All ICDP Publications with Abstracts
From parent-sysfolder "Publications" + 2 folder-levels deep
478.
Tube-wave monitoring at Mallik field: Comparing modeled and experimental time-lapse responses
SEG Technical Program Expanded Abstracts,
25
(1)
3240-3244
2006
ISSN: 10523812
Publisher: Society of Exploration Geophysicists
Keywords:▾
Geophysical prospecting; Shear waves; Tubes (components), Cross-well; Data display; First arrival; P- and S-waves; Pre-processing; Time-lapse response; Tube waves; Wave forms, Acoustics
ISBN:
9781604236972
Abstract: ▾ We apply tube-wave monitoring method to a time-lapse cross-well dataset from Mallik field. Raw waveforms are used for analysis thus avoiding any smearing of 4D response introduced by pre-processing. We perform extensive modeling that includes effects of a source borehole and confirms nature of most prominent arrivals as being tube-wave related. Modeling proves that strongest conversion of tube wave into P and S waves occurs at the sharp acoustic boundary. Data displays clear time-lapse changes in tube-wave related arrivals, while shows no change in first arrivals. Modeling suggests that to explain the data the reservoir changes have to occur at a deeper interval than previously anticipated, below the perforations. Excellent agreement between modeled and experimental data provides us with good confidence in our results. This study represents first application of tube-wave monitoring concept. © 2005 Society of Exploration Geophysicists.
477.
Transmission signatures of gashydrate bearing micro-structures: A numerical study
SEG Technical Program Expanded Abstracts,
25
(1)
1978-1982
2006
ISSN: 10523812
Publisher: Society of Exploration Geophysicists
Keywords:▾
Gases; Geophysical prospecting; Hydration; Petroleum prospecting; Seismic waves; Wave propagation, Effective velocity; Gas hydrate bearing sediments; Gas hydrate saturations; High velocity; Micro-scale; Pore space; Sediment grains; Wave propagation simulation, Gas hydrates
ISBN:
9781604236972
Abstract: ▾ This work is inspired by the observation, that gas hydrate bearing sediments have a high velocity in combination with high attenuation. We study numerically the influence of different gas hydrate locations within the pore space on transmitted p-waves. From the wave propagation simulations on the micro-scale it can be seen, that different positions of the gas hydrate in the pore space results in almost the same effective velocities and attenuation, as long as the gas hydrate had contact to the sediment grains. This changes in the case of a suspension, here the attenuation increases and the effective velocity decreases. The resulting p-wave versus gas hydrate saturation plot is in a qualitatively good agreement with experimental results obtained for the Mallik 2L-38 well. © 2005 Society of Exploration Geophysicists.
476.
The Sino-Korean-Yangtze suture, the Huwan detachment, and the Paleozoic-Tertiary exhumation of (ultra)high-pressure rocks along the Tongbai-Xinxian-Dabie Mountains
Special Paper of the Geological Society of America,
40345 – 75
2006
ISSN: 00721077
Publisher: Geological Society of America
Keywords:▾
China; Dabie Mountains; Henan; Tongbai; Xinxiang; Cooling; Deformation; Fission reactions; Geochronology; Metamorphic rocks; Phosphate minerals; Silicate minerals; Structural geology; Zircon; Holocenes; Huwan detachment; Qinling-dabie-sulu orogen; Sino-korean-yangtze suture; Ultrahigh pressure; exhumation; orogeny; Paleozoic; suture zone; Tertiary; ultrahigh pressure metamorphism; Buildings
Abstract: ▾ There are three sutures in the Qinling-Dabie-Sulu orogen in the Tongbai-Xinxian (northern Hong'an)-northern Dabie area: the Silurian Sino-Korean craton-Erlangping intra-oceanic arc suture, the Silurian Erlangping arc-Qinling unit (microcontinent) suture, and the Early Triassic Qinling unit-Yangtze craton suture. We resolve the controversy regarding the age of the Sino-Korean craton-Yangtze craton collisionby recognizing that there was Paleozoic collision between the Qinling unit and the Sino-Korean craton and Mesozoic collision between the Qinling unit and the Yangtze craton. The Qinling unit constitutes a long and narrow microcontinent that extends through the Qinling-Dabie area and probably into the Sulu area. Its common characteristics are the Mesoproterozoic (ca. 1.0 Ga) Jinningian orogeny, ca. 0.8-0.7 Ga arc formation and rifting, and Late Silurian-Early Devonian (ca. 400 Ma) arc magmatism with concomitant regional contact metamorphism up to granulite-facies conditions (peak: 680-740 °C at 0.9-1.1 GPa). A common Proterozoic history links the Qinling microcontinent to the Yangtze craton. Its 400 Ma arc, forearc basin, and its separation from the Yangtze craton by the partly oceanic Huwan mélange make the Qinling microcontinent distinct. The forearc basin sits on the southern part of the 400 Ma arc and underlying Proterozoic continental basement, and detrital geochronology ties it to the Qinling microcontinent basement and its arc. The Huwan mélange is a subduction-accretion complex containing elements of the Qinling microcontinent and its arc, the Paleotethyan ocean floor, and possibly the Yangtze craton. Quartz eclogites (540-590 °C, 2.1 GPa) signify ca. 315 Ma subduction. Devonian to Permian eclogite zircon ages, 40 Ar/ 39 Ar and Rb/Sr mineral ages in the forearc and its basement, and static, Permian blueschist metamorphism in the upper-plate basement testify to subduction throughout the late Paleozoic. The ̃10-km-wide Huwan detachment bounds the high- and ultrahigh-pressure rocks of the Xinxian-Hong'an block (pressure peak at older than 240 Ma) along their northern margin. It is partly responsible for exhumation of the high- and ultrahighpressure rocks, but the entire basement core of Hong'an-Dabie orogen is also strongly deformed. The Huwan shear-zone high-strain deformation indicates passage of rocks through the lithosphere by subhorizontal N-S extension and vertical contraction, showcased by condensed Triassic isograds (420 °C and ̃0.4 GPa in the hanging wall and ̃530 °C and 2.2 GPa in the footwall). The Huwan detachment produced Triassic crustal exhumation rates of 1.9-1.4 mm/yr; synkinematic phengite grew as early as ca. 235 Ma, and the main retrograde deformation occurred at 224-195 Ma. The Tongbai-Xinxian area shows a massive 130-115 Ma cluster of cooling ages, reflecting regional cooling after granitoid injection and regional Cretaceous heating. Apatite fission-track ages cluster at 80-55 Ma and signify cooling related to transtension that coincided with rifting marked by Late Cretaceous-Eocene red bed deposition throughout eastern China. Exhumation rates of for the last 70 m.y. have been slow: ̃0.06 mm/yr. The India-Asia collision reactivated the orogen in the Eocene, particularly along the Tanlu fault zone and locally along fault zones in Tongbai-Xinxian. © 2006 Geological Society of America.
475.
Slip zone and energetics of a large earthquake from the Taiwan Chelungpu-fault Drilling Project
Nature,
444
(7118)
473-476
2006
ISSN: 00280836
Publisher: Nature Publishing Group
Keywords:▾
Drilling platforms; Earthquake effects; Energy dissipation; Interfacial energy; Project management; Surface phenomena, Near field; Rupture processes; Seismic fracture energy; Slip zone, Seismology, borehole; Chi-Chi earthquake 1999; drilling; fault gouge; fault slip; fault zone; grain size; seismometry; surface energy, article; earthquake; energy transfer; geology; particle size; priority journal; surface property; Taiwan; thickness, Asia; Eurasia; Far East; Taiwan
Abstract: ▾ Determining the seismic fracture energy during an earthquake and understanding the associated creation and development of a fault zone requires a combination of both seismological and geological field data. The actual thickness of the zone that slips during the rupture of a large earthquake is not known and is a key seismological parameter in understanding energy dissipation, rupture processes and seismic efficiency. The 1999 magnitude-7.7 earthquake in Chi-Chi, Taiwan, produced large slip (8 to 10 metres) at or near the surface, which is accessible to borehole drilling and provides a rare opportunity to sample a fault that had large slip in a recent earthquake. Here we present the retrieved cores from the Taiwan Chelungpu-fault Drilling Project and identify the main slip zone associated with the Chi-Chi earthquake. The surface fracture energy estimated from grain sizes in the gouge zone of the fault sample was directly compared to the seismic fracture energy determined from near-field seismic data. From the comparison, the contribution of gouge surface energy to the earthquake breakdown work is quantified to be 6 per cent. ©2006 Nature Publishing Group.
474.
The origin and age of biogeochemical trends in deep fracture water of the witwatersrand basin, south africa
Geomicrobiology Journal,
23
(6)
369-414
2006
ISSN: 01490451Keywords:▾
biogeochemistry; dissolved inorganic carbon; groundwater; methanogenesis; microbial community; sulfate-reducing bacterium, Africa; South Africa; Southern Africa; Sub-Saharan Africa; Witwatersrand, Methanobacteriales
Abstract: ▾ Water residing within crustal fractures encountered during mining at depths greater than 500 meters in the Witwatersrand basin of South Africa represents a mixture of paleo-meteoric water and 2.0–2.3 Ga hydrothermal fluid. The hydrothermal fluid is highly saline, contains abiogenic CH4 and hydrocarbon, occasionally N2, originally formed at ∼250–300◦C and during cooling isotopically exchangedO and Hwith minerals and accrued H2,4He and other radiogenic gases. The paleo-meteoric water ranges in age from ∼10 Ka to >1.5 Ma, is of low salinity, falls along the global meteoric water line (GMWL) and is CO2 and atmospheric noble gas-rich. The hydrothermal fluid, which should be completely sterile, has probably been mixing with paleo-meteoric water for at least the past∼100 Myr, a process which inoculates previously sterile environments at depths >2.0 to 2.5 km. Free energy flux calculations suggest that sulfate reduction is the dominant electron acceptor microbial process for the high salinity fracture water and that it is 107 times that normally required for cell maintenance in lab cultures. Flux calculations also indicate that the potential bioavailable chemical energy increases with salinity, but because the fluence of bioavailable C, N and P also increase with salinity, the environment remains energy-limited. The4He concentrations and theoretical calculations indicate that the H2 that is sustaining the subsurface microbial communities (e.g. H2-utilizing SRB and methanogens) is produced by water radiolysis at a rate of ∼1nMyr−1. Microbial CH4 mixes with abiogenicCH4 to produce the observed isotopic signatures and indicates that the rate of methanogenesis diminishes with depth from∼100 at<1 kmbls, to<0.01nMyr−1 at>3 kmbls. Microbial Fe(III) reduction is limited due to the elevated pH. The δ13C of dissolved inorganic carbon is consistent with heterotrophy rather than autotrophy dominating the deeper, more saline environments. One potential source of the organic carbon may be microfilms present on the mineral surfaces. © Taylor & Francis.
473.
The Lake Petén Itzá Scientific Drilling Project
Scientific Drilling,
1
(3)
25-29
2006
ISSN: 18168957472.
The Cretaceous-Paleogene (K-P) boundary at Brazos, Texas: Sequence stratigraphy, depositional events and the Chicxulub impact
Sedimentary Geology,
184
(1-2)
77-109
2006
Abstract: ▾ Two cores from Brazos, Texas, spanning the Cretaceous-Paleogene (K-P) boundary, are investigated by a multidisciplinary approach aiming at unraveling environmental changes and sequence stratigraphic setting. In addition, the sedimentology of the K-P event deposit and its correlation with the K-P boundary is studied. Foraminifera and nannofossil stratigraphy indicates that both cores include a latest Maastrichtian (Zone CF1-CF2) and earliest Danian (P0, Pα and P1a) shale sequence with a sandy and Chicxulub ejecta-bearing event deposit at the K-P boundary; a hiatus of unknown duration may be present by the unconformable base of the event deposit. Planktic foraminifera as well as calcareous nannofossil abundance and diversity both decline abruptly above the event deposit (K-P mass extinction), whereas benthic foraminifera show a pronounced faunal change but no mass extinction. Mineralogical and geochemical proxies suggest that-except for the sandwiched K-P event deposit-no facies change took place across the K-P boundary and no evidence for adverse an- or dysoxic sedimentary conditions following the Chicxulub impact was observed. Therefore, the interval bracketing the K-P event deposit is considered as highstand systems tract. Increased coarse detritus input and low planktic/benthic (P/B) foraminifera ratios during the earliest Paleocene (P0 and Pα) both suggest an increased coastal proximity or relative sea-level lowering, although the K-P mass extinction of planktic foraminifera might have influenced the P/B ratios as well. Consequently, the sandy shales of the early Paleocene are considered as late regressive highstand or as lowstand deposit. During P1a, shales assigned as transgressive systems tract overlie a pyrite- and glauconite-rich bioturbated transgressive surface or type-2-sequence boundary. The smectite-dominated clay assemblage, with minor illite, kaolinite and chlorite indicates semiarid-humid climates with no obvious shifts across the K-P boundary. The magnetic susceptibility signature during the Maastrichtian reveals a subtle cyclic (or rhythmic) pattern, whereas a high-amplitude cyclic pattern is present during the early Danian. The K-P event deposit shows a succession of high-energetic debris flows and turbidites derived from multiple source areas, followed by a period of decreasing current energy. Deposition was likely triggered by multiple tsunami or tempestites followed by a prolonged period of reworking and settling. The Chicxulub ejecta at the base of the K-P event deposit consists of Mg-rich smectite-as well as Fe-Mg-rich chlorite-spherules. Their mineralogical composition points to target rocks of mafic to intermediate composition, presumably situated in the northwestern sector of the Chicxulub impact structure. Besides these silicic phases, the most prominent ejecta components are limestone clasts, accretionary carbonate clasts, and microspar, suggesting that the Texas area received ejecta also from shallow, carbonate-rich lithologies at the impact site on the Yucatán carbonate platform. The excellent correlation of Chicxulub ejecta at Brazos with ejecta found in the K-P boundary layer worldwide - along with the associated mass extinction - provides no evidence that Chicxulub predated the K-P boundary and allows for unequivocal positioning of the K-P boundary at the event deposit. © 2005 Elsevier B.V. All rights reserved.
471.
The 2005 Lake Malawi Scientific Drilling Project
Scientific Drilling,
217-19
2006
470.
Temperature dependence of elastic P- and S-wave velocities in porous Mt. Unzen dacite
Journal of Volcanology and Geothermal Research,
153
(1-2 SPEC. ISS.)
136-147
2006
ISSN: 03770273Keywords:▾
Anisotropy; High temperature effects; Metamorphic rocks; Porosity; Seismology; Volcanic rocks; Volcanoes, Pulse transmission technique; Silica-rich rocks; Volcano seismic activity, Elastic waves, dacite; elasticity; high temperature; P-wave; S-wave; seismic anisotropy; seismology; volcanic rock; volcanology, Asia; Eurasia; Far East; Japan; Kyushu; Nagasaki [Kyushu]; Unzen Volcano
Abstract: ▾ Laboratory measurements of elastic properties of volcanic rocks are crucial for the modelling of volcano seismic activity. Compared to the large database reported in the literature for sedimentary, igneous and metamorphic rocks, the data set for volcanic rocks is limited and mostly restricted to basalts. Data for more silica-rich rocks are sparse. In particular, velocity data for silica-rich volcanic rocks measured at elevated temperature are lacking. We measured the elastic P- and S-wave velocities and the velocity anisotropy of porous dacitic rocks from Unzen Volcano, Japan, exhibiting an open porosity of 3.3 to 24.3 vol.%. The measurements were done at temperatures of up to 600 °C and confining pressures of 100 MPa, corresponding to depths of ∼ 3000-4000 m. Samples with even higher porosities failed at the required pressures. The measurements were carried out in a cubic multi-anvil pressure apparatus, using the pulse transmission technique. In contrast to low-porosity magmatic and metamorphic rocks, the seismic velocities of the investigated volcanic rocks increased and the velocity anisotropies decreased with increasing temperature, due to further sample compaction. There is a close relationship between velocity, density and porosity. The higher the density (and the lower the porosity) the higher are the P- and S-wave velocities. These results can contribute to a better understanding of the propagation of seismic energy through the volcanic edifice. © 2005 Elsevier B.V. All rights reserved.
469.
Target delamination by spallation and ejecta dragging: An example from the Ries crater's periphery
Earth and Planetary Science Letters,
252
(1-2)
15-29
2006
Abstract: ▾ Subhorizontal shear planes (detachments) are observed in bedded limestones in the periphery of the Ries impact crater, Germany. These detachments occur at 0.8-1.8 crater radii distance from the crater center beneath deposits of the continuous ejecta blanket. Striations on detachment planes and offsets of markers indicate top-outward shearing with radial slip vectors. Detachments were found at depths between a few meters and more than 50 m beneath the target surface. The displacements along these faults range from meters to decameters and decrease with increasing depth and distance from the crater center. With increasing crater distance, detachment horizons tend to climb to shallower levels. Cross-cutting relationships to faults associated with the crater collapse indicate that detachment faulting started prior to the collapse but continued during crater modification. Numerical modeling of the cratering process shows that near-surface deformation outside the transient crater is induced by two separate mechanisms: (i) weak spallation by interference of shock and release waves near the target surface and (ii) subsequent dragging by the deposition of the ejecta curtain. Spallation causes an upward and outward directed motion of target material that increases in magnitude toward the target surface. It leads to decoupling of the uppermost target layers in the early cratering stage without totally disintegrating the rock. The subsequent arrival of the oblique impact shower of the ejecta curtain at the target surface delivers a horizontal momentum to the uppermost target area and results in a second horizontal displacement increment by dragging. With increasing depth this effect vanishes rapidly. Spallation decoupling and subsequent ejecta dragging of near-surface rocks is probably a general cratering mechanism around craters in layered targets with weak interbeds. © 2006 Elsevier B.V. All rights reserved.
468.
Structure of the San Andreas fault zone at SAFOD from a seismic refraction survey
Geophysical Research Letters,
33
(7)
2006
ISSN: 00948276Keywords:▾
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
Abstract: ▾ 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.
467.
Structure of the San Andreas Fault at SAFOD from P-wave tomography and fault-guided wave mapping
Geophysical Research Letters,
33
(13)
2006
ISSN: 00948276Keywords:▾
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
Abstract: ▾ 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.
466.
Strain partitioning and stress rotation at the North Anatolian fault zone from aftershock focal mechanisms of the 1999 Izmit MW = 7.4 earthquake
Geophysical Journal International,
166
(1)
373-385
2006
ISSN: 0956540XKeywords:▾
aftershock; fault zone; focal mechanism; Kocaeli earthquake 1999; strain analysis; stress analysis, Anatolia; Eurasia; Turkey
Abstract: ▾ We investigate aftershock focal mechanisms of the Mw=7.4 Izmit earthquake of 1999 August 17, on the western North Anatolian fault zone (NAFZ). Spatial clustering and the orientation of 446 fault plane solutions are analysed. The Izmit mainshock occurred as a right-lateral slip on an EW-trending near-vertical fault plane. Aftershock clusters define four individual fault segments. Focal mechanisms surrounding the epicentres of the Izmit and subsequent Düzce mainshock (Mw= 7.1, 1999 November 12) indicate predominantly strike-slip but also normal faulting. Aftershocks in the area between the Izmit and Düzce segments are mainly related to EW-oriented normal faulting delineating a small pull-apart structure. Beneath the easternmost Sea of Marmara, alignments of aftershocks suggest branching of the NAFZ into three or more active segments that differ significantly in terms of their focal mechanism characteristics. The distribution of aftershock focal mechanisms corresponds to fault segmentation of the NAFZ in the Izmit-Düzce region produced by coseismic slip. Areas with large amounts of coseismic slip show aftershocks that are predominantly strike-slip, but low-slip barriers show mostly normal faulting aftershocks. Stress tensor inversions of the aftershock focal mechanisms show rotations of the local stresses following the Izmit mainshock. In the Izmit-Sapanca area, the maximum horizontal compressive stress axis is horizontally rotated counter-clockwise by 8° with respect to the coseismic and long-term regional stress field. Towards the eastern end of the rupture (Karadere-Düzce area), stresses are rotated clockwise. We conclude that the Izmit earthquake caused significant stress partitioning along the rupture. The direction of stress rotation is related to the orientation of the individual fault segments along the NAFZ. © 2006 The Authors Journal compilation © 2006 RAS.
465.
Stable isotopes in bulk carbonates and organic matter in recent sediments of Lake Qinghai and their climatic implications
Chemical Geology,
235
(3-4)
262 – 275
2006
ISSN: 00092541Keywords:▾
Asia; China; Eurasia; Far East; Qinghai; Qinghai Lake; carbon isotope; carbonate; climate conditions; dissolved inorganic carbon; lacustrine deposit; oxygen isotope; sediment chemistry; stable isotope; total organic carbon
Abstract: ▾ Multi-proxy indices on annual/decadal scales during the past 600 years were developed from the surface sediments of Lake Qinghai based on 210Pb and 137Cs geochronology. The δ13Ccarb, δ18Ocarb, and total carbonate content are consistent with one another in trends and their variations have been mainly ascribed to regional temperature. It is suggested that the strong evaporation contributes in modifying δ18O of lake water and δ13C of the dissolved inorganic carbon of the surface water, and appears to be responsible for the covariance between δ13Ccarb and δ18Ocarb. Photosynthesis of the aquatic plants appears to have played a significant role in determining δ15N of organic matter. The discrimination of 14N and 15N during photosynthesis may have triggered a positive linkage between temperatures and δ15Norg (after removal of the stratigraphic trend). Variations of total organic carbon (TOC), C/N ratio, and δ13Corg have been ascribed to local precipitation. TOC (after removal of the stratigraphic trend) is positively correlated with the C/N ratio (r = 0.45, α < 0.01), and negatively correlated with δ13C of organic matter (δ13Corg) (r = - 0.55, α < 0.01); while δ13Corg is negatively correlated with the C/N ratio (r = - 0.48, α < 0.01). The climatic significance of these multi-proxy indices has been verified by comparing with the meteorological records and the climates inferred from tree rings in adjacent regions. © 2006 Elsevier B.V. All rights reserved.
464.
Source scaling relationships of microearthquakes at Parkfield, CA, determined using the SAFOD pilot hole seismic array
Geophysical Monograph Series,
17081-90
2006
ISSN: 00658448
Publisher: Blackwell Publishing Ltd
DOI:10.1029/170GM10
Keywords:▾
Energy dissipation; Seismic waves; Seismology, Corner frequency; Large earthquakes; Micro-earthquakes; Moment independents; Scaling relationships; Source parameters; Spectral ratios; Tectonic earthquakes, Earthquakes
ISBN:
9781118666272; 9780875904351
Abstract: ▾ 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.
463.
Mineralogical characterization of protolith and fault rocks from the SAFOD Main Hole
Geophysical Research Letters,
33
(21)
2006
ISSN: 00948276Keywords:▾
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
Abstract: ▾ 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.
462.
HOTSPOT: The snake river scientific drilling project-tracking the yellowstone hotspot through space and time
Scientific Drilling,
1
(3)
56 – 57
2006
ISSN: 18163459461.
Mapping stress and structurally controlled crustal shear velocity anisotropy in California
Geology,
34
(10)
825-828
2006
ISSN: 00917613DOI:10.1130/G22309.1
Keywords:▾
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
Abstract: ▾ 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.
460.
Chesapeake bay impact structure deep drilling project completes coring
Scientific Drilling,
1
(3)
34-37
2006
ISSN: 18168957459.
Downcore variations of total sulphur for VER98-1-14
458.
Downcore variations of the SIRM/$\kappa$LF for CON01-603-2, CON01-604-2, CON01-605-3, VER98-1-1, VER98
457.
Downcore variations of rock magnetic parameters (ARM, S-ratio and HIRM) for CON01-603-2 and VER98-1-14
456.
Down-core variations of the SIRM for CON01-603-2.
455.
Discovery of a 25-cm asteroid clast in the giant Morokweng impact crater, South Africa
Nature,
441
(7090)
203-206
2006
Abstract: ▾ Meteorites provide a sample of Solar System bodies and so constrain the types of objects that have collided with Earth over time. Meteorites analysed to date, however, are unlikely to be representative of the entire population and it is also possible that changes in their nature have occurred with time 1. Large objects are widely believed to be completely melted or vaporized during high-angle impact with the Earth2,3. Consequently, identification of large impactors relies on indirect chemical tracers, notably the platinum-group elements4. Here we report the discovery of a large (25-cm), unaltered, fossil meteorite, and several smaller fragments within the impact melt of the giant (>70 km diameter), 145-Myr-old Morokweng crater, South Africa. The large fragment (clast) resembles an LL6 chondrite breccia, but contains anomalously iron-rich silicates, Fe-Ni sulphides, and no troilite or metal. It has chondritic chromium isotope ratios and identical platinum-group element ratios to the bulk impact melt. These features allow the unambiguous characterization of an impactor at a large crater. Furthermore, the unusual composition of the meteorite suggests that the Morokweng asteroid incorporated part of the LL chondrite parent body not represented by objects at present reaching the Earth. © 2006 Nature Publishing Group.
454.
Depth versus age based on relative magnetic paleointensity correlations for all six investigated sites