Downloaded from geology.gsapubs.org on October 30, 2010 Rupture characteristics of the A.D. 1912 Mürefte (Ganos) earthquake segment of the North Anatolian fault (western Turkey) Murat Ersen Aksoy1, 2, Mustapha Meghraoui1, Martin Vallée3, and Ziyadin Çakır4 1Institut de Physique du Globe (UMR 7516) Strasbourg, 5 rue René Descartes, 67084 Strasbourg Cedex, France 2Eurasia Institute of Earth Sciences, Istanbul Technical University, Istanbul, Turkey 3Géoazur, Observatoire de la Côte d’Azur, IRD, Université de Nice-Sophia Antipolis, CNRS, Valbonne, France 4Department of Geological Engineering, Istanbul Technical University, Istanbul, Turkey ABSTRACT records allow us to deduce a focal mechanism The Ganos fault is the westernmost segment of the North Anatolian fault that generated the solution and a rupture duration model. The total 9 August 1912 Mürefte (Ganos) earthquake in western Turkey (Mw = 7.4). We study the 1912 fault length, related geometrical complexities, earthquake characteristics using coseismic fault slip and fault segmentation coupled with an and seismic characteristics are discussed to analysis of historical seismic records. Surface ruptures with small releasing and restraining determine the Sea of Marmara seismic gap. structures and 1.5–5.5 m right-lateral offsets have been measured at 45 sites of the on-land ~45-km-long fault section. Similar structures are delineated by fresh fault scarps and promi- GANOS FAULT SEGMENT nent pull-apart basins in the Sea of Marmara and Saros Bay. A second shock with Mw = 6.8 The ENE-WSW–trending Ganos fault is a occurred on 13 September 1912, implying a 20–40-km-long rupture; the damage distribu- rupture segment at the westernmost section of tion and analysis of seismic records suggest an epicenter located farther west near Saros Bay, the North Anatolian fault before it enters the near the western termination of the 9 August rupture. Modeling of seismic records reveals a North Aegean Trough from the Gulf of Saros relative source time function between the two events and indicates 40 s rupture duration, in (Fig. 1). The on-land fault appears along a nar- agreement with a 120 ± 30 km-long fault rupture for the 9 August shock. An estimated 150 row linear valley and forms geomorphic struc- ± 30 km-long rupture for the two earthquakes, combined with onshore and offshore fault seg- tures typical of strike-slip faults (releasing mentation, allow us to better constrain the western limit of the Marmara Sea seismic gap and or restraining bends and stepovers). Figure 2 the related potential for a large earthquake (sharply increased by the devastating 1999 Izmit shows the fault between Gaziköy and Saros [Sea of Marmara] seismic event). mainly delineated through stream offsets with cumulative slip varying from 10 to 600 m. The INTRODUCTION We combine on-land investigations with the off- fault segment extends offshore where recent The North Anatolian fault is a major shore fault section mapped in previous studies. high-resolution bathymetric data and multi- (~1500 km long) continental strike-slip fault The detailed fault geometry and related 1912 channel seismic data show clear fresh fault characterized by large earthquake ruptures slip distribution on land provide hints for the scarps in the Sea of Marmara and in the Aegean (Fig. 1; Barka and Kadinsky-Cade, 1988). The fault extension offshore. The analog seismic Sea (Armijo et al., 2005; Ustaömer et al., 2008). North Anatolian fault in the Sea of Marmara region (western Turkey) underwent the 1912 N 100 km Black Sea 204 488-5.0 0 25 km 1944 Mürefte earthquake (M 7.3 ± 0.35; Ambraseys 1912 1999 s 1943 1942 1967 and Jackson, 2000) and the 1999 Izmit earth- 1957 1939 40 VI Turkey 30 35 40 # quake (Mw 7.4; Barka et al., 2002) at the west- Tekirdag˘ GTT Central basin ern and eastern ends of the Sea of Marmara, T # Tekirdag˘ PUL Ganos Mt. VII tolian Fault respectively (Fig. 1). At least 250 km of the P VI VII basin h Ana # Nort North Anatolian fault might have ruptured dur- TOL VII HKJ # # # ing these 2 events, leaving a 70–150-km-long EBR VII VII # seismic gap in the Sea of Marmara (Ambraseys # VII Mürefte 9Aug . 1912 Mw 7.4 s # Marmara Sea Saro VII and Finkel, 1987; Barka et al., 2002). This large f of Fig. 2 Gul uncertainty is mostly due to the poorly known s Black Sea e VI Gelibolu Thrace VII eastern extension of the 1912 earthquake rup- # n VIII Basi ardanell Tekirdag˘ ture in the Sea of Marmara. Previous studies of Saros D Istanbul 9 Aug 1912 EQ IX Izmit the 1912 earthquake suggest 56–160 km of rup- Seismic Mw 7.4 ? gap ture length (Ambraseys and Jackson, 2000; Alti- Marmara Yalova 17.08.1999 Gökçeada ? Sea Mw 7.4 nok et al., 2003; Le Pichon et al., 2003; Altunel Çanakkale et al., 2004; Armijo et al., 2005; Karabulut et 0 50 km VI al., 2006) leaving the eastern end of rupture as 06070 3-5.7 150604 -5.2 230865-5.6 000367-5.3270375-6.6 a matter of debate. Therefore, a better constraint of the 1912 fault rupture termination and related Figure 1. Seismotectonics of western Marmara region. Focal mechanisms are from Harvard Centroid Moment Tensor catalog (http://www.globalcmt.org/; date as day-month-year [e.g., seismic characteristics is critical for seismic 060703] and Mw indicated at bottom), and A.D. 1912 solution is obtained from P-wave fi rst mo- hazard assessments in Istanbul and surrounding tions at stations PUL, GTT, EBR, TOL, and HKJ. Offshore fault trace (black lines) is modifi ed regions (Hubert-Ferrari et al., 2000). from Armijo et al. (2005) and Ustaömer et al. (2008). White and yellow stars are 9 August and We present here the 1912 earthquake fault 13 September 1912 epicenters, respectively (±0.1°; Ambraseys and Jackson, 2000). Roman numerals indicate MSK (maximum intensity in Medvedev-Sponheuer-Karnik scale) intensi- geometry and slip distribution based on aerial ties of 13 September event (Hecker, 1920). Upper left inset indicates 1912 earthquake location photographs, satellite imagery, digital elevation relative to 1939–1999 earthquake sequence. Lower right inset shows isoseismals (dashed models, bathymetry, and fi eld measurements. lines) of 9 August earthquake (EQ) and Marmara seismic gap (Ambraseys and Finkel, 1987). © 2010 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, November November 2010; 2010 v. 38; no. 11; p. 991–994; doi: 10.1130/G31447.1; 4 fi gures; Data Repository item 2010276. 991 Downloaded from geology.gsapubs.org on October 30, 2010 13 Sep. 1912 EQ 9 August 1912 Mw 7.4 Drainage 5.0 5.5 4.0 4.0 4.0 4.2 3.3 1.4 2.2 2.5 5.0 F TA1 - Wiechert Instr. 13 September 1912 M 6.8 Ridge 3.3 s 4.5 3.2 Coseismic displacement M Depression 4.0 Güzelköy 1912 EQ rupture Village 5.0 ") ") M Gaziköy 5.0 F F M F Mursallı M F F 3.2 ")M F F F 2.6 N 4.1 3.6 Yörgüç ° 1.7 ") F )" 5.2 5.4 4.7 2.0 4.0 40.7 2.5 Fault 4.6 4.0 2.4 Hos¸köy A 1.5 F 5.5 3.8 GanosF ") 5.2 F 3.5 4.5 F 4.5 0.8 Gölcük 3.9 ") 9 August 1912 TA1 Station Evres¸e ") 5.0 ") F Mürefte 0.6 Mw = 7.4 horizontal component signal F FF Yeniköy ") 5.3 0.4 3.9 Lake 0.2 s Kavak 4.3 ") Marmara Saroy ") 4.5 Ba S¸arköy 0 Sea 40.6° N Ortaköy 4.5 ") 3.2 -0.2 08km (unknown) s gnalityi 27° E 27.2° E 27.4° E -0.4 Figure 2. Ganos fault onshore and A.D. 1912 coseismic slip distribution (in meters). Yellow Veloc -0.6 Real 9 Aug. shock Reconstituted 9 Aug. shock boxes correspond to measures of this study. EQ–earthquake. -0.8 with the 13 Sep 1912 signal B 0 40Time (s) 120 160 The offshore fault zone east of Gaziköy forms a (Ambraseys and Jackson, 2000) and Mw 7.4 Relative source time function 1.2 9 Aug. 1912 restraining bend (i.e., Tekirdağ bend; 17° ± 3°) (Altunel et al., 2004), respectively. Since the 13 Sep. 1912 and can be traced in the Sea of Marmara toward 1912 Mürefte earthquake and the 1999 Izmit 1.0 the Central Basin as a nearly east-west–trending earthquake have essentially the same size (i.e., 0.8 continuous rupture (Fig. 1; Okay et al., 2004). In Mw 7.4), one may assume that both events have Saros Bay, the on-land Ganos fault extends far- a comparable rupture length (120 ± 20 km). Tak- 0.6 ther west for an additional minimum 40 km, as ing into account the 45-km-long surface rupture 0.4 shown by clear offshore fault scarps (Ustaömer onshore, ~75 km rupture should be offshore. et al., 2008; McNeill et al., 2004). A second large event, recorded in at least 0.2 Relative moment rate (/s) C 17 worldwide seismic stations, occurred on 0.0 −100 102030405060 1912 EARTHQUAKE SEQUENCE 13 September 1912 (Fig. 1; I0 = VII MSK, Time (s) The 9 August 1912 earthquake occurred along Hecker, 1920; Ms 6.8 ± 0.35, Ambraseys and the Ganos fault with an epicenter near Mürefte Jackson, 2000). Using a regional bilinear rela- Figure 3. A: Historical seismic record col- village (Fig. 1; Ambraseys and Finkel, 1987; tion, Ambraseys and Jackson (2000) estimated lected and digitized for this study.