Geo-Mar Lett (2002) 21: 183–199 DOI 10.1007/s00367-001-0088-0 Erkan Gökaşan Æ Cem Gazioğlu Æ Bedri Alpar Zeki Yaşar Yücel Æ Şükrü Ersoy Æ Oğuz Gündoğdu Cenk Yaltırak Æ Buser Tok Evidence of NW extension of the North Anatolian Fault Zone in the Marmara Sea: a new interpretation of the Marmara Sea (I_zmit) earthquake on 17 August 1999 • Received: 29 May 2001 / Accepted: 4 December 2001 / Published online: 13 February 2002 Springer-Verlag 2002 Abstract Active faults aligning in NW–SE direction and forming flower structures of strike-slip faults were observed in shallow seismic data from the shelf offshore of Avcılar in the northern Marmara Sea. By following the parallel drainage pattern and scarps, these faults were traced as NW–SE-directed lineaments in the morphology of the northern onshore sector of the Marmara Sea (eastern Thrace Peninsula). Right-lateral displacements in two watersheds of drainage and on the coast of the Marmara Sea and Black Sea are associated with these lineaments. This right-lateral displacement along the course of these faults suggests a new, active strike-slip fault zone located at the NW extension of the northern boundary fault of the Çınarcık Basin in the Marmara Sea. This new fault zone is interpreted as the NW extension of the northern branch of the North Anatolian Fault Zone (NAFZ), extending from the Çınarcık Basin of the Marmara Sea to the Black Sea coast of the Thrace Peninsula, and passing through Büyükçekmece and Küçükçekmece lagoons. These data suggest that the rupture of the 17 August 1999 earthE. Gökaşan (&) Æ C. Gazioğlu Æ B. Alpar Æ Z.Y. Yücel Istanbul University, Institute of Marine Science and Management, 34470, Vefa, Istanbul, Turkey E-mail: egokasan@istanbul.edu.tr Tel.: +90-212-5286022 Fax: +90-212-5268433 Ş. Ersoy Istanbul University, Faculty of Engineering, Geological Engineering Department, 34850, Avcılar, Istanbul, Turkey O. Gündoğdu Istanbul University, Faculty of Engineering, Geophysical Engineering Department, 34850, Avcılar, Istanbul, Turkey C. Yaltırak Istanbul Technical University Mining Faculty, Geology Department, 80626 Ayazağa, Istanbul, Turkey B. Tok Turkish Navy, Department of Navigation, Hydrography and Oceanography, 81647 Çubuklu, Istanbul, Turkey quake in the NAFZ may have extended through Avcılar. Indeed, Avcılar and I_ zmit, both located on the Marmara Sea coast along the rupture route, were strongly struck by the earthquake whereas the settlements between Avcılar and I_ zmit were much less affected. Therefore, this interpretation can explain the extraordinary damage in Avcılar, based on the newly discovered rupture of the NAFZ in the Marmara Sea. However, this suggestion needs to be confirmed by further seismological studies. Introduction The North Anatolian Fault Zone (NAFZ) is a dextral strike-slip fault zone, extending from the Karlıova region to I_ zmit Bay along Anatolia, and south of Thrace as the Ganos Fault (e.g. Ketin 1948, 1968; Şengör 1979, 1980; Şengör et al. 1985; Şengör 1987; Barka and Kadinsky-Cade 1988; Şaroğlu and Yılmaz 1991; Barka 1992; Şaroğlu et al. 1992; Yılmaz et al. 1997; Yaltırak et al. 1998; Fig. 1a). Although it has been the object of research for over 50 years, its geometry in the Marmara Sea still controversially since it was proposed by Pınar (1942) to be an E-W-directed master fault (Fig. 1a). Previous studies have generally focused on the development of four basins of the Marmara Sea, using seismic, seismologic and bathymetric data. Some authors suggest that the NAFZ is a single shear zone (Pınar 1942; Þengör 1979; Şengör et al. 1985), whereas others propose that the Marmara Sea is cut by an E-W trending graben (Ketin 1968; Crampin and Evans 1986; Smith et al. 1995). The NAFZ has also been considered as a combination of en-echelon strike-slip faults and pull-apart basins (Barka and Kadinsky-Cade 1988; Barka 1992; Wong et al. 1995; Görür et al. 1997; Armijo et al. 1999). Since the Marmara (I_ zmit) earthquake of 17 August 1999, the NAFZ has been considered with 184 185 b Fig. 1 a Geographic location of the study area on a map of the eastern Mediterranean region and on a three-dimensional morphological image of Anatolia (extracted from Algün et al. 1999). The course of the NAFZ is seen in the morphology. Dashed line NAFZ route first suggested by Pınar (1942). A Avcılar, Ç-KES Çatalca-Kocaeli erosional surface, GF Ganos Fault, İ I_ ğneada, I_ zI_ zmit, KP Kocaeli Peninsula, MS Marmara Sea, sA Study area, sCA Seismic cruise area, SM Stradnja Mountain, TP Thrace Peninsula. b Three-dimensional image of the Marmara region and the NAFZ in the Marmara Sea (Gazioğlu et al., unpublished data). The NAFZ separates two branches in the Çınarcık Basin (ÇB), along the northern and southern slopes (Okay et al. 2000; Aksu et al. 2000; I_ mren et al. 2001). On the western side, the Ganos Fault (GF) extends to the east, cutting all basins and ridges of the western Marmara Trough (MT) offshore of the Büyükçekmece Lagoon (BL; Okay et al. 2000; I_ mren et al. 2001). The inset map of the Marmara region shows the sources of the data for the three-dimensional image. Darkly shaded area shows topographic data, nonshaded area shows the multibeam bathymetric data, and lightly shaded area shows the earlier available singlebeam bathymetric data. BL Büyükçekmece Lagoon, İB I_ zmit Bay, KÇ Küçükçekmece Lagoon, l Linear slope of Çınarcık Basin. c Seismic profiles on three-dimensional image of the southern part of the area. Bold lines show the selected sections in Figs. 5A–F. ER Eastern ridge, GH Golden Horn different models in the Marmara Sea. Some authors interpreted the NAFZ as a few E-W-directed branches (Parke et al. 1999; Siyako et al. 2000). However, detailed multichannel seismic and multibeam bathymetric data indicate that an approximately E-W-directed strike-slip fault following the Marmara Trough controls the active tectonics of the Marmara Sea (Okay et al. 1999, 2000; Alpar and Yaltırak 2000; Aksu et al. 2000; I_ mren et al. 2001; Fig. 1b). Seismic, seismologic and geologic studies carried out in I_ zmit Bay after the earthquake clearly showed that the rupture of the earthquake had right-lateral strike-slip motion (Barka et al. 2000; Lettis et al. 2000; Örgülü and Aktar 2001; Gökaşan et al. 2001). The aftershocks of the earthquake were traced on the northern and southern slopes of the Çınarcık Basin, with right-lateral strikeslip movement (Örgülü and Aktar 2001). Okay et al. (2000), Aksu et al. (2000), and I_ mren et al. (2001) suggested that the rupture of the NAFZ separates two branches in the Çınarcık Basin, aligning both northern and southern slopes of the basin. According to Okay et al. (2000), the northern branch of this fault extends to the west, bending from NW to ENE off Küçükçekmece Lagoon, whereas I_ mren et al. (2001) proposed this branch dies and is replaced with reverse faulting off Küçükçekmece Lagoon, the southern branch possibly merging with the Ganos Fault in the west. To date, most of these investigations have assumed that the NAFZ does not exist in the north of the Marmara Trough. However, active faults have been observed on the Istanbul (Trakya) and Kocaeli peninsulas in the north of the Marmara Sea (Koral 1995; Koral and Şen 1995; Gökaşan et al. 1997; Koral 1998; Demirbağ et al. 1999; Oktay et al. 2002). Also, seismological records show evidence of microseismic activity on these peninsulas, particularly around the Küçükçekmece and Büyükçekmece lagoons (KOERI 2000; Gürbüz et al. 2000; Fig. 2). A major surface rupture 3 km long, and also a couple of shorter ones which formed in 1894 along the Avcılar shoreline, have been reported in historical records (Öztin 1994). Perincek (1991) interpreted a north-western-trending inactive strike-slip fault (Thrace Fault) from seismic sections in central Thrace, and suggested that this fault zone is a possible strand of the NAFZ in Thrace. Perincek (1991) also suggested that an erosional surface marking the end of deformation related to the particular fault has been folded and faulted. This author related this deformation on the erosional surface with reactivation of the Thrace fault system or consequence of the recent formation of the Marmara Sea basin (Perinçek 1991). In addition, a significant linear morphology of the NW-oriented eastern side of the northern slope of the Çınarcık Basin was replaced by a convex shape off Küçükçekmece Lagoon (Gazioğlu et al., unpublished data; Fig. 1b, c). This linear slope aligns with the NW-SE-directed right-lateral displacement on the eastern coast of the Küçükçekmece Lagoon (Fig. 1b, c). This direction is also the same as that of the parallel drainage system of the study area on the eastern Thrace Peninsula (Figs. 1a–c, 2a). The Marmara (I_ zmit) earthquake of 17 August 1999 also struck the town Avcılar close to the Küçükçekmece Lagoon, located 105.5 km from the main impact area in I_ zmit. Avcılar was badly destroyed and many people lost their lives in the area whereas the other settlements between Avcılar and the centre of the earthquake (I_ zmit) were less affected by the earthquake. After the 1999 I_ zmit earthquake, high-resolution single-channel shallow seismic data were gathered offshore of Avcılar from aboard the R/V Arar, with the aim of identifying the reasons for this large-scale destruction. In order to achieve this aim, we investigated the possible existence of active faults in the north of the Marmara Trough, as well as any relationship with the rupture of the NAFZ on the northern slope of the Çınarcık Basin, using shallowwater seismics in combination with regional bathymetric and topographic maps as well as satellite imagery. Materials and methods Seismic surveys (220 km in all) were conducted in water depths of 11–300 m in the Marmara Sea on the northern continental shelf off the Büyükçekmece and Küçükçekmece lagoons (Fig. 1c). We used a 1.25-kJ multi-electrode spark array and an 11-element, 10-m-long surface-towed hydrophone streamer. The seismic source had 30 discharging electrodes (6 kV, 30 mF), offsetting about 5 cm. Sampling interval was 186 Fig. 2 Seismic activity in the Marmara Sea and surrounding region between 1900 and 2000 (KOERI 2000) 0.25 ms, recording window length was 250 ms (two-way-travel time), and shot interval was two (about 4.1 m). These recording parameters provide information on sedimentary deposits up to 150 m below the seabed. Positioning was carried out by means of a differential geographic positioning system (GPS) with an accuracy of ±20 m. Three-dimensional surface images were extracted from Gazioğlu (2001), combining multibeam and earlier available bathymetric and topographic data. For the land surfaces, contour lines were digitised on 1:25,000 topographic maps (Fig. 1b) for every 20-m contour line, using the AutoCAD mapping software for the area. However, for closer similarities to real morphology, in certain morphological units (such as hills, rivers, lakes and coastlines) additional digitising processes were performed. Digitised lines and points were converted into shape files by means of the ArcView 3.1 software. Erdas Imagine 8.3.1 image processing software was used to generate a digital terrain model (DTM) in the form of a raster structure. The resulting DTM of the land area has a cell size of 40 m and a residual mean square (RMS) of ±13.05 m. Multibeam data were collected from the Marmara Sea from aboard the TCG Çubuklu (Fig. 1b). The single-beam data used for the area were from the multibeam data coverage. ArcView 3.2 was used to merge these two data sets. Multibeam data were preferred in overlapping data sets. The resulting DTM has a cell size of 40 m and an RMS of ca. –18.87 m (Gazioğlu 2001) in the total area, and an RMS of ca. –15.65 m in the multibeam data coverage area. The ERDAS software model was used to merge the land and sea DTMs, without any interpolation between bathymetric and topographic models. Generation of an image with grey level corresponding to elevation allows visualisation and integration with other data. Image processing software mapping between elevation and grey level is linear (minimum value of 0 to maximum value of 255). DTMs were viewed by means of Image Drape, a remote sensing and GIS technique which overlays satellite images over the models. For this purpose, the Landsat 5 TM images of May 1992 and September 1999 were used (cf. full coverage of study area). The images were converted into the same projection of UTM (Universal Transverse Mercator Projection, zone number 35) by using ground control points. Various band combinations of the Landsat images were used. 187 Results Morphology of eastern Thrace Peninsula Background information Major morphological features on eastern Thrace Peninsula include a mature erosional surface and the southwestern edge of the Strandja Mountains (Figs. 1a, 3a). The erosional surface has been recognised since the 20th century, and has been dated as Upper Miocene–Upper Pliocene (Cvijic 1908; Chaput 1936). This surface has been cut by rejuvenated streams (Fig. 3a). The largely parallel drainage patterns of the major valleys developed on Miocene rocks, and lattices on Paleozoic rocks (Erinç 2000). The Strandja Mountains also present a rejuvenated morphology with linear scarps (Fig. 3a). The scarps and drainage system generally lie in a NW–SE direction (Fig. 3a). Present study In the present study, numerous lineaments were observed for the parallel pattern drainage, linear slopes, and right-lateral displacements on the watersheds and shoreline of the Thrace Peninsula in the study area (Fig. 3a, b). In this study we focused on 18 NW-SEoriented lineaments (L1–L18) as well as two N–S and NE–SW lineament groups (L19–L20). Five of these lineaments (L1–L5), passing through the whole area continuously and cutting the others, are considered as major lineaments (Fig. 3b). There are also a number of shorter NE-SW-aligned lineaments between the NWSE-oriented lineaments which, in turn, are cut by the NW–SE ones (Fig. 3a–f). Lineament L1 can be traced from the east of the Strait of Istanbul to the west of the Terkos Lake. The NW–SE aligned linear Marmara Sea shoreline on the Kocaeli Peninsula and the Golden Horn are considered as the eastern edge of L1 (Fig. 3b). L1 follows the northern scarp of the Derbent Hill, and the source of the Alibey Stream. It passes through the Terkos Lake and combines with lineament L2 on the Black Sea coast of the Thrace Peninsula (Fig. 3b–f). The NW–SE central axis of the Küçükçekmece Lagoon and the 3.5-km-long right-lateral displacement on the Marmara Sea coast of the lagoon are taken as indicators of lineament L2 (Fig. 3a–d). L2 extends northwards following the NW–SE valley of the Koğukdere Stream. This lineament passes through a right-lateral displacement in the watershed between the Koğukdere and Sazlıkçayırdere streams (Fig. 3a, b, d), and follows the western side of this watershed (eastern side of Sazlıkçayırdere Stream) to the north (Fig. 3a–d). Scarps along the north-eastern edge of the Strandja Mountains constitute the northern extension of L2 (Fig. 3a, b, d, e). In the northern sector of these scarps, L1 joins L2 on the Black Sea coast. L2 passes through a right-lateral displacement on the Black Sea shoreline (Kıyıköy Peninsula), and it extends along the southern edge of I_ ğneada Bay to the NW corner of the study area (Fig. 3a, b, d, e). A slight right-lateral displacement on the Marmara Sea shoreline delineates the southernmost extension of lineament L3 (Fig. 3a, b). The NW–SE Haramidere stream valley and the eastern side of Büyükçekmece Basin indicate the course of lineament L3 (Fig. 3a–e). This lineament passes through a right-lateral displacement in another watershed between the Haramidere and Sazlıkçayırdere streams (Fig. 3a, b, d). The northern extension of L3 is revealed by south-west-facing scarps of the Strandja Mountains. Lineaments L4 and L5 are indicated by the northand south-facing scarps of the Çatalca Rise (Fig. 3a– d). The south-western side of the Büyükçekmece Basin and the Tepecik stream valley follow the alignment of L4. Lineaments L6 and L7 parallel lineament L1 in the north of the study area (Fig. 3a, b, d–f). L6, and are situated north of L1, starting from the southern entrance of the Strait of Istanbul and following the Alibey stream valley and the northern shoreline of Terkos Lake to eventually pass through the right-lateral displacement on the Black Sea coast of Thrace Peninsula. L7 extends from the Kocaeli Peninsula in the east from where it crosses the Bosphorus, passes through the Kağıthane stream valley to cut the Black Sea coast north-east of Terkos Lake (Fig. 3b, f). Lineament L8 follows the north-eastern slope of the Strandja Mountains until it merges with L2 (Fig. 3a, b, d, e). Lineament L9 is aligned with the Sazlıdere stream valley (Fig. 3a–f). Lineament L10 is identified as a right-lateral displacement along the northern shoreline of I_ ğneada Bay (Fig. 3a, b, d, e). Lineaments L18 represent the shorter NW-SE-oriented lineaments in Fig. 3a–f. The linear NW-SE-aligned northern slope of the Çınarcık Basin constitutes the offshore member of the lineaments (labelled as L in Fig. 1b, c). This lineament, located between the Marmara entrance of I_ zmit Bay and the south-eastern shore of Küçükçekmece Lagoon, forms a NW-SE-oriented linear offshore slope. To the south and south-west of Küçükçekmece Lagoon, this linear slope curves in an E–W direction (Fig. 1b, c). Seismic profiles on the northern shelf of the Marmara Sea Seismic data from the shelf area off the Küçükçekmece and Büyükçekmece lagoons (Fig. 1c) were interpreted stratigraphically and structurally by tracing the submarine continuation of the lineaments onshore. 188 189 b Fig. 3A–F Lineaments and some morphological features of the study area. A Simplified drainage and scarps on the morphology. B Lineaments simulating parallel pattern streams, scarps, and displacements on the shoreline. C A three-dimensional image and lineaments of the Büyükçekmece and Küçükçekmece lagoons and the Marmara Sea coast from the north. T Terkos Lake. D A three-dimensional image and lineaments of the Terkos Lake and Black Sea coast from the south-east. The three-dimensional image in the inset shows right-lateral displacements in the watersheds between the Sazlıkçayırdere, Haramidere, and Koğukdere streams. E A three-dimensional image and lineaments of the Terkos Lake and Black Sea coast from the east. F A three-dimensional image and lineaments of the Strait of Istanbul and Black Sea coast from the south-east. SoI_ Strait of Istanbul (Bosphorus) Seismic stratigraphy Three sediment sequences and a truncation are distinguished using the reflection configuration of the sediment deposits. These sequences are named lower unit, upper unit, and deltaic sequence (Figs. 4, 5a–f). Although the reflection configuration of the lower unit indicates a characteristic deformation, its initial internal parallel reflectors are observed on the seismic profiles. An angular unconformity marks the top of the lower unit, indicating an erosional period after deposition. This surface has also been reported as developed during the Würm glacial age from neighbouring areas (Demirbağ et al. 1999; Oktay et al. 2002). The parallel reflection configuration of the lower unit indicates initial deposition under low-energy conditions. However, along some narrow zones the lower unit is much more deformed than the parts on the seismic profiles called the high-deformation zone in the lower unit (Figs. 4 and 5a–f). In addition, some gas charges veiling the strata of the lower unit on seismic profiles occasionally occur in these zones (Fig. 5 c, d). Onshore deposits comprise Upper Miocene and older sediments covering the coast and hinterland of the study area (Fig. 6; Arıç 1955). The Upper Miocene sequence consists of unconsolidated gravel and sand, the base having a variable thickness, and passes through claymarl-mudstones and muddy limestone towards the top (Arıç 1955). The lower unit is interpreted as a submarine extension of the Upper Miocene and older sediments. A deltaic sequence with an oblique sigmoidal reflection configuration is observed between the upper and lower units on the truncation surface of the Marmara entrance of the Strait of Istanbul (Fig. 5a). This sequence was interpreted by Oktay et al. (2002) as delta deposits which originated during the Würm glacial period. An isopach map of this sequence is shown in Fig. 7c. The evidence indicates that the delta sequence was developed in the Bosphorus channel during the Würm glacial age before deposition of the upper unit, whereas the northern shelf of the Marmara Sea was eroded (Oktay et al. 2002). The upper unit is exposed on the truncation surface on the northern shelf of the Marmara Sea (Figs. 4, 5a–f). It is characterised by an internal parallel reflection configuration, implying low-energy depositional conditions for this unit. A truncation surface is not observed in the internal reflector. The top of this sequence forms the modern bed, suggesting that this upper unit is still actively accreting today. Deposition should thus have commenced in the course of the Flandrian transgression. The seismic profiles reveal that the upper unit is deformed together with the truncation surface and seafloor in the highly deformed, lower sequence zones, the thickness of the upper unit changing in these zones. Structural interpretation On the seismic profiles, the layers of the lower unit, which is the submarine extension of the Upper Miocene and older deposits, are observed as being gently deformed and truncated by the erosional surface during the Würm glacial age (Figs. 4, 5a–f). This gentle deformation of the lower unit is not active today. However, the lower unit is highly deformed along some narrow zones, together with the truncation surface, upper unit and seafloor (Figs. 4, 5a–f). These highly deformed zones are frequently associated with gas charging. Therefore, any faults in the deformed zone of the lower unit cutting or affecting the truncation surface, the upper sediment layers, and the seafloor may be interpreted as being active faults. These active faults are identified as fault groups on seismic profiles, and are interpreted as active fault zones. Some of them extend in a NW–SE direction, five faults being interpreted as major active fault zones named faults F1, F2, F3, F4, and F5. Faults are observed on the seismic profiles from the eastern edge of the study area, close to the Marmara Sea entrance of the Strait of Istanbul (Bosphorus; Figs. 5a, b, 7a). These faults cut layers of the upper unit, truncation surface and seafloor, and together indicate the formation of an active fault zone (Fig. 5a, b). On the other hand, small steps in the seafloor and uplifting along this zone are indicators of a strike-slip fault. Right-lateral displacement of the shoreline along the route of these faults supports this interpretation (Fig. 7a, b). This displacement also indicates the course of lineament L17, and thus lineament L17 should be an onshore extension of these faults (Figs. 7a–d, 8a). An isopach map of the delta sequence, and a palaeotopographic map of the lower sequence (Oktay et al. 2002) at the Marmara Sea entrance of the Bosphorus indicate right-lateral displacements along these faults (Fig. 7c, d). At the same time, in the hinterland extension of this fault zone, the Ayamama stream valley (L1; Fig. 3a, b) is observed to be aligned in the same direction as the active faults (Meriç et al. 1991). This suggests that the faults form a NE-SW-directed active strike-slip fault zone (fault F1). Other NE-SW-aligned faults (fault 2), which affect all units in the study area, occupy the area extending from south of the Küçükçekmece Lagoon through to the south-eastern edge of the study area (Figs. 5b, c, 190 Fig. 4A, B Detailed parts of the seismic sections. Active faults are indicated by highly deformed lower sequence zones, thickness changing and deformation at the upper sequence, as well as deformation on the truncation surface and seafloor. A Section of Fig. 5B. B Section of Fig. 5E 7a, b). These active faults cut the seafloor without any dip-slip component. The Küçükçekmece Lagoon and lineament L2 are located on the hinterland extension of these faults, along which some microseismic activity has been recorded (KOERI 2000; Figs. 2, 3b, 7a, b, 8a). Also, the shoreline is laterally displaced 3.5 km right through the south-eastern corner of the Küçükçekmece Lagoon (Fig. 3a). The contours of delta deposits and the palaeotopography of the lower sequence in the Bosphorus are right-laterally displaced along fault F2 (Fig. 7c, d). A third fault group has been traced to extend in a NW–SE direction (fault F3; Figs. 5b, c, 7a, b). A slight dip-slip movement is observed on the irregular seafloor (Fig. 5c). The shoreline is right-laterally displaced as well as the Haramidere stream valley (Fig. 3a), which 191 Fig. 5a–f Interpreted and uninterpreted seismic sections. A detailed explanation of the active faults which cut the seafloor is given in the text indicates lineament L3 developed along the course of fault F3 (Figs. 3b, 7a, b, 8a). This indicates that fault F3 is a major, active strike-slip fault zone linked to fault F2 offshore of Avcılar (Fig. 7a, b). The faults are interpreted to form a NW-SE-directed fault zone both inside and offshore of the Büyükçekmece Lagoon (fault F4; Figs. 5d, f, 7a, b). Although a narrow depression is observed on the truncation surface, the faults cut the seafloor without any dip-slip component being visible on the seismic profiles from within and south of the Büyükçekmece Lagoon (Fig. 5f). Microseismic activity observed in the Büyükçekmece Lagoon (KOERI 2000; Fig. 2) implies that these areas are seismically active. A NW-SE-directed fault (Çatalca Fault), which can be traced from the inland morphology as lineament L4 (Fig. 3a, b), has been reported from the north-west of Büyükçekmece Lagoon (Koral 1995, 1998; Figs. 3a, b, 7a, b, 8b). The last group of faults, which forms the major fault zone, is observed along the south-western edge of the study area (fault F5; Figs. 5d–f, 7a, b). These faults cut not only the truncation surface but also the seafloor. The thickness of the upper unit is affected by this fault group along its course (Fig. 5c, e, f). The linear shoreline along the west coast of Büyükçekmece Lagoon and lineament L5 may be interpreted as the hinterland extension of these faults (Figs. 3b, 7a, b, 8b) being associated with the active NW-SE-directed strike-slip fault zone. The other active faults affecting all of the stratigraphic units seem to have a strike-slip character due to 192 Fig. 5 (Contd.) 193 Fig. 5 (Contd.) their high angularity with respect to the seafloor, their dip-slip components slightly affecting the strata (Figs. 5a–f, 7a). These fault groups are numbered as F6, F8, F12, F13, F14, F15, and F16. Faults F12, F13 and F15 are bounded by the major faults F4 and F5 (Figs. 5d, e, 7a, b). Fault F6, aligned NW–SE, is located on the western side of fault F5 and connects to this fault in the SE, i.e. offshore of Büyükçekmece Lagoon. Faults F9 and F10 were originally observed by Oktay et al. (2002) and are 194 Fig. 6 Simplified lithological map of the terrestrial part of the study area recognised in our seismic profile as dip-slip faults (Fig. 5a). The isopach maps of the Marmara entrance of the Bosphorus indicate that fault F10 consists of at least seven normal subfaults which bound the Bosphorus channel (Fig. 7c, d). Fault F11 is interpreted as a strike-slip fault within a dip-slip component between faults F1 and F2 (Figs. 5b, 7a–d). Faults F7 and F17 are interpreted as dip-slip faults. With their synthetic faults they form the westernmost part of the northern slope of the Çınarcık Basin (Figs. 1b, 5c–e, 7a, b). The present shape of the slope is not only controlled and developed by the dip-slip faults (such as faults F7 and F17) but also by submarine landslides which are probably triggered by normal faulting (Fig. 5c, e). Fault F18 is interpreted as a strike-slip fault with a dipslip component along the north-eastern edge of the study area (Fig. 5a). Right-lateral displacements on the shoreline and isopach maps of the Bosphorus entrance are considered as evidence for the right-lateral activity of this fault (Fig. 7a–d). Discussion and conclusions Lineaments controlling the present morphology of the terrestrial portion of the study area should be younger than the Upper Miocene–Upper Pliocene mature erosional surface. The NW-SE-directed lineaments are more continuous and cut or bound the NE-SW-directed lineaments, indicating that the major lineament direction is aligned NW–SE in the study area. Some of the NWSE-aligned faults, along the route of lineaments L2, L10, and L13 in I_ ğneada Bay (Turoğlu 1995) and offshore of Küçükçekmece Lagoon (Yaltırak and Alpar 2000), L4 in Çatalca (Koral 1995, 1998), and L17 in the Ayamama Stream (Meriç et al. 1991), have been reported using seismic, geological and geomorphological studies. NW-SE-directed right-lateral displacements occur along the southern and northern coastlines of the Thrace Peninsula, e.g. the entrances of Küçükçekmece Lagoon, Ayamama and Haramidere streams and along the Black Sea shoreline around Kıyıköy and I_ ğneada along lineaments L2, L3, L5, L6, L10 and L17. The watersheds between the Koğukdere, Haramidere, and Sazlıkçayırdere rivers are also right-laterally displaced, coinciding to the NW-SE-directed lineaments L2 and L3. All this evidence on morphology is thought to be expressed in the NW–SE-oriented right-lateral strike-slip faults on the eastern side of Thrace Peninsula. Faults on the seismic profiles are interpreted as active right-lateral strike-slip faults, due to their flower-like expression, right-lateral displacements in delta sediments, the basement palaeosurface in the Strait of Istanbul (Oktay et al. 2002), and the coast of the Thrace Peninsula. These faults can be traced from one seismic profile to another in a NW–SE direction. Most of the major lineaments are observed along the route of the onshore extensions of the faults. They can thus be connected to each other and are interpreted as active faults (Figs. 8, 9). Some NW-SE-directed, rightlateral active strike-slip faults observed on the northern shelf of the Marmara Sea are therefore extended to the onshore area between Avcılar and I_ ğneada on the eastern Thrace Peninsula (Figs. 9, 10). A block rotation model has been suggested to explain the tectonic evolution of the Strait of Istanbul (Oktay et al. 2002). This model may also be applicable to the study area. According to this model, the NW–SE main fault zones may be considered to represent the zonebounding faults identified by Peacock et al. (1998), and the NE-SW-directed shorter ones might be the traces of block-bounding faults observed in the morphology and on seismic sections. Okay et al. (2000), Aksu et al. (2000), and I_ mren et al. (2001) identified that the northern segment of the NAFZ separates two branches in Çınarcık Basin, following the northern and southern slopes of the basin. These authors also suggested that the Ganos Fault in the western onshore region of the Marmara Sea extends to the east in an ENE–WSW direction to the offshore 195 Fig. 7A–D Active fault map of coastal and offshore portion of the study area. A Three-dimensional view of faults with some interpreted sections. B Two-dimensional view of the active faults on the northern shelf of the Marmara Sea. C Isopach map of the deltaic sequence in the Marmara Sea entrance of the Bosphorus, and active faults which are correlated with the right-lateral displacements on contours and variations in thickness values of this sequence (Oktay et al. 2002). Faults 1, 2, 11, and 18 align with these variations. D Palaeotopographic map of Upper Miocene and older sediments in the Marmara Sea entrance of the Bosphorus, and active faults which are indicated by right-lateral displacements on contours and variations in depth values of the contours (Oktay et al. 2002). Faults 1, 2, 11, and 18 align with these variations region of Küçükçekmece and Büyükçekmece lagoons (Fig. 10). Recent seismological studies focused on the Marmara Sea support these suggestions. A dominance of strike-slip motions is observed along both northern and southern slopes of the Çınarcık Basin, resulting from the focus of aftershocks of the 17 August 1999 I_ zmit earthquake (Örgülü and Aktar 2001), and ENE– WSW-oriented microseismic activity aligning the suggested offshore extension of the Ganos Fault was observed by Gürbüz et al. (2000). Okay et al. (2000) suggested that the northern branch bends off Küçükçekmece Lagoon and merges with the Ganos Fault. Thus, this merged branch consists of the major segment of the NAFZ in the Marmara Sea, whereas the southern branch dies at the western edge of the Çınarcık Basin. However, I_ mren et al. (2001) proposed that the northern branch dies off Küçükçekmece Lagoon, whereas the southern one merges with the Ganos Fault and forms the major segment of the NAFZ in the Marmara Sea. 196 Fig. 8A, B Three-dimensional combined images with seismic profiles and morphologic images. Lineaments L2, L3, L4, L5, and L17 align with the faults F1, F2, F3, F4, and F5 as their onshore extension along the eastern Thrace Peninsula. A Combined image for Fig. 5b and morphologic image around Küçükçekmece Lagoon. B Combined image for Fig. 6b and morphologic image around Büyükçekmece Lagoon We contend that, instead of dying or bending offshore of Küçükçekmece Lagoon, the northern branch extends in a NW direction through the steps of the northern slope whereas the linear slope becomes curved, and forms the faults (F1–F18) on the northern shelf and the lineaments (L1–L20) on the Thrace Peninsula (Figs. 9, 10). In this case, the southern branch may merge the Ganos Fault and form the major segment of 197 Fig. 9 Three-dimensional image showing the combination of faults observed on the shelf with lineaments on terrestrial morphology and the northern branch of the NAFZ in the Çınarcık Basin of the Marmara Sea controlling the linear northern slope of the Çınarcık Basin. D Dip-slip movement caused by submarine landslides and normal faults Fig. 10 The NW extension of the North Anatolian Fault Zone between Avcılar and I_ ğneada. This fault is modelled as the NW (eastern Thrace) extension of the northern branch of the NAFZ in the Çınarcık Basin, considering the active faults on the northern shelf of the Marmara Sea, lineaments on the eastern Thrace Peninsula, and right-lateral displaced shoreline of the eastern Thrace Peninsula (base map extracted from Taymaz et al. 2001). Epicentres of microseismic activity on eastern Thrace Peninsula support the existence of the faults. The southern branch of the NAFZ in the Çınarcık Basin is presumed to merge with the Ganos Fault (dashed line), and they consist of the major branch of the NAFZ in the Marmara Sea and extend westwards into the northern Aegean Sea. Thin dashed lines show the other possible active faults due to morphology and microseismic activity the NAFZ in the Marmara Sea, as suggested by I_ mren et al. (2001). In the light of this new interpretation, we propose a different explanation of the Marmara (I_ zmit) earthquake on 17 August 1999. Aftershock locations of the earthquake show that the rupture may have extended at least onto the northern slope of the Çınarcık Basin (Örgülü and Aktar 2001). Avcılar, which is located 105.5 km away from the earthquake centre (I_ zmit), was also seriously affected by the event, whereas other settlements between I_ zmit and Avcılar were much less affected. 198 According to historical records (Öztin 1994), some surface ruptures occurred during the Marmara earthquake of 10 July 1894, which is thought to have originated around I_ zmit. This indicates that Avcılar has also been affected by earthquakes in the past. Avcılar is located along the NW continuation of the northern boundary fault of the Çınarcık Basin, which contained the aftershocks of the earthquake on 17 August 1999 (Örgülü and Aktar 2001). This phenomenon can be explained by extending the rupture of this earthquake through to Avcılar. The other settlements were not damaged during the earthquake, because the rupture between I_ zmit and Avcılar is located in the Marmara Sea. We therefore suggest that the rupture of the 17th August 1999 earthquake was not only activated between Düzce and Hersek (Barka et al. 2000; Lettis et al. 2000) but also along the fault section between Hersek and Avcılar. On the basis of this interpretation, we should call the 17 August 1999 earthquake the Düzce-Avcılar earthquake. The serious damage limited to the Avcılar region indicates that this supposed rupture on 17 August 1999 ended in Avcılar. Although microseismic activity aligns with some ruptures of this newly suggested fault in the eastern Thrace Peninsula (Figs. 2, 10; KOERI 2000; Gürbüz et al. 2000 ; Taymaz et al. 2001), and the first results of the Ocean Bottom Seismogram (OBS) data express microseismic activity on the Thrace and Kocaeli peninsulas after the earthquake on 17 August 1999 (Taymaz et al. 2001), the suggestion of this study needs to be confirmed by further seismological studies. If these arguments and interpretations are correct, then the fault zone between Avcılar and I_ ğneada should be considered as the Thrace extension of the northern boundary fault of the NAFZ in the Çınarcık Basin, and that the Düzce-Avcılar portion of this fault released its stress during the 17 August 1999 earthquake. The area between Avcılar and I_ ğneada should therefore be considered as a new hazard zone for future earthquakes in the Marmara region, including the western part of the Marmara Sea. Acknowledgement We wish to express our sincere thanks to the officers and crew of the research vessel Arar of the University of Istanbul, and the TCG-Çubuklu of the Turkish Navy, Department of Navigation, Hydrography and Oceanography for their guidance and assistance in seismic and bathymetric data acquisition. We are grateful to F.Y. Oktay and Y. Yılmaz for their valuable suggestions. We also thank O. Algan, Ö. Huvaz, T. Ustaömer, S. Gözenç, C. Güneysu, E. Doğan, T. Taymaz, and O. Tan for their valuable suggestions and for providing the topographic and seismologic maps. This work was supported by the Research Fund of the University of Istanbul. Project number YP-91/12122001. References Aksu AE, Calon TJ, Hicott RN (2000) Anatomy of the North Anatolian Fault Zone in the Marmara Sea, Western Turkey: extensional basins above a continental transform. GSA Today 10(6):3–7 Algün O, Köse O, Küpçü R, Tunalı E (1999) Türkiye Sayısal Yükseklik modeli. 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