Bulletin of the Seismological Society of America, Vol. 99, No. 4, pp. 2127–2146, August 2009, doi: 10.1785/0120080285
Reassessment of Probabilistic Seismic Hazard in the Marmara Region by Erol Kalkan, Polat Gülkan, Nazan Yilmaz, and Mehmet Çelebi
Abstract In 1999, the eastern coastline of the Marmara region (Turkey) witnessed increased seismic activity on the North Anatolian fault (NAF) system with two dam- aging earthquakes (M 7.4 Kocaeli and M 7.2 Düzce) that occurred almost three months apart. These events have reduced stress on the western segment of the NAF where it continues under the Marmara Sea. The undersea fault segments have been recently explored using bathymetric and reflection surveys. These recent findings helped scientists to understand the seismotectonic environment of the Marmara basin, which has remained a perplexing tectonic domain. On the basis of collected new data, seismic hazard of the Marmara region is reassessed using a probabilistic approach. Two different earthquake source models: (1) the smoothed-gridded seismicity model and (2) fault model and alternate magnitude-frequency relations, Gutenberg–Richter and characteristic, were used with local and imported ground-motion-prediction equations. Regional exposure is computed and quantified on a set of hazard maps that provide peak horizontal ground acceleration (PGA) and spectral acceleration at 0.2 and 1.0 sec on uniform firm-rock site condition (760 m=sec average shear wave velocity in the upper 30 m). These acceleration levels were computed for ground mo- tions having 2% and 10% probabilities of exceedance in 50 yr, corresponding to return periods of about 2475 and 475 yr, respectively. The maximum PGA computed (at rock site) is 1.5g along the fault segments of the NAF zone extending into the Marmara Sea. The new maps generally show 10% to 15% increase for PGA, 0.2 and 1.0 sec spectral acceleration values across much of Marmara compared to previous regional hazard maps. Hazard curves and smooth design spectra for three site conditions: rock, soil, and soft-soil are provided for the Istanbul metropolitan area as possible tools in future risk estimates.
Introduction The Marmara region houses one-third of Turkey’s popu- Istanbul province, which is located in the heart of the Mar- lation and is one of the most tectonically active regions in mara region. Today, this most-populated city in Europe is Eurasia. In the last century, this region (Fig. 1) experienced under threat of much publicized forecasted earthquakes. unusual seismic activity with nine strong events having M ≥ Two fault systems located south and southeast of Istanbul 7:0 (M moment magnitude). In 1999, two destructive (the Yalova fault segment and the northern boundary fault) earthquakes (Kocaeli and Düzce) occurred in the eastern part have the potential to rupture (Parsons et al., 2000; Hubert– of the Marmara region on the North Anatolian fault (NAF) Ferrari et al., 2000). Based on a renewal model, the probabil- system. This strike-slip fault system cuts across northern ity of occurrence of M 7.0 and greater earthquakes in the Turkey for more than 1200 km and accommodates Marmara region that could directly influence the Istanbul ∼25 mm=yr right-lateral slip between the Anatolian and metropolitan area was computed as 44 18% in the next Eurasian plates (Straub et al. 1997; McClusky et al. 2000). 30 yr (Parsons, 2004). As implied by the level of seismic Since 1939, the NAF system has produced nine large earth- risk, critical assessment of the regional seismic hazard is quakes in a consistently westward-propagating sequence of paramount importance to facilitate and support a wide (Fig. 2); the only exception is the most recent large earth- range of earthquake engineering applications. quake (Düzce) that occurred east of the second most-recent The current regulatory seismic zoning map of Turkey, earthquake (Kocaeli). This pattern of earthquake propagation including the Marmara region, was developed using a Bayes- was recognized long ago (Ketin, 1969; Ambraseys, 1970; ian estimation based on then available earthquake catalog Barka, 1992; Stein et al., 1997). and ground-motion-prediction equations (GMPEs) originally The Kocaeli earthquake (M 7.4) is the most recent event developed from western U.S. ground motion data (Gülkan that occurred on the south of the eastern border of the et al., 1993). This choice was dictated by the paucity of
2127 2128 E. Kalkan, P. Gülkan, N. Yilmaz, and M. Çelebi
Figure 1. Seismicity of the Marmara region based on M ≥6:0 events (A.D. 1509–1999). The numbers of earthquakes are given in parentheses. Faults-1* type faults were recently visualized using bathymetric images and seismic reflection surveys (Le Pichon et al., 2001; Armijo et al., 2002; Le Pichon et al., 2003; Armijo et al., 2005) and Faults-2** type faults are previously known faults (Saroglu et al., 1992); see Table 1 for epicenter coordinates, magnitudes, and depths of earthquakes. strong-motion records in the country at the time. In the past generation of GMPEs. The next generation of attenuation 15 yr, a number of strong-motion records have been recorded (NGA) relations (Boore and Atkinson, 2008; Campbell and in Turkey, mainly from the 1999 Kocaeli M 7.4 and Duzce Bozorgnia, 2008; Chiou and Youngs, 2008) are used in ad- M 7.2 events, and hence, prompted development of national dition to the GMPE of Kalkan and Gülkan, developed based GMPEs (e.g., Gülkan and Kalkan, 2002; Kalkan and Gülkan, on data from shallow crustal tectonic environment of Turkey. 2004; Ulusay et al., 2004). In addition, tracing of new fault The weight assigned to the local GMPE is set equal to total segments beneath the Marmara Sea has helped better under- weight of the NGA relations. standing of the seismotectonic environment of the Marmara In computing the probabilistic seismic hazard, we include basin (Le Pichon et al., 2001; Armijo et al., 2002; Le Pichon two different earthquake source models: (1) smoothed- et al., 2003; Armijo et al., 2005). gridded seismicity model and (2) fault model, similar to those Seismic hazard of the Marmara region has been studied used in the development of the 2008 USGS National Seismic previously (Atakan et al., 2002; Erdik et al., 2004). These Hazard Maps (Petersen et al. 2008). The first model assumes studies are based on broadly described fault segments under that historical large earthquakes have in general taken place the Marmara Sea and nonindigenous GMPEs from the 1990s. in locations where epicenters of smaller earthquakes have ac- In our study, the regional seismic hazard is reassessed by cumulated (Kafka and Walcott, 1998; Kafka, 2002). This considering detailed submarine faults and using the latest model is based on the earthquake catalog and characterizes
Figure 2. Westward propagating sequence of earthquakes on the North Anatolian fault system since 1939. Reassessment of Probabilistic Seismic Hazard in the Marmara Region 2129 the hazard from earthquakes between M 4.0 and 6.5. The faults The Marmara region has a complex and heterogeneous contribute to the hazard for earthquakes larger than M 6.5. The fault system as shown in Figure 4. The 1200 km long NAF hazard calculation is the sum of the smoothed-gridded seis- fault system extends from the east of the region toward the micity model (for M ≤6:5) and the fault model (for M ≤6:5). Bay of Izmit. In the east at the junction of the Marmara Sea, Seismic hazard of Marmara is computed and projected the NAF system is controlled by right-lateral strike-slip on a set of hazard maps. These hazard maps show the PGA faults, while the plate boundary changes into a transtensional and spectral accelerations (SA) at 0.2 and 1.0 sec on uniform system that has opened a deep-basin below the Marmara Sea firm-rock site condition (VS30 760 m=sec). In the interest (Okay et al., 2000). There is no evidence of a single, con- of completeness, site amplification maps are also provided to tinuous, purely strike-slip fault under the Marmara Sea, but 360 estimate the ground motion at stiff-soil (VS30 m=sec) there is a complex segmented fault system with large normal 180 and soft-soil (VS30 m=sec) sites. These acceleration components. This fault system has been identified from seis- levels were computed for 2% and 10% probabilities of ex- mic reflection surveys (Smith et al., 1995; Parke et al., ceedance in 50 yr corresponding to return periods of about 1999). In the past a series of strong earthquakes has ruptured 2475 and 475 yr, respectively. The seismic hazard exposures the NAF zone in this region. Kocaeli and Düzce were the lat- in the vicinity of Istanbul metropolitan area are also exam- est events in a westward-propagating earthquake sequence ined, and a series of hazard curves and smooth design spectra that began with the M 7.9 Erzincan earthquake in 1939 for three site categories are provided. on this fault system. This progression has since generated nine M ≥7 earthquakes. When the 1912 event that occurred in the west of the Marmara Sea is taken into account, a seis- Regional Tectonic Setting mic gap that has not ruptured for more than 200 yr is iden- For the purpose of this study, the Marmara region is de- tified (Fig. 2). This crosses close to the northern shoreline of fined as the rectangular area bounded by latitudes 39°–43° N the Marmara Sea (Barka, 1992; Stein et al., 1997). This seis- and longitudes of 26°–32° E. The area is one of the most mic gap is around 150–160 km long and possesses the po- tectonically active regions of Turkey as evidenced by the tential to generate an M ≥7 earthquake (Hubert-Ferrari et al. number of large earthquakes (M ≥6:0) that have occurred 2000). Coulomb stress calculations indicate that shear stress between 1509 and 1999 (Fig. 1). Many of these events were increased in the aftermath of the 1999 Kocaeli earthquake on M ≥7:0 and generated on or in proximity of the NAF system. the fault segments below the sea, which may indicate their Moderate to large earthquakes with M ≥6:0 have also oc- likely impact on the rupture potential (Parsons et al., 2000). curred on fault segments situated well away from the NAF. The measured slip vectors in the Anatolian plate with Figure 3 depicts the distribution of all distinct events with respect to the stable Eurasian plate exhibit a generally antic- M ≥4:0 that occurred in the period 1901–2004. The figure lockwise rotation and an increase in total displacement to- also shows the breakdown of the number of earthquakes ward the west caused by the increasing westward pull of within each magnitude interval. the Hellenic subduction system located southwest of Turkey
Figure 3. Seismicity of the Marmara region during the 103 years between 1901 and 2004. The numbers of earthquakes are given in parentheses. 2130 E. Kalkan, P. Gülkan, N. Yilmaz, and M. Çelebi
Figure 4. Active faults in the Marmara region (NAF).
(McClusky et al., 2000). This dynamic structure pushes compiled from different sources (see Data and Resources the Marmara region in a northerly path. The NAF system section) that use different intensity scales: (1) Earthquake within the Marmara region can account for 22 3 mm=yr Research Department, General Directorate of Disaster Affair right-lateral slip (Straub et al., 1997; McClusky et al., 2000). of Turkey; (2) Kandilli Observatory, Boğaziçi University; On the basis of Le Pichon et al. (2001), we assigned (3) International Seismological Centre of UK; and (4) the 23 mm=yr slip-rate to major faults below the Marmara Sea; U.S. Geological Survey. Magnitude scales of all events were for the rest of the fault segments, the slip-rate distribution converted to moment magnitude (M) through a set of empiri- from Global Positioning System (GPS) measurements given cal equations derived based on Turkish earthquakes (Yenier in Straub et al. (1997) were used. The slip-rate distributions et al., 2008). These equations are listed in descending order assigned to fault segments agree with tectonic (Barka and of priority of the scales selected to convert to M. Kadinsky–Cade, 1988; McClusky et al., 2000; Yaltirak, 2002; Seeber et al., 2004; Aksoy et al., 2006; Motagh et al., 0 571 2 484 3 0 ≤ 5 5 2007) and kinematic simulation-based (Pulido et al., 2004) M : Ms : ; : Ms < : (1a) studies.