Structural framework of onshore and offshore Avcilar, Istanbul under the influence of the North...

Geophys. J. Int. (2011) 185, 93–105 doi: 10.1111/j.1365-246X.2011.04941.x

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Structural framework of onshore and offshore Avcılar, Istanbulunder the influence of the North Anatolian fault

Semih Ergintav,1 Emin Demirbag,1,2 Vedat Ediger,1 Ruhi Saatcılar,1,3 Sedat Inan,1

Ali Cankurtaranlar,1 Aynur Dikbas1 and Mahmut Bas4

1TUBITAK Marmara Research Center, Earth and Marine Sciences Institute, Gebze, Kocaeli, Turkey. E-mail: [email protected] Istanbul Technical University, Department of Geophysics, Ayazaga, Istanbul, Turkey3Sakarya University, Department of Geophysics, Sakarya, Turkey4 Istanbul Metropolitan Municipality, Sarachane, Istanbul, Turkey

Accepted 2011 January 5. Received 2011 January 2; in original form 2010 June 3

S U M M A R YTo investigate the onshore/offshore structural framework of the northern shelf of the Seaof Marmara, a multidisciplinary study was carried out including multichannel and single-channel seismic reflection profiling, multibeam bathymetry sounding, side-scan sonar imagingon offshore as well as land studies for S-wave velocity estimation by passive and activearray measurements and monitoring of block deformations by Global Positioning System(GPS). Based on the multidisciplinary data, possible interactions between landslides andactive faulting have been reviewed and their possible relation to the North Anatolian FaultZone (NAFZ) has been discussed.

It is observed that structurally the most deformed subsurface zone offshore coincides withthe most undulated landform due to landslides in the area. This observation leads to idea thatthe structural complexities of the area may also contribute to land sliding as much as othercontributing mechanisms such as slope instability, shallow groundwater level, lithology andliquefaction. The Catalca Fault Zone, a branch of the inactive Western Black Sea (WBS) Fault,may constitute weakness surfaces that contribute to land sliding in the deformed Miocene layersin the study area. Based on the multidisciplinary and new data sets, we show that ongoing landsliding in the study area should be related not only to common causes of landslides but alsothe structural framework of the area.

Key words: Continental neotectonics; Submarine landslides; Submarine tectonics andvolcanism; Crustal structure.

1 I N T RO D U C T I O N

A better understanding of the active structural framework of theSea of Marmara, particularly the influence of the North AnatolianFault (NAF) and its associated submarine basins and ridges, is veryimportant as the area lies in one of the highly populated regionsof Turkey (Barka 1999). After the last devastating Izmit and Duzceearthquakes (Turkey) in 1999, different research groups, by usingmarine geophysical data, investigated the main question whetherthe northern branch of the NAF (the Main Marmara Fault) is asingle thoroughgoing fault zone (Imren et al. 2001; Le Pichon etal. 2001; Demirbag et al. 2003; Kuscu 2009), or a set of faultsforming a stepover (Armijo et al. 2002; Carton 2005; Hergert& Heidbach 2010). Some researchers are also interested in thedeformation due to the NAFZ on the southern shelf (Parke et al.1999; Okay et al. 2000; Yaltırak & Alpar 2002; Kurtulus & Canbay2007; Kuscu et al. 2009) while others have focused on the Ganosfault and its submarine extension in the Tekirdag basin (Okay et al.

1999; McHugh et al. 2006). A fragment of the Western Black Seafault (WBS), the NW–SE oriented Catalca Fault Zone, is anotherimportant, now inactive, former structural feature but no data isavailable from its offshore continuation (Figs 1 and 2; Okay et al.1994; Duman et al. 2004).

Another issue is the landslides, which are observed in the northernshelf of the Sea of Marmara, especially along the shorelines and theHaramidere valley in the Avcılar Peninsula (Figs 1 and 2). Landsliding is a major problem in the area and the investigation of thelandslides is very important in terms of determination of causes andprecautions that can be taken to avoid future damage to urbanizedareas. A total of 391 landslides are mapped in the area covering60 km2 (Duman et al. 2004). Approximately half of all landslidesare distributed between Buyukcekmece and Gurpınar area of theAvcılar Peninsula, which are important local landforms in the region(Fig. 2). Based on field geological observations, Duman et al. (2004,2005a,b) have argued that the causes of the landslides are slopeinstability, shallow groundwater level, lithology and liquefaction.

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Figure 1. The topographical and bathymetric view of the study area. Topography data are provided by Istanbul Metropolitan Municipality. Bathymetry datamerged from this project and Le Pichon et al. (2001). The whole data are processed and mapped by Generic Mapping Tools (GMT; Wessel & Smith 1991).Black lines are the multichannel seismic profiles and labels on land are the GPS sites (AVCT is continuous and the others are campaign GPS sites). Inset mapshows the relationship of the West Black Sea Fault (WBSF) to active tectonic and palaeotectonic features in the region (modified from Okay et al. 1994) andstudy area in the Marmara Sea.

The first geophysical survey in the northern shelf was carried outto collect single-channel shallow seismic data, gravity cores and sur-face sediment sampling for studying Quaternary geology in the mid-1990s (Oktay et al. 2002). The second stage of data collection wasafter the disastrous earthquakes of 1999. A remarkable study, suchas the multibeam bathymetric survey and shallow seismic reflectionprofiling, was carried out by Turkish–American–Italian cooperationfor exploring possible active faults and evaluating earthquake riskin the area (Cormier et al. 2002). In that study, various types ofdata were collected including side-scan sonar images, multibeambathymetric soundings, single-channel seismic sections and gravitycores. Among the data sets, single-channel seismic sections have afew hundreds of milliseconds of recording length corresponding toroughly 100–200 m of penetration depths. In general, this is not asufficient penetration depth for a thorough investigation of structuralfeatures of the northern shelf because there is always a possibility ofconfusing the shallow deformations (e.g. landslides, irregularitiesin sediment depositions) with active faults. Therefore, deeper pene-tration is necessary to distinguish the shallow anomalies from activefaults in the seismic sections. Although there were existing multi-channel deep seismic lines collected by the Mineral Research andExploration Directorate of Turkey (MTA) before and after the 1999earthquakes (Okay et al. 1999, 2000; Imren et al. 2001; Demirbaget al. 2007), industrial lines by the Turkish Petroleum Corporation

(Aksu et al. 2000; Ates et al. 2003), the Turkish–French cooperationin 2001 (Carton 2005) and the Turkish–American cooperation stud-ies in 2008 (Sorlien et al. 2008) were not particularly designed forinvestigation of the northern shelf. These multichannel lines weremainly collected for the investigation of the northern branch of theNAF and its associated basins and ridges.

Despite the focused researches in and around the deep basins ofthe Sea of Marmara, the northern shelf with less than 100 m depth,particularly to the west of the Bosphorus exit, is relatively less in-vestigated by multidisciplinary studies. Widespread damage in theeastern part of Avcılar Peninsula during the 1999 Izmit earthquakeattracted attention of some researchers on the possibility of activefaulting in the northern shelf. Although apparent low seismicityindicates no active faulting in and around the study area (includ-ing northern shelf; Eyidogan 2006), damage to poorly constructedbuildings was high and resulted in several hundred deaths duringthe 1999 Izmit earthquake (Durukal & Erdik 2008). Parts of thisabnormally high devastation and life loss were explained by weakbedrock, which enhanced (amplified) the seismic waves (Ergin et al.2004). However, taking into consideration that vast areas of Istanbulhave similar weak bedrock, we presumed that there may be otherreasons leading to this extensive damage in the study area. Con-sidering the structural frame of the study area, the first mechanismthat comes to mind is the possible interactions of rejuvenated faults

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Avcılar under the influence of the NAF 95

Figure 2. Simplified geology map of the study area. Note the location of the Catalca Fault Zone, a fragment of now inactive the West Black Sea (WBS) faultand the distribution of the landslides along the shore lines and Haramidere riverside [simplified from Duman et al. (2004) and Ozgul et al. (2005)].

and landslides that cause massive damage to the buildings. Thuswe intended to investigate possible interactions between seeminglyinactive faults and widely observed landslides.

2 G E O L O G Y O F T H E S T U DY A R E A

The study area is located on the boundary between Strandja Mas-sif (Palaeozoic–Mesozoic) and Istanbul Zone (Palaeozoic), andboth are overlain by Eocene and younger sediments of Thracebasin (e.g. Okay et al. 2001). The Strandja Massif is representedby phyllite, schist and granitoids, which crop out in the CatalcaFault Zone, essentially a horst bounded by steeply dipping faults(Fig. 2). The metamorphism age is Late Jurassic–Early Cretaceous(Okay et al. 2001, Sunal et al. 2006). The Istanbul Zone is repre-sented by a thick turbiditic sandstone-shale sequence of Carbonif-erous age, which crop out northeast of the Kucukcekmece Lake(Fig. 2). Middle–Upper Eocene aged limestone units are transgres-sive on the Catalca metamorphics in the west and on the Car-boniferous flysch in the east; they cover a major dextral strike-slip fault zone, the West Black Sea Fault (WBSF), with a totalslip of more than 100 km (Okay et al. 1994). The Eocene lime-stones pass up into a clastic series of Upper Eocene–Oligocenein age. A major unconformity separates these Eocene–Oligocenesediments from the overlying Upper Miocene fluviatile to lacus-

trine deposits, represented by marn, limestone, shale and siltstone(Sakınc et al. 1999; Duman et al. 2004). The Miocene deposits areconfined to the coastal region (Fig. 2) and are the sites of all thelandslides.

The WBSF was probably active from Cretaceous to Mid-Eocene.In Mid-Eocene, the activity of the WBSF was probably ceasedduring the collision of the Istanbul Zone with the Sakarya conti-nent (Okay et al. 1994). On the other hand, the Catalca Fault Zonecontrols the Eocene sedimentation indicating some dip-slip activ-ity during the Middle to Late Eocene. The active NAF passes ca.10 km of the coast (Okay et al. 1999; Le Pichon et al. 2001) form-ing an east–west trending segment about 110 km long. It is believedto be active since the Late Miocene–Pliocene (Barka & Hanckok1985; Aksu et al. 2000).

Marine geophysical data from this study show that the northernshelf area is characterized by a thin blanket of Pliocene/Quaternarydeposits underlain by older strata of Miocene and Oligocene age.The older strata are usually parallel layered with gentle slopes andcut by various order of faults. The seismic sections display strongstructural anomalies, particularly offshore Avcılar Peninsula, wherethe older strata are highly deformed. Hence, folding and frequentfaulting are observed from seismic reflection sections. To the west,the Miocene and Oligocene strata are gently folded and show nomajor deformation.

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3 M AT E R I A L S A N D M E T H O D S

New offshore data collected in 2007 include: set of multichannel,high resolution, moderate penetration seismic reflection data, multi-beam bathymetry sounding, side-scan sonar imaging and single-channel seismic profiling. Onshore studies include: determinationof S-wave velocity structure at depth with active/passive array sur-face wave measurements, monitoring of local block deformationswith Global Positioning System (GPS). The multichannel seismicdata acquisition is carried out for the first time at west of the Bospho-rus in the northern shelf of the Sea of Marmara to investigate off-shore structural features such as Catalca Fault Zone and the shallowdeformations as the result of possible submarine landslides. In thispaper, we focus on the multichannel reflection data of the projectand use the other data sets as supplementary sources to interpret thestructural features offshore the Avcılar Peninsula.

3.1 Multichannel and single channel seismic datacollection: equipment and parameters

The seismic data were collected on 14 profiles with total of 234 kmof length. Data acquisition parameters are selected so that they pro-vide moderate penetration with high resolution for a conventionalseismic reflection study (Pritchett 1990). A single generator-injectorairgun with total of 180 (75 + 105) cubic inches is used as seis-mic source. Shots are fired every 12.5 m interval controlled by adifferential GPS. The seismic signals are detected by a 96-channelstreamer. The receiver interval is selected as 6.25 m for smoothand frequent spatial sampling. The sampling rate in time is 1 msand the record length is 3 s. Note that 1 ms sampling rate in timeresults in 500 Hz Nyquist frequency. Most of the seismic lines are innorth–south direction and closely spaced offshore Avcılar Peninsulaand two east–west lines are located parallel to the shoreline (Fig. 1).

The 6.25 m small receiver interval provides high Nyquistwavenumber for avoiding spatial aliasing. The 96 channels record-ing with 12.5 m of shot and 6.25 m receiver intervals provides 24 foldcommon-depth-point (CDP) data. The bandwidth of the recordedsignal is observed approximately between 10 and 250 Hz. The dom-inant frequency of the recorded seismic signal is around 110 Hz.The vertical and lateral resolution at the sea bottom are calculatedas 3.5 m based on one-fourth of the dominant wavelength and22.6 m based on the radius of the Fresnel zone, respectively, byaccepting 110 Hz dominant frequency, 1500 m s−1 P-wave velocityand a typical water depth of 75 m in the northern shelf (Yılmaz1987). The average velocity at 1 s two-way traveltime is about2000 m s−1 according to velocity analysis; therefore, the penetra-tion depth of the multichannel seismic data is about 1000 m at 1 stwo-way traveltime.

In addition, single-channel seismic data were collected along thesame profiles in Fig. 1 simultaneously with the multichannel seismicdata, using engineering equipment (Seabed). The bandwidth of thesignal is 3.5 kHz. The sampling rate and shot interval are selectedas 0.1 ms and 250 ms, respectively. The penetration depth of thesingle-channel seismic data is about 30 m.

3.2 Multichannel and single channel seismic dataprocessing: priorities, techniques and parameters

To produce geologically meaningful seismic sections, we used oneof industry standard seismic data processing packages installed atTUBITAK Marmara Research Center. Our data processing priority

is to enhance the signal-to-noise (S/N) ratio by removing all kinds ofnoise and to increase the available resolution in the raw data. We pro-cessed the data by conventional means as follows. Manual editingfor noisy and dead seismic traces on shot gathers; bandpass filteringfor periodic noise; line geometry definition for relative positioningof the shots, receivers and CDP points; spherical divergence cor-rection; sorting the shot data to CDP order data; pre-stack multipleelimination based on f −k filtering; stacking velocity analysis onthe CDP super-gathers; normal-moveout (NMO) correction; mut-ing of stretched signals on NMO corrected CDPs; full offset andnear offset partial-stacking; post-stack finite difference migrationin the time-domain. The final sections are plotted with and withoutautomated gain correction.

The velocity analysis is carried out on the CDP super-gathersto build a 2-D stacking velocity field for every seismic line. Thenumber of CDPs to form a super gather is selected as three withinthe limits of calculated lateral resolution based on the radius ofthe Fresnel zone in the sea bottom (22.6 m) and spatial samplinginterval determined by the distance between individual CDP traces(3.125 m). The velocity analysis is applied for every 100 CDPstations and every 50 CDP stations where the subsurface structureis complex. The stacking velocity range is between 1500 and 2900m s−1 for the first 1 s of the data and it usually varies smoothly bothhorizontally and vertically. To suppress the sea bottom multiples, weapplied the following techniques: f –k filtering, p-Radon, predictivedeconvolution and multichannel predictive deconvolution. In somesections, multiples seem very strong and we couldn’t remove them(e.g. Fig. 5). Partial stacking with nine near offset traces and fullfold stacking provided about three and five times improvement inthe S/N ratio, respectively.

A high-band filter, which has a 300 Hz cut-off, is applied to singlechannel data to increase S/N ratio and reduce the swelling effect,after the normalization of the amplitudes in different traces.

3.3 Investigation of S-wave velocity structure

A series of geophysical surveys and geological investigations werecarried out to obtain regional information on the macro scalebedrock properties with depth. These surveys include ambient mi-crotremor array (SPACK, f–k), active source surface wave measure-ments (MASW) and Horizontal-to-Vertical Spectral Ratio (HVSR)from single-station measurements to determine S-wave velocityprofile (Park et al. 1999; Chavez-Garcia et al. 2005). Following thedata collection and the processing step in the same way as explainedby Zor et al. (2010), combined Rayleigh wave dispersion curve pro-duced by processing the array measurements in a broad frequencyrange (0.5–20 Hz) and the HVSR curve obtained from the singlestation measurements for each site. Then, with a combined inver-sion (Dispersion + HVSR) technique, quantitative information onthe shear wave velocity structure estimated as a function of depthin the study area.

Average shear velocity down to 30 m is used by the geotechnicalengineering society to evaluate the potential site amplifications andthe classifications for the different seismic design and building codes(e.g. Turkish Building Code, 2008; NEHRP, 1997). In the case ofhaving inverted velocity model deeper than 30 m, the depth wherethe layer velocity is greater than a certain velocity can be attributedto the sediment-bedrock depth (Olsen 2000; Zor et al. 2010).

Since we obtained commonly in our velocity models, we choosethe certain velocity in this study as 1000 m s−1 described as firmto hard rock in NEHRP classification scheme (Class B in NEHRP;BSSC 1997). For each site, the depth where the layer velocity is

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greater than 1000 m s−1 is extracted from the velocity model, whichwe call it as the bedrock depth.

3.4 Geodetic measurements

Eight geodetic measurement sites were constructed in the studyarea (Fig. 1) to monitor the local deformations. GPS campaignswere conducted at each station, two sessions of 10 h are observedand four campaigns; one every 3 months. After that, two extracampaigns realized for the estimation of the long-term reliabilityof the estimated GPS velocities within 2 yr. To reduce the an-tenna phase pattern problems and the errors on the computationof the vertical components, we used the same receivers and anten-nas at all campaign sites (Trimble 4000 and 5700 with choke-ringand geodetic Zephyr antennas). The GPS data were processed byGAMIT/GLOBK software (King & Bock 1998; Herring 2000), us-ing the all available continuous and campaign sites in the Marmararegion (Ergintav et al. 2009). Details about the processing methodare described in Ergintav et al. (2009).

All sites are in the Eurasian Plate and the velocities were esti-mated in the Eurasian reference frame (Ergintav et al. 2009). So,all velocities reflects the local anomalies as a function of grounddeformations (e.g. landslides) and the stress loading as the result

of the possible active fault branches. The velocities of the sites aregiven as supplementary Table S1 and Fig. S1 of the paper.

4 D I S C U S S I O N

4.1 Submarine and subbottom featuresof the northern shelf

In this publication, only the structural interpretations of the seismicsections are presented. The seismic sections can be discussed in twogroups based on the trend of the survey line: the first group of sec-tions is across to the shoreline and cross the shelf break. The secondgroup of sections is parallel to both the shoreline and shelf break(Fig. 1). Before making structural evaluations and discussions ontheir implications, general submarine and subbottom characteristicsof the study area need to be summarized from the bathymetry, side-scan sonar and seismic data. Generally, the bathymetry changessmoothly from coastline to 100 m, where the shelf break is lo-cated (Fig. 1). Irregular bathymetric features and narrow canyonsare observed along the slope. Rocky lineaments are observed inNNW–SSE in limited areas where the older sedimentary strata cropsout in strike direction. Deep cut narrow canyons are located on theslope and they connect to the deep basins of the Sea of Marmara.Submarine slides and small bedrock rotations are observed in the

Figure 3. Migrated seismic section MCSX04 and its interpretation. Anomaly of the Catalca Fault Zone, a fragment of the WBS fault, is observed in thisseismic section. A small trough between shots 800 and 850 is seen in the section. This trough is formed between two vertical faults connecting to each other atgreater depths. Another fault anomaly is located at shot 700, which is possible, responsible for a push up ridge between shots 550 and 800.

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slope and canyons. The older folded strata is truncated at top by anangular discontinuity and also cut by a series of NNW–SSE trend-ing faults. A thin blanket of Plio-Quaternary recent deposits coverthe older deformed sedimentary strata.

In the following paragraphs, we focus on the large and deepstructural anomalies rather than small scale and shallow features. Asignificant anomaly of the Catalca Fault Zone is observed in seismicsection MCSX04 (Figs 1 and 3). A narrow subbottom trough be-tween shots 800 and 850 is seen in the section. This trough is formedbetween two subvertical faults joining at deeper parts of the section.An important fault is located at shot 700 that is possible responsiblefor a push up ridge between shots 550 and 800 as an indicator ofa fault bending in a dextral strike-slip fault zone. Section MCS01displays another important anomaly of the Catalca Fault Zone be-tween shots 500 and 750 (Figs 1 and 4). This seismic section islocated parallel to the shoreline and mouth of Buyukcekmece gulf.Gently folded, SE-dipping strata are severely cut by succession ofvertical faults between shots 500 and 750 forming a wide trough.The single-channel seismic reflection section shows the same defor-mation field in shallow scale (Fig. 4, top section). Plio-Quaternaryrecent deposits with maximum 33 m of thickness covering the olderdeformed sedimentary strata are observed. The multichannel sec-tion MCS-EK3 is shown with its counterpart single-channel sectionin Fig. 5. During the data collection, both the single-channel andmultichannel sections were recorded simultaneously. To minimize

the multiple arrival problems, the upper part of MCS-EK3 and itscounterpart single-channel section were reinterpreted to note anactivity of the Catalca fault zone in this part of the shelf. Faultanomalies at shots 230, 250, 270 and 300 affect the sea bottom andcan be defined as possible active fault branches. Note that southwestpart of the section between shots 500 and 660 displays the northernslope and sliding material.

In each test site, depths at which the reference shear velocity,obtained from the investigation of S-velocity structure, were con-sidered as reference depth (Fig. 6a). The variation of the referencedepth in the study area shows that the eastern side of the CatalcaFault Zone is nearly 250 m deeper than the western side of the fault(Fig. 6b) and possibly controlled by the Catalca Fault Zone. Thisvelocity anomaly from land studies as well as seismic survey inthe shelf define that Catalca Fault Zone is an important southerlysegment of the now inactive WBS fault and its morphologicaland subsurface characteristics are stand still in the study area. Theunits showing reflections down to about 1000 m depths in shore-parallel seismic sections are the oldest units in Oligocene to UpperEocene age based on comparison to natural gas borehole drilledby Turkish Petroleum Corporation in the area (the location can-not be given due to proprietary reasons). Fault activity displacingOligocene aged strata indicates that the activity on the Catalca FaultZone did not cease in Middle Eocene as implied by earlier workers(Okay et al. 1994) or it was rejuvenated after Middle Eocene.

Figure 4. (a) Top figure shows the corresponding single-channel shallow section. (b) Migrated seismic section MCS01 and its interpretation. This sectiondisplays an anomaly of the Catalca Fault Zone between shots 500 and 750. Gently folded, SE-dipping strata are cut by successive subvertical faults betweenshots 500 and 750.

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Figure 5. Migrated seismic section MCS-EK3 (bottom panel) and its interpretation of the upper part (middle panel) to minimize the multiple arrival problems.Vertical fault anomalies between shots 150 and 300 in this section are evidences of the Catalca Fault Zone in this part of the shelf. Southwest part of the sectionbetween shots 500 and 660 displays the northern slope and sliding material. Top panel shows the corresponding single-channel shallow section displaying stepsin the bathymetry and thin sedimentary coverage, which is an indicator of activity of the Catalca Fault Zone on offshore.

Seismic section MCS02 (Figs 1 and 7) clearly displays stronglydeformed older substrata covered by thin recent deposits betweenshots 3750 and 4550. Strong folding and successive faulting isobserved particularly across Avcılar peninsula, which is geographi-cally delimited by Buyukcekmece and Kucukcekmece lakes. In thisseismic section, we calculated maximum apparent slope of 25◦ inthe layers by converting the time units into depth units in the verticalaxis using an average velocity of 2000 m s−1. Rocky outcrops ofpossible Oligocene age seem as the surface markers of these layersand the evidence of the relatively strong materials in the depths andmarked on the single-channel seismic section, clearly (see Fig. 4top section). The folded layers are cut by vertical faults trendingin NNW–SSE direction as indicated by small to moderate vertical

offsets in the layers. NW continuations of the faults that cause blocktilting coincide with the Haramidere valley, which suggest that theformation of this valley might be structurally controlled (Fig. 7,bottom panel).

Seismic section MCS-EK9 is located close to the Kucukcekmeceand displays a few shallow small normal faults terminating at about100 ms depths shore (Figs 1 and 8). The extension of these faultscoincides with western bank of Lake Kucukcekmece, where theland sliding is observed (Fig. 2). Furthermore, some faults cuttingthe horizontal strata at greater depths at shot locations 150, 250,350 and 450 are observed in the section. The layers between shots150 and 250 are depressed and this is more pronounced at greaterdepths indicating the depression is delimited by faults.

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Figure 6. Correlation of shear velocity variation around the Catalca Fault Zone: (a) Distribution of S-wave velocity measurement sites. Numbers indicate thedepths of reference shear wave velocity (See the text). (b) Depth variation of reference shear wave velocity along the AA′ profile. Note that Catalca Fault Zonecauses a vertical offset in the S-wave velocity variation.

One of the important observations in the seismic sections is adome shaped subsurface feature offshore Avcılar district as seen insection MCSX03 (Figs 1 and 9). The dome is located to the southof the section and the older strata shows sharp upslope bending atshot 375 and the layers rest on the dome further south. The internalreflection configuration of the dome is not layered. Note that thereis also no surface indication of the dome in the bathymetry in thenorthern shelf (Fig. 1).

4.2 Multidisciplinary data and their interpretation

The northern shelf area is characterized by a thin blanket ofPliocene/Quaternary deposits underlain by Miocene and Oligoceneage older strata. The older strata is usually parallel layered withgentle slopes and cut by various importance of faults. Near the shelf

break, the older strata are carved by long steep canyons connectingto deep basins of the Sea of Marmara. Slumping and sliding are inprogress towards to canyon axes and to deep basins.

The dome shaped subbottom feature offshore Avcılar district addsmore complexity by causing the older strata bend upward and thelayers rest on at east, west and northern walls of the dome. It is wherethe older strata have the most observed dipping layers. The internalreflection configuration of the dome is not layered; no reflectionsare seen in the sections. This may lead to interpretation that thedome is a non-reflective zone. Furthermore, the dome is located tothe north of the NAFZ and it may be the northern smaller part of alarger portion located south the NAF now belonging to the CentralRidge of the Sea of Marmara separating the Cınarcık and CentralBasins. The dome can also be the reason of the aseismic zone inthis region (Imren et al. 2001; Eyidogan & Gureli 2006).

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Figure 7. Migrated seismic section MCS02 and its interpretation. This section is the most striking section in this study. It clearly displays strongly deformedolder substrata covered by thin recent deposits between shots 3750 and 4550. Strong folding and successive faulting is observed particularly offshore Avcılardistrict. The folded layers are cut by faults extending in NNW–SSE direction causing small to moderate vertical separations in the layers. Enlarged section(bottom panel) shows tilted blocks among faults indicating a structural background in formation of the Haramidere valley.

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Figure 8. Migrated seismic section MCS-EK9 and its interpretation. Thissection displays shallow normal faults terminating at about 100 ms depths.The extension of these faults coincides with western bank of KucukcekmeceLake where land sliding is observed (see Fig. 2). Some other faults cuttingthe strata at shot locations 150, 250 and 350 are observed in the section. Thelayers between shots 150 and 250 are depressed and this depression is morepronounced at greater depths indicating fault controlled mechanism.

The continuation of inferred faults from the sea area towardthe shelf and land closely matches with location of the alignedlandslides. One of the main questions is to find any activity alongthis high deformation zone to mitigate earthquake hazard in theregion. In the past, based on available seismological catalogues,researchers assumed seismic activity in the study area (Oktay et al.2002), but earthquake locations in the catalogues can have errors as

Figure 9. Migrated seismic section MCSX03 and its interpretation. A domeshaped subsurface feature offshore Avcılar district is observed in this section.Older strata show sharp uprise bending at shot 375 and the layers rest on thedome further south. Internal reflection configuration of the dome indicatesnon-reflective zone.

high as 4–5 km and contain many quarry blasts as well (Eyidogan &Gureli 2006). New catalogues from the local organizations (Inanet al. 2007) include no earthquakes for this area. However, thesecatalogues are based on weak station distributions and geometries.Hence, a local seismological network is needed to fill the gaps ofavailable seismogical catalogues to catch any microseismologicalactivity.

Strong structural anomalies in the seismic sections, particularlyoffshore Gurpınar part of Avcılar Peninsula, are observed. The olderstrata are tilted and faulted. It is also noted that the most affectedzone from landslides comes across the most deformed subsurfacezone offshore Avcılar district. This leads to idea that the landslides

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Figure 10. The horizontal GPS velocity field w.r.t. Eurasian Plate and the location of non-reflective zone, cut by the fault zone. Seismic lines shown on themap provide a link between the seismic sections and the structural features in this figure. Note the correlation between the locations of the heavy landslidesand the fault zone.

in this area may also be related to the structural complexities ofthe area as well as common causes of landslides. As one of theimportant structural anomalies, marked by seismic sections andshear wave velocity anomalies, the Catalca Fault Zone with itszone of secondary features now constituting weakness surfaces maycause instability thus contributing to landsliding in the deformedolder Miocene layers in Gurpınar region.

Another contributing mechanism of the land sliding may be theinteraction of Catalca Fault Zone and the NAF in the area. Notethat the NAF is oriented in east–west direction in the area and theCatalca Fault Zone comes towards the NAF, which may consti-tute a junction offshore Avcılar Peninsula. The GPS velocity fieldindicates a biggest deformation zone in the northern part of TheSea of Marmara (Fig. 10) as the summation of the long-term re-gional deformation (stress loading as the result of the possible activebranches of NAF) and short-term local deformation field (e.g. land-slides, substance). As an important feature, the deformations hasdifferent order of magnitude between the western and eastern partof Haramidere (Fig. 2). So, this can be marker of the extension on

land and linked to the block tilting around the structural controllingvalley, observed on the seismic section MCS02 (Fig. 7).

Interpretation of seismic sections suggest that the dome structurehas been cut by a series faults in the east of Catalca Fault Zone andthis pattern can be explained by the right lateral motion of NAF.Under the continuous right lateral motion along the NAF, this landdeformation zone may propagate toward east through time. Fig. 10summarizes these features. What is apparent is that this relativelyextended area is further sliced down by the north–south orientedfaults (determined from offshore seismic) and this interaction hasled to north–south aligned heavy landslides that are observed alongthe east and west sides of the Avcılar peninsula and Haramidereriverside, as a localized response of the wider extended area.

5 C O N C LU S I O N

We have shown that there is evidence from offshore seismic reflec-tion data as well as geological, geodetical, geophysical land data

C© 2011 The Authors, GJI, 185, 93–105



that ongoing landsliding in the Avcılar Peninsula is interrelated tothe structural framework of the area and this relation maybe moredominant as the cause of landslides than commonly believed andreported causes (e.g. slope instability, shallow groundwater leveland liquefaction).

We believe that the study area, located to the north of the MainMarmara Fault of NAFZ, which seems to show aseismic characterdeserves further studies. Thus, future plans include establishmentof dense seismological network, conducting additional GPS surveysand obtaining and processing more PSInSAR images to better revealthe activity of suspected faults and shallow deformation in the formof landslides.

A C K N OW L E D G M E N T S

The authors thank Istanbul Metropolitan Municipality andTUBITAK Marmara Research Center for the financial support. Wealso thank the research team in Seislab of Institute of Marine Sci-ences in Dokuz Eylul University (DEU) under the leaderships ofthe Prof. Dr. Gunay Ciftci and Assoc. Prof. Dr. Derman Dondururfor their unlimited support in data collection stage of surveys, thecaptains and crews of R/V Koca Piri Reis for their professional andeffective assistance, which ensured the completion of the cruisesunder difficult conditions in very heavy marine traffic. We thankProf. Dr. Aral Okay (ITU) for his valuable contribution and criti-cism. Finally, we thank the Department of Ground and EarthquakeSurvey of the Istanbul Metropolitan Municipality (namely, HikmetKaraoglu, Kemal Duran, Ahmet Emre Basmacı and Salim Gumus)for continuous interest and support in this study.

R E F E R E N C E S

Aksu, A.A., Calon, T.J., Hiscott, R.N. & Yasar, D., 2000. Anatomy of theNorth Anatolian Fault Zone in the Marmara Sea, western Turkey: exten-sional basins above a continental transform. GSA Today, 10(6), 3–7.

Armijo, R., Meyer, B., Navarro, S., King, G. & Barka, A., 2002. Asymmetricslip partitioning in the sea of Marmara pull-apart: a clue to propagationprocesses of the North Anatolian fault, Terra Nova, 14, 80–86.

Ates, A., Kayıran, T. & Sincer, I., 2003. Structural interpretation of the Mar-mara region from aeromagnetic, seismic and gravity data, Tectonophysics,367, 41–99.

Barka, A., 1999. The 17 August 1999 Izmit earthquake, Science, 285,1858–1859.

Barka, A. & Hanckok, 1985. Tectonic interpretation of enigmatig structuresin the North Anatolian fault zone, J. Struct. Geol., 5, 217–220.

BSSC, 1997. NEHRP Recommended Provisions for Seismic Regulations forNew Buildings, Part 1—Provisions, 1997 edn, 337 pp., Building SeismicSafety Council, Washington, D.C..

Carton, H., 2005. Three-dimensional seismic images of an active-pull apartbasin: the Cınarcık Basin along the North Anatolian fault, Lithos ScienceReport 7, 63–72.

Chavez-Garcia, F.J., Rodriguez, M. & Stephenson, W.R., 2005. An alterna-tive approach to the SPAC analysis of microtremors: exploiting stationar-ity of noise, Bull. seism. Soc. Am., 95, 277–293.

Cormier, M.H., Polania, A., Cagatay, N., Seeber, N., Gorur, N., Gasperini,L., Emre, O. & the scientific parties of expeditions Marmara (2000) andMarmara (2001), 2002, Bends of the North Anatolian Fault in the IzmitGulf, Eastern Marmara Sea: possible control on the structural evolution ofthe fault and the propagation of the earthquake ruptures, in Proceedingsof the 1st International Symposium of the Faculty of Mines (ITU) on EarthSciences and Engineering, 16–18 May 2002, Abstracts, Istanbul, p. 7.

Demirbag, E., Rangin, C., Le Pichon, X. & Sengor, A.M.C., 2003. Inves-tigation of the tectonics of the Main Marmara Fault by means of deeptowed seismic data, Tectonophysics, 361, 1–19.

Demirbag, E., Kurt, H., Dusunur, D., Sarıkavak, K. & Cetin, S., 2007. Con-structing a 3D structural block diagram of the Central Basin in MarmaraSea by means of bathymetric and seismic data, Mar. Geophys. Res., 28,343–353.

Duman, T.Y. et al., 2004. Istanbul Metropolu Batısındaki (Kucukcekmece-Silivri-Catalca Yoresi) Kentsel Gelisme Alanlarının Yerbilim Verileri.Maden Tetkik ve Arama Genel Mudurlugu (MTA) Ozel Yayın Serisi – 3,Ankara.

Duman, T.Y., Can, T., Gokceoglu, C. & Nefeslioglu, H.A., 2005a. Landslidesusceptibility mapping of Cekmece area (Istanbul, Turkey) by conditionalprobability, Hydrol. Earth Syst. Sci. Discuss., 2, 155–208.

Duman, T.Y., Can, T., Ulusay, R., Kecer, M., Emre, O., Ates, S. & Gedik, I.,2005b. A geohazard reconnaissance study based on geoscientific infor-mation for development needs of the western region of Istanbul (Turkey).Environ. Geol., 4, 871–888.

Durukal, E. & Erdik, M., 2008. Physical and economic losses sustained bythe industry in the 1999 Kocaeli, Turkey earthquake, Natural Hazards,46(2), 153–178.

Ergin, M., Ozalaybey, S., Aktar, M. & Yalcın, M. N, 2004. Site Amplificationat Avcılar – Istanbul, Tectonophysics, 391, 325–334.

Ergintav, S. et al., 2009. Seven years of postseismic deformation follow-ing the 1999, M = 7.4 and M = 7.2, Izmit-Duzce, Turkey earthquakesequence, J. geophys. Res., 114, B07403, doi:10.1029/2008JB006021.

Eyidogan, H., 2006. Analysis of Micro-Seismicity of Istanbul Greater CityArea and Active Faults, in Proceedings of the International Workshop on“Comparative studies of the North Anatolian Fault (northwest Turkey)and the San Andreas Fault (southern California)”. ITU, Istanbul, Turkey,Abstract book, p. 54.

Hergert, T. & Heidbach, O., 2010. Slip –rate variability and distributeddeformation in the Marmara Sea fault system, Nat. Geosci., 3, 132–135.

Herring, T.A., 2000. GLOBK: Global Kalman Filter VLBI and GPS analysisprogram Version 10.0. MIT, Cambridge, MA.

Imren, C., Le Pichon X., Rangin C., Demirbag E., Ecevitoglu B. & GorurN., 2001. The North Anatolian Fault within the Sea of Marmara: a newinterpretation based on multi-channel seismic and multi-beam bathymetrydata, Earth planet. Sci. Lett., 186, 143–158.

Inan, S., Ergintav, S., Saatcılar, R., Tuzel, B. & Iravul, Y., 2007. Turkeymakes major investment in earthquake research, EOS, Trans. Am. geophys.Un., 34, 333–334.

King, R.W. & Bock, Y., 1998. Documentation for the GAMIT analysissoftware, release 9.7. MIT, Cambridge, MA.

Kurtulus, C. & Canbay, M.M., 2007. Tracing the middle strand of the NorthAnatolian Fault Zone through the southern Sea of Marmara based onseismic reflection studies, Geo-Mar. Lett., 27, 27–40.

Kuscu, I., 2009. Cross-basin Faulting and Extinction of Pull-apart Basins inthe Sea of Marmara, NW Turkey, Turkish J. Earth Sci., 18, 3, 331–349.

Kuscu, I., Okamura, M., Matsuoka, H., Yamamori, K., Awata, Y. & Ozalp,S., 2009. Recognition of active faults and stepover geometry in GemlikBay, Sea of Marmara, NW Turkey, Mar. Geol., 260, 90–101.

Le Pichon, X. et al., 2001. The active Main Marmara Fault, Earth planet.Sci. Lett., 192, 595–616.

McHugh, C.M.G., Seeber, L., Cormier, M., Dutton, J., Cagatay, N., Polonia,A., Ryan, W.B.F. & Gorur, N., 2006. Submarine earthquake geologyalong the North Anatolia Fault in the Marmara Sea, Turkey: a model fortransform basin sedimentation, Earth planet. Sci. Lett., 248, 661–684.

Okay A.I., Sengor A.M.C. & Gorur N., 1994. Kinematic history of the open-ing of the Black Sea and its effect on the surrounding regions. Geology,22, 267–270.

Okay, A.I., Demirbag, E., Kurt, H., Okay, N. & Kuscu, I., 1999, An active,deep marine strike-slip basin along the North Anatolian Fault in Turkey.Tectonics, 18, 129–147.

Okay A.I., Kaslılar-Ozcan A., Imren C., Boztepe-Guney A., Demirbag E.& Kuscu, I., 2000. Active faults and evolving strike-slip basins in theMarmara Sea, northwest Turkey: a multichannel seismic reflection study,Tectonophysics, 321, 189–218.

Okay, A.I., Satır, M., Tuysuz, O., Akyuz, S. & Chen, F., 2001. The tectonicsof the Strandja Massif: late Variscan and mid-Mesozoic deformation andmetamorphism in the north Aegean. Int. J. Earth Sci., 90, 217–233.

C© 2011 The Authors, GJI, 185, 93–105



Oktay, F.Y., Gokasan, E., Sakınc, M., Yaltırak, C., Imren, C. & Demirbag,E., 2002. The effects of the North Anatolian Fault Zone on the latestconnection between Black Sea and Sea of Marmara. Mar. Geol., 190,367–382.

Olsen, K.B., 2000. Site amplification in the Los Angeles Basin from three-dimensional modeling of Ground Motion, Bull. seism. Soc. Am., 90(6B),S77–S94.

Ozgul, N. et al., 2005. Istanbul il alanının genel jeoloji ozellikleri, IBBDeprem ve Zemin Inceleme Mud., 79 pp., Istanbul.

Park, C.B., Miller, R.D. & Xia, J., 1999. Multichannel analysis of surfacewaves (MASW), Geophysics, 64, 800–808.

Parke, J.R., Minshull, T.A., Anderson, G., White, R.S., McKenzie, D.P.,Kuscu, I., Gorur, N. & Sengor, A.M.C., 1999, Active faults in the Seaof Marmara, western Turkey, imaged by seismic reflection profiles, TerraNova, 11, 223–227.

Pritchett, W.C., 1990. Acquiring Better Seismic Data, Chapman and Hill,London.

Sakınc, M., Yaltırak, C. & Oktay, F.Y., 1999. Palaeogeographical evolu-tion of the Thrace Neogene Basin and the Tethys–Paratethys relationsat northwestern Turkey (Thrace), Palaeogeog. Palaeclimat. Palaeoecol.,153, 17–40.

Sorlien, C.C. et al., 2008. The seismic stratigraphic record of Quaternarydeformation across the North Anatolian Fault system in Southern Mar-mara Sea, Turkey, AGU Fall Meeting, San Francisco, CA. EOS, Trans.Am. geophys. Un., 89(53), T21A-1915.

Sunal, G., Natal’in, B., Satır, M. & Toraman, E., 2006. Paleozoic mag-matic events in the Strandja Masif, NW Turkey, Geodinamica Acta, 19,283–300.

Wessel, P. & Smith, W.H.F., 1991. Free software helps map and display data,EOS, Trans. Am. geophys. Un., 72(41), 441.

Yaltırak, C. & Alpar, B., 2002. Evolution of the middle strand of North-ern Anatolian Fault and shallow seismic investigation of the southernMarmara Sea (Gemlik Bay), Mar. Geol., 190, 307–327.

Yılmaz, O., 1987. Seismic Data Processing. Investigations in Geophysics,Vol. 2, Society of Exploration Geophysicists, Tulsa, OK.

Zor, E., Ozalaybey, S., Karaaslan, A., Tapırdamaz, M.C., Ozalaybey, S.,Tarancıoglu, A. & Erkan, B., 2010. Shear-wave velocity structure ofthe Izmit Bay area (Turkey) estimated from active-passive array sur-face wave and single-station microtremor methods, Geophys. J. Int., 182,1603–1618.

S U P P O RT I N G I N F O R M AT I O N

Additional Supporting Information may be found in the online ver-sion of this article:

Table S1. Table of east and north GPS velocity components (EVel, N Vel) and 1-sigma uncertainties (Sig E, Sig N), correlationbetween E and N velocity components (RHO) in a Eurasia-fixedreference frame.

Figure S1. GPS velocities, in Eurasia-fixed reference frame. Dom-inant deformations are in our study area of the northern part of Seaof Marmara. Note that, all sites are in the Eurasian Plate.

Please note: Wiley-Blackwell are not responsible for the content orfunctionality of any supporting materials supplied by the authors.Any queries (other than missing material) should be directed to thecorresponding author for the article.

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