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Bulletin of MTA (2013) 147: 1-17 Bulletin of the Mineral Research and Exploration http://bulletin.mta.gov.tr THE SEGMENT STRUCTURE OF SOUTHERN BRANCH OF THE NORTH ANATOLIAN FAULT AND PALEOSEISMOLOGICAL BEHAVIOUR OF THE GEMLİK FAULT, NW ANATOLIA Selim ÖZALPa,*, Ömer EMREb and Ahmet DOĞANc a General Directorate of Mineral Research and Exploration, Dept. of Geological Researches, 06800, Ankara, Turkey b Fugro-Sial Geoscience Consulting and Engineering Ltd Co., Farabi Sk., No: 40/4, Çankaya, Ankara, Turkey c General Directorate of Combating to Desertification and Erosion Control, Ankara, Turkey ABSTRACT Keywords: The North Anatolian Fault (NAF), which is an intra-continental transform fault, is divided North Anatolian into two branches as Northern and Southern branches in Marmara Region. The southern Fault-Southern Branch, Gemlik Fault, branch which separates from each other by rightward stepovers between Bandirma and paleoseismology. Dokurcun valley is formed by three main fault segments as Geyve, İznik and Gemlik from East to West. The length of fault segments ranges between 40 and 57 km and GPS data in Southern branch propose a 5 mm/year slip rate. Two surface faulting events were observed during paleoseismological excavations which had been carried out on 40 km long Gemlik segment and these can be correlated with earthquakes that occurred in 1857 and 1419. The recurrence interval between the last two earthquakes in Gemlik Fault is 438 years. Findings indicate that Gemlik fault was also included into surface faulting of the earthquake in 1419 which its presence is known in İznik fault. At least 95 km long multi-segment surface faulting developed in this earthquake. Based on 5 mm/years slip rate, the cumulative offset amount slip rate of NAF was approximately estimated as 3 meters in 595 years in region between İznik Lake and Dokurcun Valley. Accordingly; it can be stated that the southern branch of the North Anatolian Fault has a potential to trigger a large earthquake as well as the Northern branch. 1. Introduction a triggering factor of a next earthquake transferring the stress to a neighboring segment (Ambraseys, The North Anatolian Fault (NAF) is 1.600 1970; Toksöz et al., 1979; Barka and Kadinsky-Cade, km long, right lateral strike slip, intra continental 1988; Stein et al., 1996). Earthquake in 1967 ended transform fault and causes Anatolian plate to up towards western part of Mudurnu valley where the move westward with respect to Eurasia (Figure 1) fault is divided into two branches and the following (Ketin, 1957; Şengör, 1979). This transform fault is İzmit earthquake in 1999 August 17th (Mw: 7,4) and divided into two branches as Northern and Southern Düzce earthquake in 1999 November 12th occurred on branches in Eastern Marmara Region. NAF, in world the Northern branch of the fault. There is a consensus earthquake literature, is known by large earthquakes that the seismic hazard has increased in northern that occurred within the last century and migrates branch of the Sea of Marmara region after the eastward. Between the years 1939 and 1999, total of occurrence of these earthquakes (Stein et al., 1996; 1100 km surface faulting has developed in nine large Parsons et al., 2000; Erdik et al., 2004). earthquakes that had developed in NAF between Erzincan and the Sea of Marmara. Within this Historical records within last 2000 years and earthquake series, each earthquake segment has been paleoseismological data have revealed the presence * Corresponding author: S. ÖZALP, [email protected] 1 Paleoseismology of Gemlik Segment, North Anatolian Fault 24° 32° 40° 48° 44° Belgrade Bucharest EURASIAN 44° PLATE Sofia Black Sea Figure2 Caucasians 40° İstanbul Anatolia Sea of North n Fault Marmara 24 mm/year 40° Aegean Ankara Sea Athena Zagros ANATOLIAN 10 mm/year 36° Bitlis Antalya PLATE EAFZ Thrust Helen 15 mm/year 33 mm/year ARABIAN 36° T rench Belt Strabo PLATE Pliny Cyprus Med ite rr ge Dead Sea Fault an d 32° ean Ri AFRICAN Mediterranean PLATE Damascus 24° 32° 40° 32° Figure 1 – Active tectonic map of Turkey. Triangles filled with red color: active subduction zones; empty triangles: active thrust belts; thick lines: strike slip faults; thin lines with smallFigur indents:e normal1, Özalp faults. etBig al. dark arrows and values nearby indicate the movement direction of lithospheric plates and GPS velocities according to Eurasia, respectively (modified from Okay et al., 2000; GPS velocity values were taken from Reilinger et al., 2006). of earthquake recurrences on the Northern branch of 2000; Kalafat et al., 2011). Total geological offsets that NAF similar to the ones in the last century (Ambraseys, have developed in NAF Zone within last 4-5 million 1988, 2002; Ambraseys and Finkel, 1991; Guidoboni years are compatible with GPS data and explain that et al., 1994; Şengör et al., 2005). Despite that, data the slip rate on Northern branch is 3-3.5 times higher on the segment structure and paleoseismological than Southern branch (Sipahioğlu and Matsuda, 1986; behavior of the Southern branch are very limited. Şaroğlu et al., 1987; Koçyiğit, 1988; Emre et al., NAF which is divided into two main branches as 1998; Armijo et al., 2002; Meade et al., 2002; Emre North and South in the west of Bolu City is observed and Awata, 2003; Yildirim and Emre, 2004). as a broad zone in Marmara Region (Figure 2). Recent GPS data indicate that plate movements in Marmara Paleoseismological studies carried out on Region were basically caused by the Northern branch the Northern Branch in recent years suggest that of NAF and these data suggest a velocity of 24±1 mm/ recurrence interval of large earthquakes on which year on this branch (McClusky et al., 2003; Reilinger surface faulting that had developed are in between et al., 2006). According to GPS data, the Southern 150 to 300 years (Toda et al., 2001; Emre et al., 2002; branch which has a slip velocity of 5 mm/year causes Rockwell et al., 2001, 2009; Hartleb et al., 2003, 1/4 portion of the horizontal displacements in NAF 2006; Kozaci et al., 2009, 2011; Özaksoy et al., 2010). Zone (Meade et al., 2002). Historical records and It is known that destructive earthquakes have been instrumental period data within the last century occurring on the Southern branch as well during last indicate the presence of a more intense seismicity on 2000 years (Ambraseys and Finkel, 1991; Ambraseys the Northern branch (Figure 2, Table 1) (Ambraseys, and Jackson, 2000; Ambraseys, 2002). Large 1988, 2002; Ambraseys and Finkel, 1991; Guidoboni earthquakes which had formed surface faulting were et al., 1994; Ambraseys and Jackson, 2000; Taymaz, defined in a limited number of paleoseismological 2 Bulletin of MTA (2013) 147: 1-17 studies. However, these data (Ikeda et al., 1989; (McKenzie, 1972, 1978; Şengör, 1979, 1980; Jackson Barka, 1992; Yoshioka and Kuşcu, 1994; Uçarkuş, and McKenzie, 1984; Şengör et al., 1985). As a result 2002; Özalp et al., 2003) are not sufficient to reveal of these continental collisions, North Anatolian Fault the recurrence interval and the earthquake behavior (NAF) and East Anatolian Fault (EAF), transform fault of large earthquakes on the Southern branch. In some systems, which are two big recent tectonical structures other studies which do not have a paleoseismological of the country have revealed. NAF (Ketin, 1948) is basis, it is suggested that the recurrence interval of the margin of an active plate which is approximately earthquakes on the Southern branch at a magnitude 1600 km long with a character of the right lateral of M ≥ 7,0 is between 600 – 1000 years (Utkucu et transform strike slip (Şengör, 1979-1980) and passes al., 2011). The southern branch of NAF which is across Anatolia in E-W directions. Although recent approximately 250 km long between Dokurcun and deformations are observed along a narrow zone at Bandirma is one of the most significant earthquake central and eastern parts of NAF, these deformations source zone of the region. After earthquakes in 1999 starting from Bolu City get a broad zonal structure it is known that the earthquake hazard has greatly towards west (Barka and Kadinsky-Cade, 1988; increased in this zone (Langridge et al., 2002). The Barka, 1992, 1997; Armijo et al., 1999). The NAF in segment structure of the Southern branch which Eastern Marmara Region, between Bolu and the Sea is between Dokurcun Valley and Gemlik Bay was of Marmara, is formed by two branches (Northern and investigated in this study, the result of trenching Southern). The Southern branch of NAF, known as carried out on Gemlik segment was presented and the Central branch in literature and separated from the the earthquake hazard of the Southern branch was main fault in Dokurcun Valley, (Barka and Kadinsky- discussed comparing historical earthquakes with Cade, 1988) includes into Southern Marmara region paleoseismological data. in which the bending kinematics is dominant between NAF and West Anatolian Extensional Tectonical 2. Recent Tectonical Features Regime on western side (Figure 2) (Koçyiğit and Özacar, 2003; Emre et al., 2005; 2011a, b). Recent The study area is located in NW Anatolia which slip rate on this branch of the fault is 5 mm/year, is one of the most seismically active regions of approximately (Meade et al., 2002). Turkey. The neotectonics of Turkey begins with continent-continent collision in Middle-Late Miocene The Southern branch, which is 250 km long between Arabian-African plates and Eurasian plate between Bandirma and Dokurcun valley, is formed N 0 25 50 km Figure 2 – Active faults of Marmara Region and the distribution map of large earthquake (Ms>6.8) epicenters that have occurred within last 2000 years. Faults that ruptured in 20th century were shown in red line (Ambraseys and Finkel, 1991; Şaroğlu et al., 1992; Emre et al., 1998; Ambraseys, 2002; Armijo et al., 2002; Tan et al., 2008).