than anywhere else in the US of a M 6 or greater earthquake. Parkfield instrumentation is still largely in place, although there have been losses due to attrition as well as improvements made possible by new technology. Most Parkfield data sets are now available via the Internet, and all others may be obtained upon request from individual investigators. Detailed seismic monitoring has shown that events with identical seismograms, recurring in exactly the same locations, account for a high proportion of the background seismicity at Parkfield. Geophysical studies have revealed that fault zone seismic and electrical properties are consistent with high fluid content. The rate of interseismic slip on the SAF changed significantly in late 1992 or early 1993, during a period of relatively high seismic activity. The strain-rate change, measured by borehole tensor strainmeters and the two-colour electronic distance-measuring network, was also manifested as shortened recurrence intervals of repeating microearthquakes. Whether or not the accelerated deformation turns out to be an intermediate-term precursor to the next M 6 Parkfield earthquake, documenting the variation of interseismic strain rates with time has important implications for fault dynamics and seismic hazard estimation. Two possible instances of pre-earthquake signals have been recorded at Parkfield: water-level and strain changes over a period of three days prior to the nearby 1985 Mw 6.1 Kettleman Hills, California, earthquake and anomalous electromagnetic signals prior to the M 5 earthquake near Parkfield on 20 December 1994. Future work planned at Parkfield includes a National Science Foundation proposal to construct an SAF Observatory at Depth (SAFOD), as part of the Earthscope initiative. The Observatory will consist of a 4-km-deep borehole to penetrate the SAF and a shallow microearthquake cluster on Middle Mountain, directly above the hypocenter of the 1966 Parkfield earthquake. 2000 English 00113891 Current Science 79 1226-1236 US Geological Survey, 5400 MacArthur Blvd., Vancouver, WA 98661, United States 9 https://www.scopus.com/inward/record.uri?eid=2-s2.0-0010383618&partnerID=40&md5=2e48052b9901f718e8a2291ab4dc3091 cited By 28 E. Roeloffs