Analysis and interpretation of the aftershock sequence of the August 17, 1999, Izmit (Turkey) earthquake

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1 Journal of Seismology 6: , Kluwer Academic Publishers. Printed in the Netherlands. 287 Analysis and interpretation of the aftershock sequence of the August 17, 1999, Izmit (Turkey) earthquake O. Polat 1, H. Eyidogan 2, H. Haessler 1,A.Cisternas 1 & H. Philip 3 1 ULP IPGS,5rueRené Descartes, Strasbourg Cedex, France; 2 ITU Maden Fakültesi Jeofizik Mühendisligi Bölümü, Maslak Istanbul, Turkey; 3 Université Montpellier II, Laboratoire de Géophysique, Montpellier, France Received 25 March 2001; accepted in revised form 20 October 2001 Key words: aftershock analysis, Izmit earthquake, Izmit fault, Marmara region, seismotectonic, stress tensor Abstract A micro-seismic field experiment has been carried out in the Marmara Sea region. The analysis of the events before and after the August 17, 1999 Izmit (Turkey) earthquake has been completed events have been well located out of a total of 3165 recorded within the period from July 15 to November 2, % of the aftershocks with magnitude greater than 4 have occurred within the first 6 days after the main-shock. Earthquakes of the Izmit sequence are distributed in the first 15 km of the earth crust, but major events are located in between 5 km and 15 km depth. The seismicity pattern defines a rupture plane extending for about 150 km in an E-W direction. The rupture is extremely linear but segmented, and its complexity increases towards the western end manifesting bifurcation. A stress analysis has been carried out both at the western end and all along the aftershock zone. 96 selected aftershocks, registered between August 21 and October 22, were chosen in order to compute their focal mechanisms and obtain information about the stress regime after the Izmit earthquake. Strike-slip and normal faulting mechanisms are dominant. The numerous strike-slip mechanisms are compatible with a dextral motion on an EW oriented vertical fault plane. The best stress tensor solution shows a regime in extension with a well-defined σ 3 axis oriented approximately N35. Introduction The Marmara region is a transition zone between the strike-slip regime of the North Anatolian Fault (NAF) Zone to the east, and the extension regime of the Aegean Sea to the west. Tectonics in this region is characterized by the splitting of the NAFZ into three branches running more or less in an E-W direction (Figure 1). Activity of the NAF during the 20 th century began with the destructive Erzincan earthquake (Mw = 8.2, according to our estimation) in 1939 in northeast Turkey, and it migrated westwards by a series of earthquakes in 1942, 1943, 1944, 1951 and 1967 (Barka, 1996; Stein et al., 1997; Toksöz et al., 1979). Previously, there had been an important rupture along the western end of the NAF, across the Gallipoli peninsula, in 1912 (Ms = 7.4). Thus, before 1999, a seismic gap remained between the 1967 rupture zone and the broken 1912 segment. Part of this seismic gap has been filled up by the Izmit (Kocaeli) earthquake on August 17, 1999 (Mw = 7.6, Delouis et al., 2002; Harvard CMT), but there still remains a gap between the western end of the Izmit rupture and the Gallipoli peninsula, that represents a high risk for the city of Istanbul with more than 10 million inhabitants. After the study of the 1992 Erzincan earthquake (Fuenzalida et al., 1997), we centered our attention on the Marmara Sea and performed a micro-seismicity field experiment there in 1995 (Gürbüz et al., 2000), within a Turkish-French cooperation. We followed this work in 1999 by deploying a network of 20 short period stations. The installation of this network was completed on July 15, 1999 one month before the destructive Izmit earthquake. The first results of this experiment are under publication (Polat et al., 2002).

2 288 Figure 1. Tectonic features of the Marmara Sea region, together with the USGS DEM 30sec topography (after Barka, 1992). The northern branches of the North Anatolian Fault are seen as scarps in the relief southeast of Istanbul, and across the deep basins on the northern half of the Marmara Sea in the west. It cuts the Gelibolu peninsula and goes into the Saros basin (the political boundary of the city of Istanbul is drawn in gray color). AP: Armutlu peninsula, C: Çinarcik, CD: Çatal Delta, D: Degirmendere, GB: Gemlik Bay, GP: Gelibolu peninsula, HD: Hersek Delta, IB: Izmit Bay, PI: Prince Islands, SL: Sapanca Lake, Y: Yalova, 1967: Mudurnu Valley rupture zone (M 7.1). This paper presents the final results of the 1999 Marmara Sea experiment. We show here the analysis of the aftershocks of the Izmit earthquake, and the results of the stress tensor inversion both for the whole region and for the western part of the Izmit bay. We also show the distributions of magnitudes and depths, as well as field observations along the surface ruptures of the Izmit fault. Aftershock analysis of the 1999 Izmit earthquake A seismic monitoring field experiment was carried out in the Marmara Sea region between July 15 and November 2, Here, we show the complete analysis of the events before and after the August 17, 1999 Izmit earthquake. The phase arrivals from seismograms have been read for events that were well recorded by the 20 short period stations. We performed a statistical analysis in order to retain the best-located events. We selected events with a RMS error smaller or equal to 0.5 sec, and with at least 7 P and 3 S phase arrivals. Based on these criteria, 1446 events were well located over a total of 3165 recorded (Figure 2). Most of the aftershocks follow a succession of linear trends between Şenköy (Çinarcik) and Gölyaka, with concentration of activity NE of the Armutlu peninsula near Yalova and Çinarcik, east of Izmit Bay, and Akyazi. Roughly, aftershocks follow a trend of N85 from Yalova to Izmit, a trend of N105 from Izmit to Akyazi crossing Sapanca Lake, and take a NE direction from Akyazi to Gölyaka. The epicenter distribution is less precise at the eastern end due to weaker station coverage. Three important aftershock clusters have been observed at the west of Hersek delta: a cluster at the south of the Prince Islands on the NW-SE direction, another one on the E-W direction of the NAFZ and a third one between Şenköy and Yalova. We also recorded some seismic activity NW of the Marmara Island following an event of magnitude (Md) 5.0 on September 20, There is also a small seismic linear cluster activated between Iznik Lake and Gemlik bay. Based on the seismicity analysis, 67% of the aftershocks with duration magnitudes greater or equal to 4 occurred within the first 6 days following the main-shock (Figure 2 inset). Among these, 55 events have magnitudes between 3.5 and 4.0, and 395 earthquakes have magnitudes between 3.0 and 3.5. About half (725) of the well located aftershocks have magnitudes between 2.5 and 3.0. Finally, 271 events have magnitudes between 2.0 and 2.5.

3 289 Figure 2. After-shock distribution of the 1999 Izmit earthquakes (August 17-October 23, 1999) as recorded by a local network. The figure shows the stations that were used for the location. Most of them functioned since July 15, Aftershocks with magnitudes greater or equal to 4 can be seen in the inset of the figure. Three clusters are seen, together with the epicenter (large star). See Figure 1 for the names of places. Located events are examined week by week in order to better understand the time evolution of the aftershocks (Figure 3). We could not observe any remarkable seismic activity before the Izmit main shock except for one or two events west of Gölcük and Sapanca Lake. Recorded events within the month before the Izmit earthquake have been located mostly within the northern half of the Marmara Sea region (Figure 3a). Figure 3b shows the Izmit earthquake which is located N E (large star), and its aftershocks during the time interval August. The total number of located events during this period is 331. We observe clusters near Yalova, near Izmit and east of Sapanca Lake. The main elongation of the Yalova cluster is to the north, in the direction of Prince Islands with a concentration to the south of Büyükada Island. Seismic activity is globally aligned E-W but a cluster east of Sapanca Lake is oriented NE- SW. Figure 3c includes 296 events in the period going from 25 to 31 August. There we observe clearly three cluster zones: Armutlu, Izmit and Akyazi. We did not record any remarkable aftershock activity within the segments Hersek delta-izmit city and Izmit city- Akyazi. Figure 3d shows 206 aftershocks within the week of September 1 to 8. Again we observe a gap along Izmit Bay and the clusters at Yalova, Izmit and east of Sapanca Lake. Figure 3e includes 168 events from September 9 to September 16. The same clusters are present, but seismicity becomes more diffuse. 125 events have been located within the week of September 17 to 24 (Figure 3f). Seismic activity decreases as expected but the Yalova cluster is still well defined, the Izmit Bay quietness zone still present, and a Md = 5.0 shock generates a new small cluster NW of Marmara Island. Figure 3g includes 77 events in the period September 25 to 30. All of the activity is observed near the epicentral area and near Yalova, except for a few events at the far east and far west. Figure 3h shows the located aftershock distribution from October 1 to 23 for about 3 weeks. 231 events have been located within this period. The Yalova, Izmit and Akyazi clusters are well defined as well as a small cluster near Marmara Island in the west. Finally Figure 3i shows the total distribution of activity during the period August 17 to October 23. The three main orientations mentioned above intersect at the epicenter of the main shock (large star), and near Akyazi. The epicenters corresponding to the activity before the main shock are included as empty squares for comparison. A very important feature could be observed when we considered a statistical picture of the evolution of seismicity as a function of time. Figure 4 shows these data for the period July 15-October 23, It is possible to detect a 18 days quiescence period where the level of background seismicity lowers sensibly just prior to the main shock. Indeed, the average number

4 290 Figure 3. Seismicity of the Marmara Sea region before and after the Izmit earthquake (large star) displayed by time windows one week long each. BI: Büyükada Island, G: Gölcük, IL: Iznik Lake, MI: Marmara Island, see also Figure 1. a) Seismicity before the main shock (empty rectangles) July 15-August 16. b) August. c) August. d) 1 8 September. e) 9 16 September. f) September. g) September. h) 1 23 October. i) Cumulated seismicity: a) to h).

5 291 Figure 4. Histogram of the number of events with time for the time period between 15 July 1999 and 23 October A quiescence period of 18 days is observed just before the main shock. The hyperbola shows the least squares adjusted Omori s law. of earthquakes during quiescence is 4.4 times smaller than that of previous days. Then comes the main shock and the aftershock sequence that decreases according to the well known Omori law. Some deviations are due to the sequel of large aftershocks of magnitude about 5 (August 31, September 9, 18, 20, October 5). Depth distribution along the Izmit aftershock zone We retained as the best solution the one combining low RMS and the higher possible number of P and S arrivals. In order to avoid bad quality and poorly constrained hypocenters we submitted the hypocentral determinations to a sorting based on the following criteria: RMS less than or equal to 0.5 s, total number of phases (P+S) taken into account greater or equal to 7, including at least three S-phases, horizontal and vertical errors less than or equal to 2 km and 5 km respectively. Based on these criteria we obtained 1316 hypocenters over a total of 2405 by using a depth sweeping procedure (Delouis, 1996) with the Hypoinverse location program (Klein, 1978). We present the corresponding epicenter and depth distributions in Figure 5. The depth of the main shock was fixed at 15 km, following the maximum depth of the aftershocks (Delouis at al., in press, give 12 km; KOERI gives 18 km). As we mentioned before, the Izmit Bay quietness zone is clearly observed in the Figures 5a and 5b. The hypocentral distribution does not extend east of Akyazi because the poor station coverage there makes accurate depth determination difficult. The Izmit aftershocks lie above a depth of 15 km, and almost all of them (90%) are located within a band zone between 5 km and 15 km depth, as can be seen on the E-W depth-cross section in Figure 5b. The upper 5 km of the crust shows a low seismic activity. A similar observation is also reported by Özalaybey et al. (2002). This low activity zone can be observed between 27.5 E and 30.0 E. However, most hypocenters are located closer to the surface from

6 292 Table 1. Focal mechanism of 96 events along the aftershock zone and west of Izmit bay Date y/m/d Time h:m Lat. N Lon. E Azim.( ) Dip ( ) Rake ( ) Mag. Ref : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :

7 293 Table 1. Continued Date y/m/d Time h:m Lat. N Lon. E Azim.( ) Dip ( ) Rake ( ) Mag. Ref : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : Magnitude is taken from KOERI.

8 294 Figure 5. a) Epicenters. b) Hypocentral distribution of the Izmit aftershocks (the depth of the main shock is fixed at 15 km, at the lower depth of aftershocks). The boundaries of the three parallel depth-cross sections (A-A, B-B, C-C ) are seen on Figure 5a. Izmit bay to Akyazi. Near this city, earthquakes lie above a depth of 10 km. The largest clusters of aftershocks are located between Yalova and south of Prince Islands. The deepest earthquake of the entire sequence has been determined here with a depth a 23 km on August 21, 1999 (Md = 2.6). Further west, a small hypocentral cluster is located from 6 to 14 km depth, below the NW margin of Marmara Island, except for one or two events, near the surface (Figure 5b). In order to illustrate the spatial distribution of the hypocenter locations region by region, we show three parallel-cross sections (A-A, B-B, C-C ) across the Izmit aftershock zone (Figure 5a and 6). The first section (A-A ) is between the longitudes Eand E and it contains 808 hypocenters. The events are at a depth of about 15 km south of Prince Islands. Further south, we observe depths between 5 and 15 km to the SW of Darica peninsula. Seismicity increases sharply near Yalova and Cinarcik. The cross section B- B shows 135 depths between the longitudes E and E. We observe seismic activity only to the west of the Hersek delta along the Izmit bay. Although most of the events are located from 5 km down to 15 km in depth, few hypocenters are within the first

9 Figure 6. Depth distribution of the Izmit aftershocks on three NS cross-sections (A-A, B-B, C-C ). The number of events in each section is 808, 135 and 176 respectively. 295

10 296 Figure 7. a) Map of 96 focal mechanisms obtained for the period August 21-October 22, 1999 all along the aftershock zone. The mechanisms are shown in an equal area projection on the lower hemisphere. Most of the mechanisms show a dominant strike slip or normal component except for four clear reverse faults (N 31, 39, 83, 87). b) Stress tensor parameters, shape factor R (equal to 1.6 ± 0.1) and likelihood value (94%). Maximum values are normalized to 1. The score indicates the normalized number of polarities consistent with the stress tensor. The error ellipses correspond to one standard deviation. 5 km. Finally, further east, the C-C cross section shows 176 events between the longitudes Eand E. Hypocenters start near the surface north of Izmit bay, then depth increases up to 12 km towards Gölcük. The depth of the events decreases from the east of Sapanca Lake to Akyazi. All depths in the C-C section are within first 10 km of the earth crust. Stress tensor and focal mechanisms inversion In order to investigate the stress field and the characteristics of faulting along the Izmit rupture zone, we used an algorithm which performs the simultaneous inversion of the orientation and shape factor R of the stress tensor and of individual focal mechanisms for a population of earthquakes (Rivera and Cisternas, 1990). The shape factor R defined by R = (σ z -σ x )/(σ y -σ x ), where σ z is the principal stress closest to the vertical, and σ x, σ y are the other two principal stresses, with the condition that σ y >σ x. State of stress along the Izmit aftershock zone A selected number of 96 events, recorded between August 21 and October 22, 1999, are used to determine the focal mechanism solutions and the present state of stress after the Izmit earthquake. All of the selected events have at least 10 first motion polarities and are inverted jointly. Figure 7a shows the epicenter distribution together with their focal mechanism solutions. A list of focal parameter is given in Table 1 and in the

11 297 Figure 7. Continued. Appendix. As expected, strike-slip and normal faulting mechanisms are dominant except for four cases of reverse faulting (N 31,39,83,87). Figure 7b presents the best stress tensor solutions, showing the shape factor R equal to 1.6 ± 0.1. The σ 3 axis is almost horizontal and oriented approximately N35,buttheσ 1 and σ 2 axes are inclined about 45, and it is difficult to say which one is closer to the vertical. Thus, the well defined orientation of σ 3 implies extension in a N35 E direction, but the stress regime is in between extension (σ 1 is closer to the vertical) and strike-slip (σ 2 is closer to the vertical). State of stress at the west of the Izmit aftershocks We also carried out a specific inversion to infer the stress regime at the western end of the Izmit aftershock zone. For this, we used the same algorithm for events located in between longitudes E and E, with a selected number of 50 earthquakes located near Çinarcik and Yalova during the period from August 21 to October 20, The same selection criteria as above have been used for this study. Figure 8a presents the results of the mechanism solutions with their epicentral distributions. The focal parameter list is given in Table 1. Two clear reverse fault solutions (Nb 39,83) are still present here, but the most dominant mechanisms are in normal faulting. Figure 8b shows the best stress tensor solutions, with a shape factor R equal to 1.8 ± 0.3 which represents an extension regime. The σ 3 axis is oriented approximately N35 as before. Although σ 1 is closer to the vertical (extension regime), it is again oblique and it is not possible to decide between extension and strike-slip regimes. Seismotectonic analysis A field study has been performed along the Izmit fault in order to relate surface ruptures to the aftershock distributions. The Izmit earthquake generated a remarkably linear set of surface ruptures about 150 km

12 298 Figure 8. a) Map of 50 focal mechanisms obtained for the period August 21-October 20, 1999 at the western part of the Izmit Bay. Most of the events are located near Çinarcik and Hersek delta. b) Stress tensor parameters for the region (see Figure 7b for definitions), shape factor R (1.8 ± 0.3) and likelihood (96%). long, with a dominant dextral strike-slip character (Barka et al., 2000; Delouis et al., 2002; Gülen et al., 2002; Polat et al., 2002). These ruptures could be observed mainly east of Izmit up to Gölyaka, but also in the Degirmendere-Gölcük region and in the military airport at Çatal delta (Figure 9, Figure 8a). The main branches of the Izmit fault rupture observed in land are: 1. Degirmendere-Tiktik, 2. Tiktik-Sapanca Lake, 3. Sapanca Lake-Akyazi, 4. Akyazi-Gölyaka. Seventy-six reliable measurements (Table 2) were made on the Izmit fault segments from Yalova to Gölyaka (Figure 10). The maximum horizontal offset of 5 m was measured in the Navy base of Gölcük and near Arifiye (east of Sapanca Lake) with a rupture azimuth of N98 and N82 respectively. The offset of the Izmit fault rupture gradually decreases further east, as seen in Karadere east of Akyazi, and further east in Degirmentepe east of Gölyaka, with a maximum value of 1m20. The observed azimuths of the rupture are N74 in Karadere and N66 in Degirmentepe. Numerous examples of man-made structures like roads, houses, alignement of trees, walls, fences, channels, plowed fields, etc., were shifted across the fault and permitted quantitative measurements all along the Izmit surface rupture. The measured values are close to those obtained by Barka et al. (2000), though they present a more complete data set. Even though most of the observations confirmed the strikeslip character of the fault (Table 2), there is at least one place with an important branch in normal faulting. The wall of the Stadium of Gölcük was a good indicator to determine the vertical offset of 1m76. Horizontal offset is 80 cm and the azimuth of the surface rupture is N131. We considered this to be either a secondary fault or the limit of a landslide (Polat et al., 2002). Discussion and conclusions The seismic monitoring of the Marmara Sea region with a local network provided information about the

13 299 Figure 8. Continued. Figure 9. Observed (solid lines) and inferred (broken lines) surface ruptures, together with the epicenters located within the period August 17 to November 2, Four clear segments are shown: 1. Degirmendere-Tiktik. 2. Tiktik-Sapanca Lake. 3. Sapanca Lake-Akyazi. 4. Akyazi-Gölyaka. Site names (see also Figures 1 and 3): A: Akyazi, AD: Adapazari, GY: Gölyaka, H: Hendek, IZ: Izmit, K: Kullar, KD: Karadere, KM: Karamürsel, T: Topçular.

14 300 Figure 10. Field measurements along the Izmit surface rupture. The slip is shown at selected places. Site names (see also Figures 1, 3 and 8 and Table 2): Ar: Arifiye, Dt: Degirmentepe, G: Gölyaka, IB: Izmit Bay, Kd: Karadere, Tt: Tepetarla, tk: Tiktik. seismicity before, and after, the 1999 Izmit earthquake. The main results are discussed below. The Izmit earthquake generated a remarkable set of surface ruptures over 150 km (see also Barka et al., 2000). Observed deformations mostly correspond to en échelon tension gashes alternating with pressure ridges, thus showing a dominant dextral strike-slip character. Quantitative measurements of the offsets give a maximum value of 5 m. These values vary along the fault, indicating segmentation, even though the segments do not change orientation in an important way, except for the eastern Gölyaka branch. Some discussion is found in the literature between partisans of a single fault, and those who propose different branches (Imren et al., 2001; Okay et al., 2000; Gökasan et al., 2001). A multidisciplinary approach shows that the Izmit rupture cannot be considered as a long, linear, single fault. Clustering in the epicentral distribution (1446 selected events over a total of 3165), and the concentration of aftershocks around the ruptured areas at depth, also contribute to the recognition of segments. Aftershocks lie above a depth of 15 km, and almost all events (90%) are located within a band zone between 5 km and 15 km. Most of the important clusters are detected NE of Armutlu peninsula, near Yalova and Çinarcik to the east of Izmit Bay, and near Akyazi (Bolu). The clustering at the western end of the Izmit Gulf shows a complex structure, suggesting the presence of at least three branches of the NAF, and connecting the Izmit rupture to the dormant faults under the Marmara Sea. Four segments have been identified mainly from surface ruptures. The first one is located between Degirmendere and Tiktik. The second one is between Tiktik and West Sapanca Lake. A third one, goes from East Sapanca Lake to Akyazi, and the fourth one runs from Akyazi to Gölyaka. A fifth segment can be deduced from the aftershock clustering, between Çinarcik and Hersek. The cluster located to the south of Prince Islands may lead to the activation of the northern part of the Marmara Sea, the last remaining gap between the 1912 Gallipoli earthquake and the sequence of ruptures that began at Erzincan in Three aligned pull-apart basins, corresponding to segments of the NAF, may break over there, independently, or as a whole. We could not observe any significant seismic activity along the rupture zone of the Izmit earthquake before the main shock, with the exception of one or two events west of Izmit Bay and Sapanca Lake. Nevertheless, the month before the Izmit earthquake is characterized by an 18 days quiescence period, just prior to the main shock. This result is well controlled thanks to the density of our network after July 15, The time distribution of the aftershocks follows Omori s law, except for perturbations due to the activity following events of magnitude around 5, in particular the Marmara Island shock of September 20.

15 301 Table 2. Horizontal (h) and vertical (v) offset measurements along the Izmit fault No. Fault location Slip information Explanation Lat. ( ) Lon. ( ) Az ( ) H (m) V (m) West of Aerial base, Topçular Compressive fissures, Topçular Degirmendere Wall, Yüzbasilar Offset of building, Yüzbasilar Offset of wall, Yüzbasilar Yüzbasilar Wall of Navy Base Yüzbasilar Slump, Gölcük Gölcük Stadium Normal fault Gölcük Ford Auto Plant, Normal fault, Gölcük Normal fault Gölcük Normal fault Gölcük Normal fault Gölcük Gölcük Gölcük Gölcük Gölcük Gölcük Gölcük Gölcük Gölcük Gölcük Normal fault, Chicken factory, Gölcük Yuvacik Yuvacik Yuvacik Offset of Channel, Yuvacik Yuvacik Kullar Kullar Small wall Kullar Small wall Kullar Kullar Mosque Tiktik village Tiktik Tiktik Tiktik Tiktik Tiktik Tiktik Tepetarla Tepetarla

16 302 Table 2. Continued No. Fault location Slip information Explanation Lat. ( ) Lon. ( ) Az ( ) H (m) V (m) Acisu Religious School Karaburun, NW of Sapanca lake Karaburun Sapanca Sapanca Toyota Auto Plant South of Toyota Türk Çaybasi, East of Toyota, Arifiye Akyazi Camili village, Akyazi Karadere, east of Akyazi Gölyaka Gölyaka Gölyaka Gölyaka Degirmentepe village of Gölyaka Az: Azimuth of the fault rupture from North, H: horizontal slip, V: vertical slip. The calculated stress tensor obtained from the aftershocks is characterized by a stable σ 3 axis oriented N215.Theσ 1 and σ 2 axes are inclined, and not well defined by the inversion process. Thus, we may conclude that the stress regime after the Izmit earthquake is in between extension and strike-slip all along the aftershock zone. Acknowledgements We dedicate this work to the memory of Aykut Barka ( ), a great seismologist and friend. We thank to the Ministry of Foreign Affairs of France via the Embassy at Ankara (Turkey) for the financial support. This work was also supported by the CNRS-INSU (France), and the TUBITAK (Turkey) YDABCAG project number 199Y075. We thank to Gülsün Saglamer and Naci Görür for providing many facilities at the Campus of the Istanbul Technical University, and to Ahmet Mete Isikara for accessing and using the possibilities of the Kandilli Observatory and Earthquake Research Institut of Bogazici University. The authors wish to thank to Bertrand Delouis and Michel Bouchon for helpful comments on the manuscript. Appendix Focal mechanisms of 96 events after the inversion. The 95% confidence ellipse of the pole of the fault plane, and the slip vector are shown. All mechanisms are represented on the lower hemisphere equalarea projection. Black filled squares are compression polarities.

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