This file was created by the TYPO3 extension publications --- Timezone: CEST Creation date: 2026-04-11 Creation time: 14:03:26 --- Number of references 35 article WOS:000949573100037 2023 10.1029/2022GL101258 GEOPHYSICAL RESEARCH LETTERS 50 3 seismicity catalog; sea level change; hydrothermal region; strain; strainmeter; solid Earth tides P. Martínez-Garzón G. C. Beroza G. M. Bocchini M. Bohnhoff article Kılıç2020375 Given its intense seismic activity and damaging earthquake generation potential, the western part of the North Anatolian Fault constitutes a serious natural hazard. As a result, the fault is monitored with a broad range of seismological and geodetic instrumentation making it a natural laboratory environment for scientific studies. One of the long-term projects in this region is GONAF (Geophysical Borehole Observatory at the North Anatolian Fault) which is the first borehole seismometer network project in Turkey. GONAF is a joint research project that started in 2011 as joint initiative of the Turkish Ministry of Interior, Disaster and Emergency Management Presidency AFAD and GFZ and the German Research Center for Geoscience Helmholtz Center Potsdam. The aim of GONAF is to detect, examine, and monitor the microseismic activity in the region and to observe the physical processes before, during and after a large Marmara earthquake (M > 7.0) that is expected to rupture the western part of the North Anatolian Fault, below the Marmara Sea along the Princes Islands segment or even further to the west. For this purpose, the permanent GONAF observatory was established consisting of 7 borehole seismometer arrays installed down to a depth of 300 m. In this paper, we report on regional stress changes in the western part of the North Anatolian Fault Zone (NAFZ) using instrumental data and the Coulomb stress method. We also present preliminary results of the observation and evaluation of microseismic activity obtained from the GONAF observatory. For the automatic evaluation of real-time data, Seiscomp3, RTQUAKE, and Earthworm Softwares were used. Within the scope of automatic earthquake detection studies, between March, 2016 and November, 2017, a total of 2568 earthquakes were detected using the RTQUAKE software. Of these, 1459 could be analyzed. While the magnitude of the analyzed earthquakes varies between 0.8 and 4.2, the depth of these events ranges from 2 to 30 km. © 2020, Springer Nature B.V. Article 2020 English 13834649 10.1007/s10950-020-09907-6 Journal of Seismology 24 Springer 375 – 395 2 Sea of Marmara; Turkey; borehole geophysics; Coulomb criterion; earthquake damage; earthquake magnitude; geophysical method; seismograph; seismology https://www.scopus.com/inward/record.uri?eid=2-s2.0-85081586883&doi=10.1007%2fs10950-020-09907-6&partnerID=40&md5=deffd2d113aa03d9bb0505bbc7107a9d Cited by: 2 Tuğbay Kılıç Recai F. Kartal Filiz T. Kadirioğlu Marco Bohnhoff Murat Nurlu Diğdem Acarel Patricia Martínez Garzon Vedat Özsarac article Süer20201655 This study presents the results of a real-time gas monitoring experiment conducted, via the use of a quadrupole mass spectrometer, in a mofette field within the Tekke Hamam geothermal site in western Anatolia (Turkey), a tectonically active region hosting several east–west trending grabens. The study is aimed to establish a baseline gas profile of the region. Within the framework of the experiment, gas compositions (CO2, N2, O2, H2, H2S, CH4, He, and Ar) and flow rate of a mofette were monitored during two observation periods: November 2007–January 2008 and April–July 2008. During the course of monitoring, the major gas component was CO2 with concentration changing around 96 volume percent. Other gases, from the most abundant to the least, were N2, CH4, O2, H2S, Ar, H2, and He. The study produced a short-term, baseline gas profile of the region with daily/diurnal variations and temporal gas fluctuations appearing as instant signals. Although the temporal gas fluctuations did not reach the anomaly level (variations staying within the mean ± 2σ), some of the variations in more than one parameter in the gas compositions (exceeding the mean ± 1σ), accompanied by changes in the diurnal gas pulses lasting for long durations, were correlated with the seismic events selected according to the adopted seismic event elimination criteria. The variations were mainly attributed to changing gas mixing ratios in relation to porosity/permeability modifications possibly related to seismicity. Studies involving the continuous monitoring of meteorological parameters are necessary to assign these variations to geogenic events. © 2020, Springer Nature B.V. 2020 English 0921030X 10.1007/s11069-020-04238-8 Natural Hazards 104 Springer Science and Business Media B.V. 1655-1678 Department of Geological Engineering, Middle East Technical University, Ankara, 06800, Turkey; Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Potsdam, 14473, Germany; Department of Petroleum and Natural Gas Engineering, Middle East Technical University, Ankara, 06800, Turkey; Central Laboratory, Middle East Technical University, Ankara, 06800, Turkey 2 concentration (composition); environmental monitoring; gas flow; geothermal system; mass spectrometry; mixing ratio; real time; seismic method; seismicity, Anatolia; Turkey https://www.scopus.com/inward/record.uri?eid=2-s2.0-85089975052&doi=10.1007%2fs11069-020-04238-8&partnerID=40&md5=a29a0227c331214b61fd1059a1fcf1fb cited By 1 S. Süer T. Wiersberg N. Güleç J. Erzinger M. Parlaktuna article Martínez-Garzón2019 Quantifying regional earthquake cluster style is essential for providing a context for studies of seismicity patterns and earthquake interactions. Here, we identify clusters of seismicity in the Sea of Marmara region of the North Anatolian Fault, NW Turkey, using a recently derived high-resolution seismicity catalog and the nearest-neighbor earthquake cluster approach. The detected earthquake clusters are utilized for (1) determining spatial distribution of mainshock and aftershock rates and estimating the proximity to failure on different fault segments, (2) identifying fault sections having earthquake repeaters, and (3) finding areas with enhanced foreshock activity. About 6%, 70% and 24% of the events are identified as foreshocks, mainshocks and aftershocks, respectively, with the largest concentration of aftershocks and foreshocks located along the Western High and the Cinarcik Fault, respectively. The method successfully identifies regions where previous studies reported earthquake repeaters as indicator for fault creep and suggests additional repeater areas in the Gulf of Gemlik. The largest proportion of mainshocks with associated foreshocks and aftershocks are along the Western High and Cinarcik Fault segments, potentially indicating that these segments are closer to failure and have increased susceptibility to seismic triggering. Continuing studies can contribute to monitoring possible preparation phase of a large (M > 7) earthquake in the Marmara region near the Istanbul Metropolitan region. © 2019 Elsevier B.V. 2019 English 00401951 10.1016/j.tecto.2019.228176 Tectonophysics 768 Elsevier B.V. Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Section 4.2: Geomechanics and Scientific Drilling, Telegrafenberg, Potsdam, 14473, Germany; University of Southern California, Department of Earth Sciences, Los Angeles, CA 90089-0740, United States; University of Nevada, Reno, Department of Mathematics and Statistics, Reno, NV 89557, United States; Institute of Geological Sciences, Free University of Berlin, Berlin, Germany Faulting, Earthquake dynamics; Earthquake process; Fault segmentation; Metropolitan regions; North Anatolian Fault; Regional earthquakes; Seismicity clusters; Seismicity pattern, Earthquakes, aftershock; cluster analysis; earthquake catalogue; earthquake event; earthquake mechanism; earthquake trigger; fault; foreshock; North Anatolian Fault; seismicity, Gulf of Gemlik; Istanbul [Istanbul (PRV)]; Istanbul [Turkey]; Sea of Marmara; Turkey https://www.scopus.com/inward/record.uri?eid=2-s2.0-85070879394&doi=10.1016%2fj.tecto.2019.228176&partnerID=40&md5=b3ab7349182ee9562ef359ec2d3ad567 cited By 10 P. Martínez-Garzón Y. Ben-Zion I. Zaliapin M. Bohnhoff article Martínez-Garzón2019209 We analyze a large transient strainmeter signal recorded at 62.5 m depth along the southern shore of the eastern Sea of Marmara region in northwestern Turkey. This region represents a passage of stress transfer from the Izmit rupture to the Marmara seismic gap. The strain signal was recorded at the Esenkoy site by one of the ICDP-GONAF (International Continental Drilling Programme – Geophysical Observatory at the North Anatolian Fault) strainmeters on the Armutlu peninsula with a maximum amplitude of 5 microstrain and lasting about 50 days. The onset of the strain signal coincided with the origin time of a M W 4.4 earthquake offshore Yalova, which occurred as part of a seismic sequence including eight M W ≥3.5 earthquakes. The M W 4.4 event occurred at a distance of about 30 km from Esenkoy on June 25th 2016 representing the largest earthquake in this region since 2008. Before the event, the maximum horizontal strain was subparallel to the regional maximum horizontal stress derived from stress inversion of local seismicity. During the strain transient, we observe a clockwise rotation in the local horizontal strain field of about 20°. The strain signal does not correlate with known environmental parameters such as annual changes of sea level, rainfall or temperature. The strain signal could indicate local slow slip on the Cinarcik fault and thus a transfer of stress to the eastern Marmara seismic gap. © 2019 Elsevier B.V. Article 2019 English 0012821X 10.1016/j.epsl.2019.01.001 Earth and Planetary Science Letters 510 Elsevier B.V. 209 – 218 Armutlu Peninsula; Istanbul [Istanbul (PRV)]; Istanbul [Turkey]; Sea of Marmara; Turkey; Earthquakes; Offshore oil well production; Sea level; Strain measurement; Transform faults; Sea of Marmara; Seismic hazards; slow slip; Strain transients; Strain-meter; amplitude; earthquake event; earthquake rupture; fault slip; seismic hazard; strain; stress; transform fault; Fault slips https://www.scopus.com/inward/record.uri?eid=2-s2.0-85060524398&doi=10.1016%2fj.epsl.2019.01.001&partnerID=40&md5=d528516a0d81f96ff0d7739b323a8f52 Cited by: 12 Patricia Martínez-Garzón Marco Bohnhoff David Mencin Grzegorz Kwiatek Kathleen Hodgkinson Murat Nurlu Filiz Tuba Kadirioglu Recai Feyiz Kartal article Géli2018 Understanding micro-seismicity is a critical question for earthquake hazard assessment. Since the devastating earthquakes of Izmit and Duzce in 1999, the seismicity along the submerged section of North Anatolian Fault within the Sea of Marmara (comprising the "Istanbul seismic gap") has been extensively studied in order to infer its mechanical behaviour (creeping vs locked). So far, the seismicity has been interpreted only in terms of being tectonic-driven, although the Main Marmara Fault (MMF) is known to strike across multiple hydrocarbon gas sources. Here, we show that a large number of the aftershocks that followed the M 5.1 earthquake of July, 25th 2011 in the western Sea of Marmara, occurred within a zone of gas overpressuring in the 1.5-5 km depth range, from where pressurized gas is expected to migrate along the MMF, up to the surface sediment layers. Hence, gas-related processes should also be considered for a complete interpretation of the micro-seismicity (~M &lt; 3) within the Istanbul offshore domain. © 2018 The Author(s). 2018 English 20452322 10.1038/s41598-018-23536-7 Scientific Reports 8 Nature Publishing Group Ifremer, Département Ressources Physiques et Ecosystèmes de Fond de Mer (REM), Plouzané, F-29280, France; CEREGE, Aix Marseille Univ., CNRS, IRD, INRA, Coll. France, Aix-Marseille, France; Lamont-Doherty Earth Observatory, Palisades, NY, United States; Universidad de Los Andes, Bogotà, Colombia; ALomax Scientific, Mouans-Sartoux, 06370, France; Kandilli Observatory and Earthquake Research Institute, Boǧaziçi University, Istanbul, Turkey; Istanbul Technical University, Istanbul, Turkey; Ocean and Earth Science, National Oceanography Centre, Southampton, United Kingdom; School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham, United Kingdom; Mineral Research and Exploration General Directorate, MTA, Ankara, Turkey; Institute for Marine Science and Technology, Dokuz Eyiul Universitesi, Izmir, Turkey; Helmholtz-Centre Potsdam German Centre for Geosciences GFZ, Section 4.2 Geomechanics and Rheology, Telegrafenberg, Potsdam, 14473, Germany; Freie Universität Berlin, Department of Earth Sciences, Malteser Strasse 74-100, Berlin, 12249, Germany; Institute of Marine Science, ISMAR-CNR, Bologna, Italy; Istituto Nazionale di Geofisica e Vulcanologia, INGV, Roma, Italy; Faculty of Environmental Science and Engineering, Babes-Bolyai University, Cluj-Napoca, Romania 1 article; earthquake; sediment; Turkey (republic); Marmara Sea https://www.scopus.com/inward/record.uri?eid=2-s2.0-85046363781&doi=10.1038%2fs41598-018-23536-7&partnerID=40&md5=b6cd6460e388083d7538e68a8f64e9b6 cited By 17 L. Géli P. Henry C. Grall J.-B. Tary A. Lomax E. Batsi V. Riboulot E. Cros C. Gürbüz S.E. Işlk A.M.C. Sengor X. Le Pichon L. Ruffine S. Dupré Y. Thomas D. Kalafat G. Bayrakci Q. Coutellier T. Regnier G. Westbrook H. Saritas G. Çifçi M.N. Çağatay M.S. Özeren N. Görür M. Tryon M. Bohnhoff L. Gasperini F. Klingelhoefer C. Scalabrin J.-M. Augustin D. Embriaco G. Marinaro F. Frugoni S. Monna G. Etiope P. Favali A. Bécel article Malin2018 Article 2018 10.1038/s41598-018-34563-9 Scientific Reports 8 1 https://www.scopus.com/inward/record.uri?eid=2-s2.0-85055907317&doi=10.1038%2fs41598-018-34563-9&partnerID=40&md5=d7ee2cf1f3e5b2e42d8b1e7f252bf193 Cited by: 21; All Open Access, Gold Open Access, Green Open Access Peter E. Malin Marco Bohnhoff Felix Blümle Georg Dresen Patricia Martínez-Garzón Murat Nurlu Ulubey Ceken Filiz Tuba Kadirioglu Recai Feyiz Kartal Tuğbay Kiliç Kenan Yanik article Raub2017232 We studied spatiotemporal b-value variations along the North Anatolian Fault Zone (NAFZ) in northwestern Turkey with a focus on the combined 1999 Izmit and Düzce rupture and the eastern Sea of Marmara. We used a local seismicity catalog of the Izmit-Düzce region covering a time span from 2.5 years prior to the Izmit until 14 months after the Düzce mainshock and a four-year hypocenter catalog in the eastern Sea of Marmara. We consistently calculated moment magnitudes to ensure a homogeneous dataset and applied strict quality criteria. This allows studying variations of b-values throughout the region and at different stages of the seismic cycle. With a standard gridding technique b-value maps, depth sections and time series were calculated which reveal a very heterogeneous b-value distribution in the study area. The variety of b-value observations cannot be interpreted unambiguously, given that the b-value most likely depends on a combination of fault-zone characteristics like local stress conditions, heterogeneity of the crust and damage distribution. By presenting a comprehensive set of possible interpretations we point out that a biased discussion of the results towards stress or another individual parameter may lead to erroneous conclusions. Furthermore, the applied data discretization scheme influences the appearance of the final b-value distribution leading to potential misinterpretations. © 2017 Elsevier B.V. 2017 English 00401951 10.1016/j.tecto.2017.05.028 Tectonophysics 712-713 Elsevier B.V. 232-248 Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences, Section 4.2: Geomechanics and Rheology, Telegrafenberg, Potsdam, Germany; Free University Berlin, Dpt. of Earth Sciences, Malteserstrasse 74-100, Berlin, 12249, Germany Geophysics; Physics, B value; B-value analysis; Damage distribution; Data discretization; North Anatolian Fault Zone; Northwestern Turkey; Sea of Marmara; Turkey, Seismology, earthquake catalogue; earthquake hypocenter; earthquake magnitude; earthquake rupture; fault zone; seismicity; spatiotemporal analysis, Anatolia; Sea of Marmara; Turkey https://www.scopus.com/inward/record.uri?eid=2-s2.0-85020018377&doi=10.1016%2fj.tecto.2017.05.028&partnerID=40&md5=19c1df075981262c5667883dc537c16b cited By 8 C. Raub P. Martínez-Garzón G. Kwiatek M. Bohnhoff G. Dresen article Bohnhoff2017332 Discriminating between a creeping and a locked status of active faults is of central relevance to characterize potential rupture scenarios of future earthquakes and the associated seismic hazard for nearby population centres. In this respect, highly similar earthquakes that repeatedly activate the same patch of an active fault portion are an important diagnostic tool to identify and possibly even quantify the amount of fault creep. Here, we present a refined hypocentre catalogue for the Marmara region in northwestern Turkey, where a magnitude M up to 7.4 earthquake is expected in the near future. Based on waveform cross-correlation for selected spatial seismicity clusters, we identify two magnitude M ~ 2.8 repeater pairs. These repeaters were identified as being indicative of fault creep based on the selection criteria applied to the waveforms. They are located below the western part of the Marmara section of the North Anatolian Fault Zone and are the largest reported repeaters for the larger Marmara region. While the eastern portion of the Marmara seismic gap has been identified to be locked, only sparse information on the deformation status has been reported for its western part. Our findings indicate that the western Marmara section deforms aseismically to a substantial extent, which reduces the probability for this region to host a nucleation point for the pending Marmara earthquake. This is of relevance, since a nucleation of the Marmara event in the west and subsequent eastward rupture propagation towards the Istanbul metropolitan region would result in a substantially higher seismic hazard and resulting risk than if the earthquake would nucleate in the east and thus propagate westward away from the population centre Istanbul. © The Authors 2017. 2017 English 0956540X 10.1093/gji/ggx169 Geophysical Journal International 210 Oxford University Press 332-339 GFZ German Research Centre for Geosciences, Section 4.2 'Geomechanics and Rheology', Potsdam, D-14473, Germany; Department of Earth Sciences, Free University Berlin, Berlin, D-12249, Germany; Bestec GmbH, Landau, D-76829, Germany 1 Creep; Faulting; Geophysics; Hazards; Locks (fasteners); Nucleation; Seismic response; Seismology; Transform faults, Earthquake hazard; Metropolitan regions; North Anatolian Fault Zone; Northwestern Turkey; Nucleation points; Rupture propagation; Selection criteria; Waveform cross correlation, Earthquakes, active fault; creep; earthquake catalogue; earthquake magnitude; earthquake rupture; fault zone; seismic hazard; seismicity; transform fault, Marmara [Turkey]; Sea of Marmara; Turkey https://www.scopus.com/inward/record.uri?eid=2-s2.0-85037094140&doi=10.1093%2fgji%2fggx169&partnerID=40&md5=7e0ba7bff71e3d26320c08be28b95a14 cited By 39 M. Bohnhoff C. Wollin D. Domigall L. Küperkoch P. Martínez-Garzón G. Kwiatek P.E. Malin article Bohnhoff201719 The Marmara section of the North Anatolian Fault Zone (NAFZ) runs under water and is located less than 20 km from the 15-million-person population center of Istanbul in its eastern portion. Based on historical seismicity data, recurrence times forecast an impending magnitude M > 7 earthquake for this region. The permanent GONAF (Geophysical Observatory at the North Anatolian Fault) has been installed around this section to help capture the seismic and strain activity preceding, during, and after such an anticipated event. © Author(s) 2017. 2017 English 18168957 10.5194/sd-22-19-2017 Scientific Drilling 22 Copernicus GmbH 19-28 GFZ German Research Centre for Geosciences, Section 4.2 'Geomechanics and Rheology', Potsdam, 14473, Germany; Free University Berlin, Department of Earth Sciences, Berlin, 12249, Germany; AFAD Disaster and Emergency Management Presidency, Earthquake Department, Ankara, 06510, Turkey; Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokohama, 236-0001, Japan; UNAVCO, Boulder, CO 80301, United States Earthquakes; Geophysics, Historical seismicity; Istanbul; North Anatolian Fault; North Anatolian Fault Zone; Population centers; Recurrence time; Sea of Marmara; Under water, Observatories https://www.scopus.com/inward/record.uri?eid=2-s2.0-85020071261&doi=10.5194%2fsd-22-19-2017&partnerID=40&md5=e4e0d68689713ac4e38dd786936bedf6 cited By 13 M. Bohnhoff G. Dresen U. Ceken F.T. Kadirioglu R.F. Kartal T. Kilic M. Nurlu K. Yanik D. Acarel F. Bulut H. Ito W. Johnson P.E. Malin D. Mencin article Najdahmadi2016931 We image velocity contrast (bimaterial) interfaces along the Karadere Fault of the North Anatolian Fault Zone, toward the eastern part of the 1999 Izmit Mw 7.4 rupture in NW Turkey, using waveforms recorded by a local seismic network. Applying an automatic procedure for identification and picking of fault zone head waves (FZHW) and direct P arrivals, and manually revising the picks through particle motion analysis, we identify two different groups of FZHW as well as fault zone reflected waves (FZRW). The first group of FZHW has a moveout with respect to the direct P arrivals with distance traveled along the fault, indicating a deep bimaterial interface down to the base of the seismogenic crust with an average velocity contrast of ~3.4%. The second group of FZHW has a constant time difference from the direct P arrivals and is associated with a shallow local interface bounding a low-velocity damage zone or basin structure that extends to a depth of 4-5 km. While the first group of FZHW exists on the slower crustal block, the second group of FZHW and the FZRW are present generally on both sides of the fault. These phases add to the richness and complexity of the early P waveforms observed at stations close to a large fault. The relatively low velocity contrast across the Karadere Fault compared to values to the west may have helped stopping the Izmit rupture. ©2016. American Geophysical Union. All Rights Reserved. 2016 English 21699313 10.1002/2015JB012601 Journal of Geophysical Research: Solid Earth 121 Blackwell Publishing Ltd 931-950 Geomechanics and Rheology, Helmholtz-Centre Potsdam, GFZ German Centre for Geosciences, Potsdam, Germany; Institute of Geological Sciences, Free University Berlin, Berlin, Germany; Department of Earth Sciences, University of Southern California, Los Angeles, CA, United States 2 fault zone; North Anatolian Fault; P-wave; wave reflection; waveform analysis, Turkey https://www.scopus.com/inward/record.uri?eid=2-s2.0-84959097882&doi=10.1002%2f2015JB012601&partnerID=40&md5=c7c04b4e755d5f6c76d05371737e9269 cited By 26 B. Najdahmadi M. Bohnhoff Y. Ben-Zion article Bohnhoff2016147 Constraining the maximum likely magnitude of future earthquakes on continental transform faults has fundamental consequences for the expected seismic hazard. Since the recurrence time for those earthquakes is typically longer than a century, such estimates rely primarily on well-documented historical earthquake catalogs, when available. Here we discuss the maximum observed earthquake magnitudes along different sections of the North Anatolian Fault Zone (NAFZ) in relation to the age of the fault activity, cumulative offset, slip rate and maximum length of coherent fault segments. The findings are based on a newly compiled catalog of historical earthquakes in the region, using the extensive literary sources that exist owing to the long civilization record. We find that the largest M7.8-8.0 earthquakes are exclusively observed along the older eastern part of the NAFZ that also has longer coherent fault segments. In contrast, the maximum observed events on the younger western part where the fault branches into two or more strands are smaller. No first-order relations between maximum magnitudes and fault offset or slip rates are found. The results suggest that the maximum expected earthquake magnitude in the densely populated Marmara-Istanbul region would probably not exceed M7.5. The findings are consistent with available knowledge for the San Andreas Fault and Dead Sea Transform, and can help in estimating hazard potential associated with different sections of large transform faults. © 2016 The Authors. 2016 English 00401951 10.1016/j.tecto.2016.02.028 Tectonophysics 674 Elsevier B.V. 147-165 Helmholtz-Centre Potsdam GFZ German Centre for Geosciences, Telegrafenberg, Potsdam, 14473, Germany; Department of Earth Sciences, Free University Berlin, Malteser Strasse 74-100, Berlin, 12249, Germany; Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740, United States Earthquakes; Hazards; Seismology; Strike-slip faults; Structural geology; Transform faults, Continental transform; Earthquake magnitudes; Fault zone; Historical seismicity; North Anatolian Fault Zone, Fault slips, continental margin; earthquake catalogue; earthquake magnitude; fault zone; hazard assessment; North Anatolian Fault; San Andreas Fault; seismic source; seismicity; seismology; transform fault, Dead Sea; Istanbul [Turkey]; Marmara [Turkey]; Turkey https://www.scopus.com/inward/record.uri?eid=2-s2.0-84959922037&doi=10.1016%2fj.tecto.2016.02.028&partnerID=40&md5=b549247ada651bd06909b427f8ba32b9 cited By 67 M. Bohnhoff P. Martínez-Garzón F. Bulut E. Stierle Y. Ben-Zion article Raub2016912 Using the first dataset available from the downhole Geophysical Observatory of the North Anatolian Fault, we investigated near-surface seismic-wave propagation on the Tuzla Peninsula, Istanbul, Turkey. We selected a dataset of 26 seismograms recorded at Tuzla at sensor depths of 0, 71, 144, 215, and 288 m. To determine near-surface velocities and attenuation structures, the waveforms from all sensors were pairwise deconvolved and stacked. This produced low-noise empirical Green’s functions for each borehole depth interval. From the Green’s functions, we identified reflections from the free surface and a low-velocity layer between ∼90 and ∼140 m depth. The presence of a low-velocity zone was also confirmed by a sonic log run in the borehole. This structure, plus high near-surface P- and S-wave velocities of ∼3600–4100 and ∼1800 m=s, lead to complex interference effects between upgoing and downgoing waves. As a result, the determination of quality factors (Q) with standard spectral ratio techniques was not possible. Instead, we forward modeled the Green’s functions in the time domain to determine effective Q values and to refine our velocity estimates. The effective QPvalues for the depth intervals of 0–71, 0–144, 0–215, and 0–288 m were found to be 19, 35, 39, and 42, respectively. For the S waves, we obtained an effective QS of 20 in the depth interval of 0–288 m. Considering the assumptions made in our modeling approach, it is evident that these effective quality factors are biased by impedance contrasts between our observation points. Our results show that, even after correcting for a free-surface factor of 2, the motion at the surface was found to be 1.7 times greater than that at 71 m depth. Our efforts also illustrate some of the difficulties of dealing with site effects in a strongly heterogeneous subsurface. © 2016, Seismological Society of America. All rights reserved. Article 2016 English 00371106 10.1785/0120150216 Bulletin of the Seismological Society of America 106 Seismological Society of America 912 – 927 3 Istanbul [Turkey]; Turkey; Tuzla [Istanbul]; Seismic waves; Seismology; Shear waves; Time domain analysis; Wave propagation; Well logging; Impedance contrast; Interference effects; Istanbul , Turkey; Low velocity layers; Low velocity zones; North Anatolian Fault; Observation point; P- and S-wave velocities; Green function; North Anatolian Fault; P-wave; S-wave; seismic wave; seismogram; shallow water; wave propagation; wave velocity; Velocity https://www.scopus.com/inward/record.uri?eid=2-s2.0-85011960981&doi=10.1785%2f0120150216&partnerID=40&md5=993696c90cd17045892ceabb07420b66 Cited by: 9; All Open Access, Green Open Access Christina Raub Marco Bohnhoff Bojana Petrovic Stefano Parolai Kenan Yanik Recai Feyiz Kartal Tuğbay Kiliç article Bohnhoff2016132 The North Anatolian Fault Zone (NAFZ) in NW Turkey as one of the most active and best studied strike-slip faults provides a unique opportunity to study earthquake related relaxation processes through analyzing co- and postseismic deformation. We study the spatial and temporal distributions of seismicity related to the two consecutive 1999 M > 7 Izmit and Düzce earthquakes. A high-resolution aftershock catalogue including ~ 10,000 hypocenters extending along the combined rupture zone and extending from prior to the Izmit event to after the Düzce event is studied. Spatial and temporal distributions of events allow to identify distinct seismically active and inactive fault patches. Their location is related to the co- and postseismic deformation within and below the seismogenic layer, respectively. Four seismically inactive patches extending 30–50 km along the rupture zone and down to 10 km depth are identified with a systematic spatial shift between them introduced by the Düzce mainshock. The cumulative distribution of sub-areas hosting coseismic slip, aftershock clusters and postseismic creep shows that the entire upper (seismogenic) and lower (ductile) portions of the crust along the combined Izmit and Düzce rupture zone are activated between rupture initiation and a two-year postseismic period. This observation was only achieved due to the subsequent occurrence of two adjacent M > 7 strike-slip earthquakes in combination with a distinct local seismic and geodetic monitoring. Our findings suggest that a coseismically introduced lateral and vertical slip deficit is systematically compensated postseismically in both the brittle and ductile portions of the crust. © 2016 Elsevier B.V. 2016 English 00401951 10.1016/j.tecto.2016.07.029 Tectonophysics 686 Elsevier B.V. 132-145 Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences, Section 4.2: Geomechanics and Rheology, Telegrafenberg, Potsdam, Germany; Free University Berlin, Department of Earth Sciences, Malteser Strasse 74-100, Berlin, 12249, Germany Creep; Strike-slip faults, Earthquake dynamics; NW Turkey; Postseismic creeps; Seismic cycle; Seismicity; Seismicity and tectonics; Seismotectonics; Spatial analysis, Earthquakes, coseismic process; creep; earthquake rupture; Kocaeli earthquake 1999; postseismic process; seismicity; seismotectonics; slip rate; spatial analysis; spatial distribution; spatial variation; temporal distribution; temporal variation, Duzce [Turkey]; Izmit; Kocaeli [Turkey]; Turkey https://www.scopus.com/inward/record.uri?eid=2-s2.0-84983027911&doi=10.1016%2fj.tecto.2016.07.029&partnerID=40&md5=e6cc3cd34b48f65f1811a02c0c94fd72 cited By 15 M. Bohnhoff M. Ickrath G. Dresen article Ickrath20152120 We investigate spatiotemporal variations of the crustal stress field orientation along the rupture zones of the 1999 August Izmit Mw 7.4 and November Düzce Mw 7.1 earthquakes at the North Anatolian Fault zone (NAFZ) in NW Turkey. Our primary focus is to elaborate on the relation between the state of the crustal stress field and distinct seismotectonic features as well as variations of coseismic slip within the seismogenic layer of the crust. To achieve this, we compile an extensive data base of hypocentres and first-motion polarities including a newly derived local hypocentre catalogue extending from 2 yr prior (1997) to 2 yr after (2001) the Izmit and Düzce main shocks. This combined data set allows studying spatial and temporal variations of stress field orientation along distinct fault segments for the pre- and post-seimic phase of the two large earthquakes in detail. Furthermore, the occurrence of two M &gt; 7 earthquakes in rapid succession gives the unique opportunity to analyse the 87-d-long 'inter-seismic phase' between them. We use the MOTSI (first MOTion polarity Stress Inversion) procedure directly inverting first-motion polarities to study the stress field evolution of nine distinct segments. In particular, this allows to determine the stress tensor also for the pre- and post-seismic phases when no stable single-event focal mechanisms can be determined. We observe significantly different stress field orientations along the combined 200-km-long rupture in accordance with lateral variations of coseismic slip and seismotectonic setting. Distinct vertical linear segments of the NAFZ show either pure-strike slip behaviour or transtensional and normal faulting if located near pull-apart basins. Pull-apart structures such as the Akyazi and Düzce basins show a predominant normal faulting behaviour along the NAFZ and reflect clearly different characteristic from neighbouring strike-slip segments. Substantial lateral stress field heterogeneity following the two main shocks is observed that declines with time towards the post-seismic period that rather reflects the regional right-lateral strike-slip stress field. © The Authors 2015. Published by Oxford University Press on behalf of The Royal Astronomical Society. All rights reserved. 2015 English 0956540X 10.1093/gji/ggv273 Geophysical Journal International 202 Oxford University Press 2120-2132 Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences, Section 3.2: Geomechanics and Rheology, Telegrafenberg, Potsdam, Germany; Free University Berlin, Department of Earth Sciences, Malteser Strasse 74-100, Berlin, D-12249, Germany; AFAMResearch Center, IstanbulAydinUniversity, Istanbul, 34295, Turkey 3 Earthquake effects; Earthquakes; Faulting; Geophysics; Seismology; Stresses; Strike-slip faults, Earthquake dynamics; Earthquake source observations; North Anatolian Fault Zone; Seismicity and tectonics; Seismotectonic settings; Spatial and temporal variation; Spatio-temporal variation; Stress-field orientation, Fault slips, coseismic process; earthquake catalogue; earthquake rupture; focal mechanism; inverse problem; Kocaeli earthquake 1999; seismic data; seismic source; seismicity; spatiotemporal analysis; stress field; tectonics, Anatolia; Turkey https://www.scopus.com/inward/record.uri?eid=2-s2.0-84940093600&doi=10.1093%2fgji%2fggv273&partnerID=40&md5=e76b0dccf85e559e1fa20ceefe00487f cited By 14 M. Ickrath M. Bohnhoff G. Dresen P. Martínez-Garzón F. Bulut G. Kwiatek O. Germer article Prevedel20151537 Downhole sensors of different types and in various environments provide substantial benefit to signal quality. They also add the depth dimension to measurements performed at the Earths’ surface. Sensor types that particularly benefit from downhole installation due to the absence of near-surface noise include piezometers, seismometers, strainmeters, thermometers, and tiltmeters. Likewise, geochemical and environmental measurements in a borehole help eliminate near-surface weathering and cultural effects. Installations from a few hundred meter deep to a few kilometer deep dramatically reduce surface noise levels—the latter noticeably also reduces the hypocentral distance for shallow microearthquakes. The laying out of a borehole network is always a compromise of local boundary conditions and the involved drilling costs. The installation depth and procedure for a long-term downhole observatory can range from time limited installations, with a retrieval option, to permanently cemented sensors. Permanently cemented sensors have proven to be long-term stable with non-deteriorating coupling and borehole integrity. However, each type needs to be carefully selected and planned according to the research aims. A convenient case study is provided by a new installation of downhole seismometers along the shoreline of the eastern Marmara Sea in Turkey. These stations are being integrated into the regional net for monitoring the North Anatolian Fault Zone. Here we discuss its design, installation, and first results. We conclude that, despite the logistical challenges and installation costs, the superior quality of downhole data puts this technique at the forefront of applied and fundamental research. © 2015, The Author(s). 2015 English 14373254 10.1007/s00531-015-1147-5 International Journal of Earth Sciences 104 Springer Verlag 1537-1547 Helmholtz-Centre Potsdam GFZ German Centre for Geosciences, Telegrafenberg, Potsdam, 14473, Germany; Department of Earth Sciences, Free University Berlin, Malteser Strasse 74-100, Berlin, 12249, Germany; AFAD, Disaster and Emergency Management Presidency, Ankara, Turkey; IESE, University of Aukland, Aukland, New Zealand; TÜBITAK Marmara Research Center, Gebze, Turkey 6 earthquake hypocenter; earthquake mechanism; fault zone; microearthquake; North Anatolian Fault; piezometer; seismic noise; seismograph, Sea of Marmara; Turkey https://www.scopus.com/inward/record.uri?eid=2-s2.0-84940954063&doi=10.1007%2fs00531-015-1147-5&partnerID=40&md5=f3429336fb115bd9d5a14f7e0d10c2d3 cited By 12 B. Prevedel F. Bulut M. Bohnhoff C. Raub R.F. Kartal F. Alver P.E. Malin article Stierle20141878 We study potential non-double-couple (non-DC) components in aftershocks of the 1999 Izmit earthquake. The Izmit earthquake ruptured a ~140-km-long segment of the North Anatolian Fault Zone in northwestern Turkey and was followed by the Mw = 7.1 Düzce earthquake that extended the rupture further to the east. Focal mechanisms of Izmit aftershocks clearly indicate a segmentation of the rupture into several segments, one of which is the Akyazi Plain, a pull-apart structure, where significant non-DC components might be observed. The analysed earthquake catalogue containswaveforms ofmore than 4000 accurately located events observed at 35 three-component short-period seismic stations. To ensure high-quality datawith good focal coverage, we apply strict quality criteria to the aftershock catalogue reducing the number of events to only 33 aftershocks for which stable moment tensors were calculated using P-and S-wave amplitudes. The moment tensors of the 33 analysed aftershocks display significant differences in the percentage of the non-DC components for the three distinct fault segments: the Izmit-Sapanca, Karadere-Düzce and the Akyazi segments. Events located in the Izmit-Sapanca and Karadere-Düzce segments exhibit only small percentages of the non-DC components and if existent they are mainly positive. This correlates well with the predominant strike-slip stress regime along these segment and also with the main shock rupture being rightlateral strike-slip. In contrary, we found a substantial percentage of non-DC components for events below the Akyazi Plain where the Sapanca Fault splits into the Mudurnu and Karadere faults. There, the observed non-DC components are entirely positive indicating a tensional regime and ranging from 20 to 48 per cent, clearly exceeding the defined error bounds found in a synthetic study. This observation is in accordance with the post-seismic setting following the Izmit main shock that left a remarkable slip deficit of 3.5 m below the Akyazi bend. 2014 English 0956540X 10.1093/gji/ggt503 Geophysical Journal International 196 1878-1888 Potsdam German Research Centre for Geosciences (GFZ), Telegrafenberg, D-14473 Potsdam, Germany; Department of Earth Sciences, Free University Berlin, Malteser Strasse 74-100, D-12249 Berlin, Germany; Institute of Geophysics, Academy of Sciences of the Czech Republic, Boční II/1401, CZ-14131 Prague, Czech Republic 3 https://www.scopus.com/inward/record.uri?eid=2-s2.0-84894091998&doi=10.1093%2fgji%2fggt503&partnerID=40&md5=4f316585dbf98463bfc931260dae643b cited By 43 E. Stierle M. Bohnhoff V. Vavryčuk article Stierle20141869 We perform a detailed synthetic study on the resolution of non-double-couple (non-DC) components in the seismic moment tensors from short-period data observed at regional networks designed typically for monitoring aftershock sequences of large earthquakes. In addition, we test two different inversion approaches-a linear full moment tensor inversion and a nonlinear moment tensor inversion constrained to a shear-tensile source model. The inversions are applied to synthetic first-motion P- and S-wave amplitudes, which mimic seismic observations of aftershocks of the 1999 Mw = 7.4 Izmit earthquake in northwestern Turkey adopting a shear-tensile source model. To analyse the resolution capability for the obtained non-DC components inverted, we contaminate synthetic amplitudes with random noise and incorporate realistic uncertainties in the velocity model as well as in the hypocentre locations. We find that the constrained moment tensor inversion yields significantly smaller errors in the non-DC components than the full moment tensor inversion. In particular, the errors in the compensated linear vector dipole (CLVD) component are reduced if the constrained inversion is applied. Furthermore, we show that including the S-wave amplitudes in addition to P-wave amplitudes into the inversion helps to obtain reliable non-DC components. For the studied station configurations, the resolution remains limited due to the lack of stations with epicentral distances less than 15 km. Assuming realistic noise in waveform data and uncertainties in the velocity model, the errors in the non-DC components are as high as ±15 per cent for the isotropic and CLVD components, respectively, thus being non-negligible in most applications. However, the orientation of P- and T-axes is well determined even when errors in the modelling procedure are high. © The Authors 2014. Published by Oxford University Press on behalf of The Royal Astronomical Society. 2014 English 0956540X 10.1093/gji/ggt502 Geophysical Journal International 196 1869-1877 Helmholtz-Centre, Potsdam German Research Centre for Geosciences (GFZ), Telegrafenberg, D-14473 Potsdam, Germany; Institute of Geophysics, Academy of Sciences of the Czech Republic, Bocni II/1401, Cz-14131 Prague, Czech Republic; Department of Earth Sciences, Free University Berlin, Malteser Strasse 74-100, D-12249 Berlin, Germany 3 https://www.scopus.com/inward/record.uri?eid=2-s2.0-84894083144&doi=10.1093%2fgji%2fggt502&partnerID=40&md5=b9a33e74396b368bade08bede4eff29d cited By 55 E. Stierle V. Vavryčuk J. Šílený M. Bohnhoff article Acarel20141954 We analyze the ambient seismic-noise field in order to investigate the crustal structure at the North Anatolian fault zone (NAFZ) in northwest Turkey. We focus on the eastern Sea of Marmara section, where the NAFZ is in the final phase of the seismic cycle prior to an expected major (M >7) earthquake. We apply crosscorrelation analysis of the seismic ambient noise to determine the spectral dependence of the seismic velocity in order to image the crustal structure at seismogenic depth. Time-domain cross correlations are calculated for available station pairs in the target area. Interstation distances span 0.3-90 km. Here, the vertical component is analyzed in order to recover fundamental-mode Rayleigh waves in the 0.05-1.1 Hz frequency range. Group velocity dispersion curves are obtained for selected correlation paths in particular to address the azimuthal dependence of the velocity field. In the frequency band of interest, average group velocities range between ~1:8 and 3:5 km=s. Dispersion curves corresponding to the north-south-trending paths crossing the main NAFZ fault branch below the eastern Sea of Marmara show low group velocities between ~1:5 and 1:8 km=s, which is well explained by the 3-4 km-deep Çi{dotless}narci{dotless}k basin, located between the two major fault branches, the Princes Islands and Armutlu fault segments. In contrast, ray paths restricted to within the mainland of Istanbul and the Armutlu peninsulas (primarily trending east-west) show higher group velocities up to 3:2 km=s. By averaging the dispersion curves, we determine an optimized 1D S-wave velocity model for the eastern Sea of Marmara region, allowing for a significant improvement in hypocenter determination for local seismicity. 2014 English 00371106 10.1785/0120130160 Bulletin of the Seismological Society of America 104 Seismological Society of America 1954-1963 GFZ German Research Center for Geosciences, Section 3.2 Geomechanics and Rheology, Telegrafenberg, 14473 Potsdam, Germany; Free University Berlin, Institute of Geological Sciences, Berlin, Germany 4 Acoustic noise; Frequency bands; Group velocity dispersion; Light velocity; Wave propagation, Azimuthal dependence; Cross correlations; Cross-correlation analysis; Group velocity dispersion curve; Low group velocity; North Anatolian Fault Zone; Seismic velocities; Spectral dependences, Earthquakes, ambient noise; crustal structure; dispersion; earthquake hypocenter; North Anatolian Fault; S-wave; seismic noise; seismic velocity; seismicity; spectral analysis, Sea of Marmara; Turkey https://www.scopus.com/inward/record.uri?eid=2-s2.0-84905675453&doi=10.1785%2f0120130160&partnerID=40&md5=63b88298b14aa2ce2073ba76568e77ba cited By 1 D. Acarel F. Bulut M. Bohnhoff article Bohnhoff2013 Over the last century the North Anatolian Fault Zone in Turkey has produced a remarkable sequence of large earthquakes. These events have now left an earthquake gap south of Istanbul and beneath the Marmara Sea, a gap that has not been filled for 250 years. Here we investigate the nature of the eastern end of this gap using microearthquakes recorded by seismographs primarily on the Princes Islands offshore Istanbul. This segment lies at the western terminus of the 1999 Mw7.4 Izmit earthquake. Starting from there, we identify a 30-km-long fault patch that is entirely aseismic down to a depth of 10 km. Our evidence indicates that this patch is locked and is therefore a potential nucleation point for another Marmara segment earthquake - a potential that has significant natural hazards implications for the roughly 13 million Istanbul residents immediately to its north. © 2013 Macmillan Publishers Limited. All rights reserved. 2013 English 20411723 10.1038/ncomms2999 Nature Communications 4 Helmholtz-Centre Potsdam German Centre for Geosciences GFZ, Telegrafenberg, 14473 Potsdam, Germany; Department of Earth Sciences, Freie Universitat Berlin, Malteser Strasse 74-100, 12249 Berlin, Germany; Institute of Earth Science and Engineering, University of Auckland, Auckland, New Zealand; Kandilli Observatory and Earthquake Research Institute, Bogazici University, 34342 Bebek, Cengelköy, Istanbul, Turkey active fault; earthquake event; fault zone; Kocaeli earthquake 1999; microearthquake; nucleation; seismic hazard; seismograph, article; biological accident; correlation analysis; earthquake; Turkey (republic), Earthquakes; Geography; Humans; Islands; Oceans and Seas; Turkey, Istanbul [Turkey]; Sea of Marmara; Turkey https://www.scopus.com/inward/record.uri?eid=2-s2.0-84879664833&doi=10.1038%2fncomms2999&partnerID=40&md5=2f690a02f732647cb1810f8df6b91ffd cited By 94 M. Bohnhoff F. Bulut G. Dresen P.E. Malin T. Eken M. Aktar article Eken2013911 The North Anatolia Fault Zone (NAFZ) is a transform zone 1600 km in length representing the plate boundary between the westward moving Anatolian Plate and stable Eurasia. Almost the entire fault zone has failed during the last century except for the Sea of Marmara section, which is located in direct vicinity to the city of Istanbul. In this study, we investigate the crustal anisotropy along the eastern Marmara section of the NAFZ based on shear-wave splitting. We measure seismic anisotropy parameters, namely, the fast polarization direction (PD) and time delay (TD), by analyzing local seismicity recorded at selected seismographs operated throughout the eastern Sea of Marmara region. Our shear-wave splitting (SWS) observations indicate a predominant northwest-southeast-oriented PD, which is subparallel to both the orientation of the regional SHmax in northwest Turkey and the local NAFZ strike along the Princes' Islands segment. Toward the south, at the Armutlu Peninsula, we find a different PD pattern reflecting local fault strikes, SHmax as well as strain asymmetry between different crustal blocks across the fault zone. Applying strict quality criteria enables us to identify robust, preferred fast PDs, which suggests that initially observed PD heterogeneities are sometimes caused by second-order effects in the data rather than by varying PDs. Comparing TD and hypocentral depth distribution, we find the depth extent of the anisotropy is confined to the uppermost 10-km depth of crust. We combine our SWS results with those from previous studies conducted along the San Andreas fault (SAF) and NAFZ in order to investigate the relation of angular deviations of the PDs from regional SHmax and local fault strikes with fault-zone distance. We find that fast PDs are mainly controlled by the local fault structure in close proximity to a fault zone (5 and 10 km) while they are controlled by crustal stress at off-fault locations (5 and 10 km). 2013 English 00371106 10.1785/0120120156 Bulletin of the Seismological Society of America 103 911-924 Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Potsdam D-14473, Germany; Bogazici University, Kandilli Observatory and Earthquake Research Institute, Istanbul, 34684, Turkey 2 A Angular deviations; Crustal anisotropy; Hypocentral depth; Northwestern Turkey; Polarization direction; Second order effect; Seismic anisotropy; Shear wave splitting, Shear waves; Strike-slip faults, Anisotropy, anisotropy; crustal structure; fault zone; North Anatolian Fault; plate boundary; San Andreas Fault; seismicity; wave splitting, Istanbul [Istanbul (PRV)]; Istanbul [Turkey]; Sea of Marmara; Turkey https://www.scopus.com/inward/record.uri?eid=2-s2.0-84875465771&doi=10.1785%2f0120120156&partnerID=40&md5=0fe27ce1d0c193ddd4efa8ca26f658ea cited By 23 T. Eken M. Bohnhoff F. Bulut B. Can M. Aktar article Ickrath2013951 Local rotations of the stress field might serve as an indicator to characterize the physical status of individual fault segments during the seismic cycle. In this study we focus on the pre-, 2-month aftershock- and post-seismic phase of the 1999 Mw7.4 Izmit earthquake in northwestern Turkey. Using a compilation of focal mechanism data we investigate spatiotemporal changes of the stress field orientations and find distinct variations along individual fault segments. Whereas the regional stress field prior to the Izmit earthquake and following the 2-month aftershock sequence reflects a stable strike-slip regime, the early aftershock period is dominated by EW-extension below the Akyazi Basin. During the 2-month aftershock period we find significant changes from strike-slip to normal-faulting during the main shock following by a systematic backrotation to the pre-main shock stress regime. This backrotation commences first in the Akyazi Plain hosting a co-seismic slip deficit of ≤3m and propagates then further to the east towards the Karadere and Düzce faults where the Düzce Mw 7.1 main shock nucleated 87 d later. Our results confirm that spatiotemporal stress field rotations are a useful indicator for variations of the seismotectonic setting during the seismic cycle. © The Authors 2013. Published by Oxford University Press on behalf of The Royal Astronomical Society. 2013 English 0956540X 10.1093/gji/ggt409 Geophysical Journal International 196 951-956 Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences Section 3.2, Geomechanics and Rheology, Telegrafenberg, D-14473 Potsdam, Germany; Free University Berlin, Department of Earth Sciences, Malteser Strasse 74-100, 12249 Berlin, Germany 2 Earthquake dynamics; Regional stress field; Seismic cycle; Seismicity and tectonics; Seismotectonic settings; Spatial analysis; Spatio-temporal changes; Stress-field orientation, Fault slips; Stresses; Transform faults, Earthquakes, aftershock; coseismic process; focal mechanism; Kocaeli earthquake 1999; normal fault; seismicity; seismotectonics; spatial analysis; stress field; strike-slip fault; transform fault, Akyazi; Sakarya; Turkey https://www.scopus.com/inward/record.uri?eid=2-s2.0-84892472760&doi=10.1093%2fgji%2fggt409&partnerID=40&md5=00d3b3f76d7a774fc204ae814a4a403d cited By 19 M. Ickrath M. Bohnhoff F. Bulut G. Dresen article Bulut201217 We present results on imaging contrast of seismic velocities across the Mudurnu segment of the North Anatolian Fault Zone (NAFZ) in northwestern Turkey with polarization analysis of early P waveforms generated by near-fault seismicity and recorded by near-fault stations. The analysis uses changes in motion polarity from fault-normal to source-receiver directions to identify early-arriving fault zone head waves on the slow side of the fault, and measure the arrival times of the head and direct P waves. The moveout between the head and direct waves with increasing source-receiver distance along the fault provides an estimate of the average contrast of seismic velocities across the fault. The results indicate that the average contrast of P wave velocities across the Mudurnu segment of the NAFZ is at least 6%, with the south block being the faster side. The findings provide a basis for deriving improved event locations, focal mechanisms and estimated shaking hazard associated with earthquakes on the fault. The analysis technique can be used in other fault zones monitored with sparse seismic instrumentation. © 2012 Elsevier B.V.. 2012 English 0012821X 10.1016/j.epsl.2012.02.001 Earth and Planetary Science Letters 327-328 17-22 Helmholtz-Zentrum Potsdam, Deutsches Geo ForschungsZentrum, Telegrafenberg, 14473 Potsdam, Germany; Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740, United States Analysis techniques; Arrival time; Bi-material interfaces; Direct waves; Event location; Fault zone; Focal mechanism; Head waves; Imaging contrast; Moveout; Near-fault; North Anatolian Fault; North Anatolian Fault Zone; Northwestern Turkey; P waves; P-wave velocity; Polarization analysis; Seismic instrumentation; Seismic velocities; Wave forms, Polarization, Seismic waves, fault zone; focal mechanism; hazard assessment; imaging method; monitoring; normal fault; North Anatolian Fault; P-wave; seismic source; seismic velocity; seismicity; wave velocity; waveform analysis, Turkey https://www.scopus.com/inward/record.uri?eid=2-s2.0-84857467076&doi=10.1016%2fj.epsl.2012.02.001&partnerID=40&md5=fc1c425575c5cca551624355b5b2a752 cited By 47 F. Bulut Y. Ben-Zion M. Bohnhoff article Bulut20111759 We investigate earthquakes with similar waveforms in order to characterize spatiotemporal microseismicity clusters within the North Anatolian fault zone (NAFZ) in northwest Turkey along the transition between the 1999 İzmit rupture zone and the Marmara Sea seismic gap. Earthquakes within distinct activity clusters are relocated with cross-correlation derived relative travel times using the double difference method. The spatiotemporal distribution of micro earthquakes within individual clusters is resolved with relative location accuracy comparable to or better than the source size. High-precision relative hypocenters define the geometry of individual fault patches, permitting a better understanding of fault kinematics and their role in local-scale seismotectonics along the region of interest. Temporal seismic sequences observed in the eastern Sea of Marmara region suggest progressive failure of mostly nonoverlapping areas on adjacent fault patches and systematic migration of microearthquakes within clusters during the progressive failure of neighboring fault patches. The temporal distributions of magnitudes as well as the number of events follow swarmlike behavior rather than a mainshock/aftershock pattern. 2011 English 00371106 10.1785/0120100215 Bulletin of the Seismological Society of America 101 1759-1768 Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Telegrafenberg, Haus D, Potsdam D-14473, Germany; U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025-3591, United States; Boǧaziçi University, Kandilli Observatory and Earthquake Research Institute, 34342 Bebek, Çengelköy, Istanbul 34684, Turkey 4 Cross correlations; Double differences; Fault kinematics; High-precision; Mainshock; Marmara Sea; Micro-earthquakes; Microseismicity; North Anatolian Fault Zone; Northwest Turkey; Progressive failure; Region of interest; Relative location; Rupture zone; Seismic sequence; Seismotectonics; Source sizes; Spatiotemporal distributions; Temporal distribution; Travel time; Wave forms, Image segmentation, Earthquakes, accuracy assessment; aftershock; earthquake catalogue; earthquake hypocenter; earthquake magnitude; earthquake mechanism; earthquake rupture; earthquake swarm; fault zone; Kocaeli earthquake 1999; microearthquake; spatiotemporal analysis; tectonic setting, Anatolia; Sea of Marmara; Turkey https://www.scopus.com/inward/record.uri?eid=2-s2.0-79961157143&doi=10.1785%2f0120100215&partnerID=40&md5=93cbd4130014d0e7a040ea945f708b7c cited By 18 F. Bulut W.L. Ellsworth M. Bohnhoff M. Aktar G. Dresen article Hergert2010132 The slip rate along a fault controls the accumulation of strain that is eventually released during an earthquake. Along a 150-km-long stretch of the North Anatolian fault near Istanbul, Turkey, strain has been building up 2 since the last large earthquake in 1766. Estimates of the geodetic slip rates along the main Marmara fault vary widely, ranging between 17 and 27.9 mm yr-1 (refs 2-5). This slip rate is difficult to quantify because of the lack of satellite observations offshore and the complexity of the submarine fault system that includes the main Marmara fault2,6,7. Here we estimate the right-lateral slip rate on the main Marmara fault using a three-dimensional geomechanical model that incorporates these structural complexities. From our simulations we infer slip rates between 12.8 and 17.8 mm yr-1; our estimates are smaller and more variable than previous results, primarily because of slip partitioning and internal deformation. Our model results reconcile geodetic observations and geological fault slip rates8-10, which had been considered conflicting previously. We suggest that the inferred variability in slip rate on the main Marmara fault favours segmented release of seismic moment during consecutive events over the failure of the whole seismic gap in one large earthquake. © 2010 Macmillan Publishers Limited. All rights reserved. 2010 English 17520894 10.1038/ngeo739 Nature Geoscience 3 132-135 Geophysical Institute, Universität Karlsruhe (TH), Hertzstr. 16, 76187 Karlsruhe, Germany; GFZ German Research Centre for Geosciences, Telegrafenberg, 14773, Potsdam, Germany 2 accumulation rate; analytical method; earthquake; fault displacement; satellite imagery; slip rate; three-dimensional modeling, Sea of Marmara; Turkey https://www.scopus.com/inward/record.uri?eid=2-s2.0-76449107435&doi=10.1038%2fngeo739&partnerID=40&md5=a528688a3102d705fbe4fff469519d1c cited By 72 T. Hergert O. Heidbach article Görgün2010170 We investigate aftershock focal mechanisms along the eastern part of the Izmit Mw = 7.4 August 17, 1999 rupture zone during the time period August 22, 1999-October 17, 1999. Two spatial clusters of aftershock activity are analyzed representing the Karadere Fault (KF) and the Düzce Area (DA). Based on an aftershock hypocenter catalogue restricted to events with horizontal and vertical errors &lt; 2 km, we determine fault plane solutions for 221 events. The high number of focal mechanisms at the eastern Izmit rupture zone could be determined only due to the low magnitude-detection threshold of the seismic network and allows to resolve the local deformation pattern with unprecedented precision. Focal mechanisms along the Karadere Fault allow us to identify dominantly dextral strike-slip mechanisms with normal faulting components on NE-SW trending fault planes. Focal mechanisms in the Düzce Area predominantly exhibit NE-SW extensional normal faulting but also a substantial part of strike-slip faulting. Further subdivision of the data set slightly decreases for the misfit for deeper (z &gt; 10 km) events. North and east of the easternmost end of the Karadere Fault we observe a high variance in stress field orientation correlated with lower b-values. While the Karadere Fault reflects a predominant dextral strike-slip regime with normal faulting components, the Düzce Area further to the East that also hosted the forthcoming Mw = 7.2 mainshock 87 days after the Izmit earthquake can be subdivided into a dominantly NE-SW extensional normal faulting regime below the Düzce Basin (DB) and a first-order strike-slip regime along the western Düzce Fault (DF). We conclude that the Düzce Basin was set under tension by the Izmit rupture and partly released the slip deficit by extensional faulting on Karadere Fault parallel to the coseismic displacement. At the same time this area and in particular the Düzce Fault that bounds the Düzce Basin to the south reflects mostly strike-slip events representing a major asperity along the North Anatolian Fault Zone (NAFZ) before initiating the Düzce rupture 87 days after the Izmit event. © 2009 Elsevier B.V. All rights reserved. 2010 English 00401951 10.1016/j.tecto.2009.07.012 Tectonophysics 482 170-181 Helmholtz Centre Potsdam GFZ, German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany 1-4 Fault plane solutions; Izmit earthquakes; North Anatolian Fault Zone; Seismotectonics; Stress tensors, Earthquakes; Focusing; Tensors, Aircraft accidents, aftershock; earthquake catalogue; earthquake hypocenter; earthquake magnitude; earthquake rupture; extensional tectonics; fault zone; faulting; focal mechanism; seismotectonics; spatial variation; strike-slip fault; tectonic setting, Anatolia; Duzce; Izmit; Kocaeli [Turkey]; Turkey https://www.scopus.com/inward/record.uri?eid=2-s2.0-76349083599&doi=10.1016%2fj.tecto.2009.07.012&partnerID=40&md5=bca1fbb017f2edfd50d35ad5be681802 cited By 26 E. Görgün M. Bohnhoff F. Bulut G. Dresen article bohnhoff2010passive 2010 New frontiers in integrated solid earth sciences Springer 261--285 Marco Bohnhoff Georg Dresen William L Ellsworth Hisao Ito article Görgün2009507 We investigate spatial clustering of 2414 aftershocks along the Izmit Mw = 7.4 August 17, 1999 earthquake rupture zone. 25 days prior to the Düzce earthquake Mw = 7.2 (November 12, 1999), we analyze two spatial clusters, namely Sakarya (SC) and Karadere-Düzce (KDC). We determine the earthquake frequency-magnitude distribution (b-value) for both clusters. We find two high b-value zones in SC and one high b-value zone in KDC which are in agreement with large coseismic surface displacements along the Izmit rupture. The b-values are significantly lower at the eastern end of the Izmit rupture where the Düzce mainshock occurred. These low b-values at depth are correlated with low postseismic slip rate and positive Coloumb stress change along KDC. Since low b-values are hypothesized with high stress levels, we propose that at the depth of the Düzce hypocenter (12.5 km), earthquakes are triggered at higher stresses compared to shallower crustal earthquake. The decrease in b-value from the Karadere segment towards the Düzce Basin supports this low b-value high stress hypothesis at the eastern end of the Izmit rupture. Consequently, we detect three asperity regions which are correlated with high b-value zones along the Izmit rupture. According to aftershock distribution the half of the Düzce fault segment was active before the 12 November 1999 Düzce mainshock. This part is correlated with low b-values which mean high stress concentration in the Düzce Basin. This high density aftershock activity presumably helped to trigger the Düzce event (Mw = 7.2) after the Izmit Mw 7.4 mainshock. © 2009 Elsevier B.V. All rights reserved. 2009 English 00401951 10.1016/j.tecto.2009.04.027 Tectonophysics 474 507-515 Helmholtz Centre Potsdam GFZ, German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany 3-4 Aftershock distributions; B value; Crustal earthquakes; Earthquake frequency; Earthquake rupture; Fault asperities; Gutenberg-Richter law; High density; High stress; High stress concentration; Izmit earthquake; Mainshock; North Anatolian Fault Zone; Postseismic slip; Spatial cluster; Spatial clustering; Stress changes; Surface displacement, Earthquake effects; Stress concentration, Laws and legislation, aftershock; coseismic process; displacement; earthquake magnitude; earthquake rupture; earthquake trigger; fault zone; Kocaeli earthquake 1999; low velocity zone; North Anatolian Fault; postseismic process; slip rate; stress change, Duzce; Eurasia; Turkey https://www.scopus.com/inward/record.uri?eid=2-s2.0-69449085751&doi=10.1016%2fj.tecto.2009.04.027&partnerID=40&md5=c3d6f5d8ec9dbe8eaf6875a3f9de86f3 cited By 27 E. Görgün A. Zang M. Bohnhoff C. Milkereit G. Dresen article Bulut2009 The North Anatolian Fault Zone (NAFZ) below the Sea of Marmara forms a "seismic gap" where a major earthquake is expected to occur in the near future. This segment of the fault lies between the 1912 Ganos and 1999 İzmit ruptures and is the only NAFZ segment that has not ruptured since 1766. To monitor the microseismic activity at the main fault branch offshore of Istanbul below the Çinarcik Basin, a permanent seismic array (PIRES) was installed on the two outermost Prince Islands, Yassiada and Sivriada, at a few kilometers distance to the fault. In addition, a temporary network of ocean bottom seismometers was deployed throughout the Çinarcik Basin. Slowness vectors are determined combining waveform cross correlation and P wave polarization. We jointly invert azimuth and traveltime observations for hypocenter determination and apply a bootstrap resampling technique to quantify the location precision. We observe seismicity rates of 20 events per month for M < 2.5 along the basin. The spatial distribution of hypocenters suggests that the two major fault branches bounding the depocenter below the Çinarcik Basin merge to one single master fault below ∼17 km depth. On the basis of a cross-correlation technique we group closely spaced earthquakes and determine composite focal mechanisms implementing recordings of surrounding permanent land stations. Fault plane solutions have a predominant right-lateral strike-slip mechanism, indicating that normal faulting along this part of the NAFZ plays a minor role. Toward the west we observe increasing components of thrust faulting. This supports the model of NW trending, dextral strike-slip motion along the northern and main branch of the NAFZ below the eastern Sea of Marmara. Copyright 2009 by the American Geophysical Union. 2009 English 21699313 10.1029/2008JB006244 Journal of Geophysical Research: Solid Earth 114 Blackwell Publishing Ltd Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Section 3.2, Geomechanics and Rheology, Telegrafenberg D424, D-14473 Potsdam, Germany; U.S. Geological Survey, MS 977, 345 Middlefield Road, Menlo Park, CA 94025, United States; Kandilli Observatory and Earthquake Research Institute, Boǧaziçi University, 34680, Çengelköy, Istanbul, Turkey 9 azimuth; depocenter; earthquake hypocenter; earthquake mechanism; earthquake rupture; fault geometry; fault zone; focal mechanism; observational method; P-wave; seismic tomography; seismicity; seismograph; spatial distribution; strike-slip fault; tectonic setting; travel time; waveform analysis, Anatolia; Eurasia; Istanbul [Turkey]; Sea of Marmara; Turkey https://www.scopus.com/inward/record.uri?eid=2-s2.0-72049131273&doi=10.1029%2f2008JB006244&partnerID=40&md5=75acb05f6c4b9b422b937adb48b0d64f cited By 45 F. Bulut M. Bohnhoff W.L. Ellsworth M. Aktar G. Dresen article Janssen200911 We analyzed the paleostress field, ongoing deformation, meso- to micro-scale faulting, cataclasis, fault rock alteration and veining within turbidite and limestone sequences at the Ganos Fault which represent a major branch of the North Anatolian Fault Zone in NW Turkey. Fault damage was found to occur across a several kilometers wide zone. Effects of faulting are shown by localized subsidiary brittle faults and fault rock alteration in the turbidites as well as fault breccia formation in the limestone sequence. Microseismicity along the Ganos Fault cluster at two locations, the more pronounced being located offshore at a fault bend associated with a change from a transpressional to a transtensional regime. Kinematic analysis reveals a dextral strike-slip regime with components of normal and thrust faulting. Along strike paleostress orientation at the Ganos Fault is rather uniform. Deformation mechanisms and fluid inclusion data from quartz and calcite veins suggest that fault-related quartz veins were formed at temperatures between 170 and 250 °C and pressures between 40 and 120 MPa. Fault-related calcite vein growth occurred during a temperature decrease from 170 °C to 70 °C with pressures likely below 50 MPa. Fluid inclusion and stable isotope data show that the fluids are predominantly of meteoric origin and migrated upwards into the fault. Pure CH4 inclusions in quartz also suggest a biogenic or thermogenic methane origin. © 2008 Elsevier Ltd. All rights reserved. 2009 English 01918141 10.1016/j.jsg.2008.09.010 Journal of Structural Geology 31 11-28 GeoForschungsZentrum, Telegrafenberg, D-14473 Potsdam, Germany; Department of Geological and Environmental Sciences, Ben Gurion University of the Negev, Beer-Sheva, 84105, Israel; Bogazici University, Kandilli Observatory, Earthquake Research Institute, Cengelköy, 81220 Istanbul, Turkey; Istanbul Technical University, Eurasia Institute of Earth Sciences, Faculty of mines, Maslak, 34680 Istanbul, Turkey 1 Fault architecture; Fluid inclusions; Ganos fault; Seismicity; Stable isotopes; Turkey, Calcite; Carbonate minerals; Deformation; Inclusions; Isotopes; Limestone; Methane; Mineralogy; Oxide minerals; Quartz, Structural geology, deformation mechanism; fault zone; faulting; kinematics; North Anatolian Fault; paleostress; strike-slip fault; tectonic evolution, Eurasia; Turkey https://www.scopus.com/inward/record.uri?eid=2-s2.0-57849118402&doi=10.1016%2fj.jsg.2008.09.010&partnerID=40&md5=bfc208ddebdf6105fcc138a9851411d8 cited By 23 C. Janssen M. Bohnhoff Y. Vapnik E. Görgün F. Bulut B. Plessen D. Pohl M. Aktar A.I. Okay G. Dresen article dresen2008drilling 2008 English 18168957 10.2204/lodp.sd.6.10.2008 Scientific Drilling 6 ICDP-IODP Göttingen, Germany 58-59 GeoForschungsZentrum Potsdam, Telegrafenberg D-14473 Potsdam, Germany; Bogaziel University, Kandilli Observatory and Earthquake Research Institute (KOERI), Cangelkoy, Istanbul 81220, Turkey; Istanbul Technical University, Mining Faculty, Department of Geophysics, 3469 Maslak Istanbul, Turkey 1 https://www.scopus.com/inward/record.uri?eid=2-s2.0-78651545000&doi=10.2204%2flodp.sd.6.10.2008&partnerID=40&md5=509f8884df17cc1ac492c5de9ca20cbd cited By 3 Georg Dresen Marco Bohnhoff Mustafa Aktar Haluk Eyidogan article Bohnhoff200885 The most recent devastating earthquakes that occurred along the North Anatolian Fault Zone (NAFZ) in northwestern Turkey were the 1999 Izmit (Mw=7.4) and Düzce (Mw=7.1) events. In this study we present a catalog of Izmit aftershock hypocenters that was deduced from a network covering the entire 140 km long rupture of the mainshock. 7348 events with a location accuracy better than 5 km are analysed. Aftershocks were observed along the entire ruptured segment along a 20 km wide band of activity. Events are clustered in distinct regions and dominantly occur at 5 to 15 km depth. The eastern termination of the Izmit rupture is characterized by a sharp and steeply dipping boundary exactly where the Düzce mainshock initiated 87 days after the Izmit event. Relocation of the events using double-difference technology results in 4696 high-resolution hypocenters that allow resolving the internal structure of the seismically active areas with a resolution of 300 m (horizontal) and 400m (vertical). Below the Akyazi Plain, representing a small pull-apart structure at a triple junction of the NAFZ, we identify planes of activity that can be correlated with nodal planes of EW extensional normal faulting aftershocks. Along the easternmost Karadere-Düzce segment we identify the down-dip extension of the Karadere fault that hosted about 1 m of right-lateral coseismic slip. At the easternmost rupture we correlate a cloud-type distribution of seismic activity with the largest aftershocks in this area, a subevent of the Izmit mainshock and the Düzce mainshock that all have an almost identical focal mechanism. This part of the NAFZ is interpreted as a classical example of a seismic barrier along the fault. 2008 English 16807340 10.5194/adgeo-14-85-2008 Advances in Geosciences 14 European Geosciences Union 85-92 GeoForschungsZentrum Potsdam, Telegrafenberg D424, 14473 Potsdam, Germany aftershock; coseismic process; earthquake event; fault zone; faulting; Kocaeli earthquake 1999; seismotectonics; tectonic setting; triple junction, Anatolia; Duzce; Eurasia; Kocaeli [Turkey]; Turkey https://www.scopus.com/inward/record.uri?eid=2-s2.0-37749009216&doi=10.5194%2fadgeo-14-85-2008&partnerID=40&md5=654b5c1f13a5cf8932e4d7a0003ee782 cited By 4 M. Bohnhoff F. Bulut E. Görgün C. Milkereit G. Dresen article Bulut2007 We relocated part of the aftershock activity in Çιnarcιk Basin and surrounding areas that are associated with the 1999 İzmit earthquake Mw 7.4. Double difference relocation algorithm is used to relocate the aftershocks. The data set was obtained from a temporary seismic network deployed 10 days after the main shock by cooperation between Boǧaziçi University Kandilli Observatory and Earthquake Research Institute, LGIT (Grenoble), and IPGP (Paris). For a better station coverage, additional data set was obtained from a network operated by TUBITAK Marmara Research Center. Differential travel times were calculated using both arrival time readings and waveform cross correlation method. We relocated 1145 of the aftershocks and interpreted the results with emphasis on the Yalova and Tuzla cluster located within the Marmara Sea. The results show better focused seismicity patterns for the Yalova cluster, providing clear evidence for the proposed models. We present a revised location of the 1963 Çιnarcιk earthquake which took place in area of Yalova cluster. Finally we propose that the activity of Tuzla cluster represents a parallel subsidiary fault of the Main Marmara Fault. Copyright 2007 by the American Geophysical Union. 2007 English 00948276 10.1029/2007GL029611 Geophysical Research Letters 34 Department of Geophysics, Kandilli Observatory and Earthquake Research Institute, Boǧaziçi University, Istanbul 34680, Turkey 10 Aftershocks; Double difference method; Double difference relocation algorithm; Seismic network; Seismicity, Algorithms; Correlation methods; Earthquake resistance; Mathematical models; Seismic response; Seismic waves; Waveform analysis, Earthquakes, aftershock; arrival time; Kocaeli earthquake 1999; seismicity; travel time; waveform analysis https://www.scopus.com/inward/record.uri?eid=2-s2.0-34447564097&doi=10.1029%2f2007GL029611&partnerID=40&md5=bb82bb0757548c62036f26407ec902a7 cited By 21 F. Bulut M. Aktar article Bulut2007 Joint inversion for hypocentral parameters and the velocity field is nowadays a state of the art tool to obtain high-resolution images of seismically active regions. In this study, we focus on the location accuracy of aftershocks of the 1999 Mw = 7.4 İzmit (NW Turkey) earthquake. We obtained a new velocity model for the region, and depicted its improvement on absolute locations in terms of uncertainty and misfit. Two well-developed aftershock clusters located in the Akyazi area and Karadere-Düzce region, were analyzed in detail based on a waveform cross-correlation approach that allowed improving the location accuracy by a factor of 6. Relocation results reveal that hypocenters form narrow planes of activity that can be correlated with focal mechanisms of the larger aftershocks as well as nearby clouds of activity with no internal structure down to the resolved scale of ∼300 m. Copyright 2007 by the American Geophysical Union. 2007 English 00948276 10.1029/2007GL031154 Geophysical Research Letters 34 GeoForschungsZentrum Potsdam, Haus D Projektbereich 3.2, Telegrafenberg D-14473 Potsdam, Germany; Department of Geophysics, Kandilli Observatory and Earthquake Research Institute, Boǧaziçi University, 34680 Çengelköy Istanbul, Turkey 20 Earthquake effects; Mathematical models; Seismic waves; Waveform analysis, Hypocenters; Seismically active regions; Turkey, Seismology, aftershock; earthquake event; earthquake hypocenter; fault plane; focal mechanism; seismic velocity, Eurasia; Izmit; Kocaeli [Turkey]; Turkey https://www.scopus.com/inward/record.uri?eid=2-s2.0-37349111761&doi=10.1029%2f2007GL031154&partnerID=40&md5=6d5d55ef09eca82ae5e06f5e406c2cd7 cited By 41 F. Bulut M. Bohnhoff M. Aktar G. Dresen article Bohnhoff2006373 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. 2006 English 0956540X 10.1111/j.1365-246X.2006.03027.x Geophysical Journal International 166 373-385 GeoForschungsZentrum Potsdam (GFZ), Telegrafenberg, 14473 Potsdam, Germany 1 aftershock; fault zone; focal mechanism; Kocaeli earthquake 1999; strain analysis; stress analysis, Anatolia; Eurasia; Turkey https://www.scopus.com/inward/record.uri?eid=2-s2.0-33745608736&doi=10.1111%2fj.1365-246X.2006.03027.x&partnerID=40&md5=005771f77ddfa70804488f5c8818bd3c cited By 92 M. Bohnhoff H. Grosser G. Dresen