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Microseismicity at the North Anatolian Fault in the Sea of Marmara offshore Istanbul, NW Turkey

Microseismicity at the North Anatolian Fault in the Sea of Marmara offshore Istanbul, NW Turkey
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  University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln USGS Sta -- Published ResearchUS Geological Survey 1-1-2009 Microseismicity at the North Anatolian Fault in theSea of Marmara oshore Istanbul, NW Turkey  Fatih Bulut GFZ German Research Center for Geosciences Marco Bohnho  GFZ German Research Center for Geosciences  William L. Ellsworth U.S. Geological Survey Mustafa Aktar  Kandilli Observatory and Earthquake Research Institute Georg Dresen GFZ German Research Center for Geosciences is Article is brought to you for free and open access by the US Geological Survey at DigitalCommons@University of Nebraska - Lincoln. It has beenaccepted for inclusion in USGS Sta -- Published Research by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Bulut, Fatih; Bohnho, Marco; Ellsworth, William L.; Aktar, Mustafa; and Dresen, Georg, "Microseismicity at the North AnatolianFault in the Sea of Marmara oshore Istanbul, NW Turkey" (2009). USGS Sta -- Published Research. Paper 393.hp://pub/393  Microseismicity at the North Anatolian Fault in the Sea of Marmaraoffshore Istanbul, NW Turkey Fatih Bulut, 1 Marco Bohnhoff, 1 William L. Ellsworth, 2 Mustafa Aktar, 3 and Georg Dresen 1 Received 9 December 2008; revised 28 April 2009; accepted 8 June 2009; published 3 September 2009. [ 1 ]  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. Thissegment of the fault lies between the 1912 Ganos and 1999  I : zmit ruptures and is theonly NAFZ segment that has not ruptured since 1766. To monitor the microseismicactivity at the main fault branch offshore of Istanbul below the C¸ınarcık 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 temporarynetwork of ocean bottom seismometers was deployed throughout the C¸ınarcık 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 C¸ınarcık Basin merge to one single master fault below  17 km depth. On the basis of a cross-correlation technique we group closely spacedearthquakes 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 thenorthern and main branch of the NAFZ below the eastern Sea of Marmara. Citation:  Bulut, F., M. Bohnhoff, W. L. Ellsworth, M. Aktar, and G. Dresen (2009), Microseismicity at the North Anatolian Faultin the Sea of Marmara offshore Istanbul, NW Turkey,  J. Geophys. Res. ,  114 , B09302, doi:10.1029/2008JB006244. 1. Introduction [ 2 ] The North Anatolian Fault Zone (NAFZ) representsone of the largest plate-bounding transform faults separatingtheAnatoliaandEurasianplatesandextendingfor   1600km between Eastern Anatolian and the Northern Aegean. West-ward movement of Anatolia has developed in the framework of the northward moving Arabian plate and the Hellenicsubduction zone where the African lithosphere is subducted below the Aegean. Current right-lateral slip rate along thefault is 20–30 mm/a [e.g.,  Barka , 1992,  McClusky et al. ,2000] repeatedly producing major strike-slip earthquakes but also N-S extensional normal faulting events south of theMarmara region. During the 20th century, the NAFZ hasruptured over 900 km of its length [  Ambraseys , 1970;  Barka ,1999]. A series of large earthquakes starting in 1939 near Erzincan in eastern Anatolia propagated westward towardthe Istanbul-Marmara region in northwestern Turkey wherethe 1999  I : zmit earthquake occurred (Figure 1a). West of the I : zmit rupture a ‘‘seismic gap’’ spans along a >100 km longsegment below the Sea of Marmara [e.g.,  To¨kso¨z et al. ,1979;  Stein et al. , 1997;  Reilingeret al. , 2000] (see Figure 1a).The Marmara segment connects the 1912 Ganos and 1999 I : zmit ruptures and has not ruptured since 1766. Assumingcurrent slip rates it may have accumulated a slip deficit of up to 4–5 m. It is believed being capable of generating aM    7.4 earthquake [  Hubert-Ferrari et al. , 2000] or anumber of smaller normal faulting events [  Armijo et al. ,2002]. However, it could even rupture in a large single event [  Le Pichon et al. , 1999]. Dynamic modeling of earthquakesfor the Marmara segment on that zone indicates that thenucleation location has a significant effect on size, rupture propagation and final slip of an upcoming event  [ Oglesbyet al. , 2008].[ 3 ] The two most recent major earthquakes in NWTurkey occurred in 1999 (Mw7.4  I : zmit and Mw7.1 Du¨czeevents, Figure 1a) rupturing a   200 km long segment of  NAFZ east of the Marmara region [e.g.,  Tibi et al. , 2001;  Barka et al. , 2002;  Aktar et al. , 2004]. Both main shocks JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, B09302, doi:10.1029/2008JB006244, 2009 1 Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Section 3.2, Geomechanics and Rheology, Potsdam, Germany. 2 U.S. Geological Survey, Menlo Park, California, USA. 3 Kandilli Observatory and Earthquake Research Institute, Bog˘azic¸iUniversity, Istanbul, Turkey.Copyright 2009 by the American Geophysical Union.0148-0227/09/2008JB006244$09.00 B09302  1 of 16     This article is a U.S. government work, and is not subject to copyright in the United States.  had a (pure) dextral strike-slip mechanism reflecting theoverall characteristic of the NAFZ. In the case of the  I : zmit event the rupture extended from the eastern Sea of Marmarato the Du¨zce area. There, the subsequent Du¨zce event occurred 87 days later extending the rupture by another 50 km to the East (see Figure 1a). The western end of the1999 rupture is located in the eastern Sea of Marmara belowthe C¸ınarcık Basin (CB) [e.g.,  Wright et al. , 2001]. Therupture may have extended to just south of the PrincesIslands [  Bouchon et al. , 2002;  O¨  zalaybey et al. , 2002] that are located within   20 km distance to the city of Istanbulwith its >12.5 million inhabitants. In contrast,  Pinar et al. [2001] argue that the rupture did not enter the C¸ınarcık Basin but terminated close to Hersek, west of the  I : zmit Gulf. The  I : zmit event ruptured in distinct segments and thesame segmentation was observed from analysis of after-shock focal mechanisms [  Bohnhoff et al. , 2006]. At thewestern end of the  I : zmit rupture a branching of the NAFZis observed with streaks of similar normal or strike-slipfaulting mechanisms along individual fault branches belowthe C¸ınarcık Basin [ O¨ rgu¨lu¨ and Aktar  , 2001;  O¨  zalaybey et al. , 2002;  Karabulut et al. , 2002]. The estimated 30-year  probability for an event M  7 below the Sea of Marmara is35–70% [e.g.,  Wright et al. , 2001;  Parsons et al. , 2000;  Parsons , 2004].[ 4 ] In this study, we present results of a microseismicmonitoring campaign conducted throughout the C¸ınarcık Basin and on the Prince Islands at the northern escarpment of the C¸ınarcık Basin. Our objective is to determine theseismotectonic setting along the eastern part of the Marmara‘‘seismic gap’’ based on microseismic recordings withunprecedented low magnitude detection threshold. Resultsare related to existing structural information available for the area and discussed in the light of an expected major earthquake at this segment of the NAFZ. 2. PIRES: A Seismic Array on the Prince Islands [ 5 ] Earthquake hypocenters in the Marmara regionobtained by the regional seismic network operated byKandilli Observatory and Earthquake Research Institute(KOERI hereafter) broadly define the major branches of  Figure 1.  (a) Tectonic map of Anatolian-Aegean region (modified from  U.S. Geological Survey [2000]): Westward movement of Anatolian plate causes destructive earthquakes along North AnatolianFault Zone (black line). Rupture zones associated with destructive earthquakes are represented bydifferent colors. In this study we focused on the westernmost part of 1999 ( I : zmit 17 August 1999 Mw7.4 and Du¨zce 12 November 1999 Mw 7.2) rupture zone in which a destructive earthquake is expected tooccur in the next decades. (b) Fault plane solution for M > 5.0 size earthquakes occurred within thevicinity of Sea of Marmara region since 1943 (compiled from  O¨ rgu¨lu¨ and Aktar,  [2001],  Pinar et al. [2003], and  S¸engo¨r et al.  [2005]). Fault lines taken from Turkey General Directorate of Mineral Researchand Exploration, and  Armijo et al.  [2005]. B09302  BULUT ET AL.: MICROSEISMICITY IN THE SEA OF MARMARA2 of 16 B09302  the NAFZ in NW Turkey (see Figure 2a). However, themain branches of the NAFZ are located offshore in the Seaof Marmara where there are no near stations, resulting in acatalog magnitude of completeness (Mc) of Mc    2.6.Hypocenter precision is also limited by the station geome-try, making it difficult to relate seismicity to individual fault structures.[ 6 ] In order to improve the detection threshold and hypo-centeraccuracyformicroseismicitybelowtheC¸ınarcıkBasinwe first deployed a temporary network of eight ocean bottomseismometers extending throughout the C¸ınarcık Basin be-tween November 2005 and January 2006 (Figure 2b). The position of each OBS on the seafloor was determined usingtwo-way traveltimes of acoustic signals sent from the boat tothe OBS while circumnavigating the instrument. The clock drift was linearly interpolated from the measured drift after recovery of the instrument. Timing systems of two of theOBS stations were out of order. From these stations only theS-P times were used. We obtained an average station azi-muthal gap of 137   combining the OBSs with the KOERIland stations. Combined network allowed us to locate 17well-constrained microearthquakes along the basin duringthe 55-day period of OBS campaign (M < 2.5, err < 5.0 km,RMS < 0.3). This observation confirmed that currently a Figure 2.  (a) Seismic activity in eastern Marmara region based on KOERI catalog for the time periodJanuary 2000 to November 2005. (b) Seismic activity in C¸ınarcık Basin after the analysis of combineddata from OBS (squares), PIRES (stars), and KOERI seismic networks for the time period of November 2005 to December 2007. Segmentation of NAFZ in the Sea of Marmara (gray lines) is obtained from  Armijo et al.  [2005]. B09302  BULUT ET AL.: MICROSEISMICITY IN THE SEA OF MARMARA3 of 16 B09302  significant amount of microseicmic activity occurs alongthe C¸ınarcık Basin segments of NAFZ.[ 7 ] In a second step designed to improve long-term mon-itoring we installed a seismic array (Prince Islands Real-timeEarthquake System, PIRES) on the two outermost PrinceIslands, Sivriada and Yassiada. PIRES consists of two sub-arrays of five stations on each island and is located less than5 km to the north of the surface trace of the main fault branch(Figure 3). Both subarrays include a cross-shaped distribu-tion of stations with an aperture of    300 m. The averagestation spacing within each PIRES subarray is 191 m.Establishing the PIRES network was a major logistic task since none of the islands are inhabited or connected byregular ferry traffic. To allow secure long-term operationand to protect stations from unauthorized access, concretehousings were constructed. Stations are operated autono-mouslyandrunonsolarpanels.PIRESwentintooperationinautumn 2006 with nine short-period stations equipped withMARK L4–3C seismometers of 1 Hz natural frequency andonebroadbandsensoroftypeSTS2.Allstationsareequippedwiththreecomponentsensors.Dataaresampledat200Hz.Inthis study, we analyze recordings covering the time period4 October 2006 to 31 December 2007. Events were detectedusing a STA/LTA (short-term average/long-term average)trigger.Eventwindows areextracted once thesignal-to-noiseratio (SNR) simultaneously exceeds a given threshold at aminimumofsixstations.Forabettercoverageoflocaleventsthe database includes additional data recorded at selectedland stations of the regional seismic network (operated byKOERI).[ 8 ] The data is recovered by periodic exchange of harddisk storage every three months. A waveform exampleobtained by PIRES station is shown in Figure 4a (localevent at 7.1 km epicentral distance). During the recording period considered in this study we analyzed a total of 416detected events that were evaluated using the procedure asdiscussed in the following. 3. Array Processing [ 9 ] Many of the events detected by the PIRES array werenot observed on the regional seismic network. As a conse- Figure 3.  Configuration of PIRES subarrays on two outer Princes Islands (left) Yassıada and (right)Sivriada located at   5 km distance to the northern slope of C¸ınarcık Basin. Seismic stations are indicated by red triangles. Circles indicate the well-located hypocenters detected by PIRES arrays. B09302  BULUT ET AL.: MICROSEISMICITY IN THE SEA OF MARMARA4 of 16 B09302
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