Nat. Hazards Earth Syst. Sci., 17, 2365–2381, 2017 https://doi.org/10.5194/nhess-17-2365-2017 © Author(s) 2017. This work is distributed under the Creative Commons Attribution 3.0 License. Planar seismic source characterization models developed for probabilistic seismic hazard assessment of Istanbul Zeynep Gülerce1, Kadir Bugra˘ Soyman1, Barı¸sGüner2, and Nuretdin Kaymakci3 1Department of Civil Engineering, Middle East Technical University, Ankara, 06800, Turkey 2Department of Nuclear Safety, Turkish Atomic Energy Authority, Ankara, 06510, Turkey 3Department of Geological Engineering, Middle East Technical University, Ankara, 06800, Turkey Correspondence: Zeynep Gülerce ([email protected]) Received: 27 March 2017 – Discussion started: 2 May 2017 Revised: 9 October 2017 – Accepted: 19 October 2017 – Published: 22 December 2017 Abstract. This contribution provides an updated planar seis- 1 Introduction mic source characterization (SSC) model to be used in the probabilistic seismic hazard assessment (PSHA) for Istanbul. The North Anatolian fault zone (NAFZ), one of the most It defines planar rupture systems for the four main segments active fault systems in the world, extends for more than of the North Anatolian fault zone (NAFZ) that are critical for 1500 km along northern Turkey (Fig. 1b). NAFZ was rup- the PSHA of Istanbul: segments covering the rupture zones tured progressively by eight large and destructive earth- of the 1999 Kocaeli and Düzce earthquakes, central Mar- quakes (Mw > 6:5) in the last century. Earthquakes that oc- mara, and Ganos/Saros segments. In each rupture system, curred between 1939 and 1967 had ruptured approximately the source geometry is defined in terms of fault length, fault 900 km of a uniform trace in the east, whereas the 1999 width, fault plane attitude, and segmentation points. Activity Kocaeli and Düzce earthquakes ruptured a total fault span rates and the magnitude recurrence models for each rupture of approximately 200 km where the NAFZ is divided into system are established by considering geological and geode- a number of branches in the west. The northern strand of tic constraints and are tested based on the observed seismic- the NAFZ is submerged beneath the Marmara Sea to the ity that is associated with the rupture system. Uncertainty in west of the rupture zone of the 1999 Kocaeli earthquake, the SSC model parameters (e.g., b value, maximum magni- introducing major uncertainties into segment location, con- tude, slip rate, weights of the rupture scenarios) is consid- tinuity, and earthquake recurrence (Fig. 1a). In 2004, Par- ered, whereas the uncertainty in the fault geometry is not in- sons compiled a catalog of large-magnitude (M > 7) earth- cluded in the logic tree. To acknowledge the effect of earth- quakes occurred around the Marmara Sea for the time pe- quakes that are not associated with the defined rupture sys- riod of AD 1500–2000. Based on the rupture zones of these tems on the hazard, a background zone is introduced and the large-magnitude events, four main segments for the northern seismicity rates in the background zone are calculated us- strand of the NAFZ around Marmara Sea were proposed by ing smoothed-seismicity approach. The state-of-the-art SSC Parsons (2004): (1) the Ganos segment, which combines the model presented here is the first fully documented and ready- rupture zones of August 1776 and 1912 earthquakes; (2) the to-use fault-based SSC model developed for the PSHA of Is- Prince Island segment, which includes the rupture zones of tanbul. 1509 and May 1766 earthquakes; (3) the Izmit segment, de- fined for the rupture zones of the 1719 and 1999 earthquakes; and (4) the Çınarcık segment, defined for M ∼ 7 floating earthquakes (independent normal-fault earthquakes that may have occurred on different fault segments in or around the Çınarcık Basin). Parsons (2004) noted that 10 May 1556 (Ms D 7:1), 2 September 1754 (M D 7:0), and 10 July 1894 (M D 7:0) earthquakes were assigned locations in the Çınar- Published by Copernicus Publications on behalf of the European Geosciences Union. 2366 Z. Gülerce et al.: Planar seismic source characterization models for Istanbul 27 26 29 (a) 28 30 31 ISTANBUL 41 Hersek- Sapanca- 41 TEKIRDAG rmara (S4) Ç Central Ma ina Gölcük Akyazi Karadere rci zce 2 (D k (S3) (S2.1) Izmit (S2.2) (S2-3) (S1) Dü 2) S. Ganos (S6) armara (S5) Çin Düzce (D1) W M arcik Saros (S7) Abant GALLIPOLI GEYVE BANDIRMA Subordinate faults BURSA Earthquakes within the buffer zone 40 Earthquakes out of the buffer zone All earthquakes used in the analysis Other earthquakes 29 26 28 30 31 27 42° 27°STUDY AREA 33° 39° NÇF (S3) (d) S4 T O L I A N A N A F Y N O R T H A U SÇF S2.1 L T N Z O N E X S4 19 NÇF (S3) 9 17 28° S2.1 (b) SÇF 36° (e) X (SW) Y(NE) 27 26 28 29 30 31 0 41 41 ZONE 2 2 ZONE 1 Sediments 4 ZONE 3 5 km 40 40 Basement (c) 6 wo-way-travel-time (s) 29 26 28 27 30 31 Figure 1. (a) Major branches of the North Anatolian fault zone, defined rupture systems and the instrumental seismicity (Mw > 4) in the study area. The buffer zones used for source-to-epicenter matching are shown around the rupture systems. (b) Simplified active tectonic scheme of Turkey (modified from Emre et al., 2013). Thick lines are the North Anatolian and East Anatolian fault zones; thin lines are other active faults. (c) Distribution of the declustered seismicity used to calculate the b values. Zone 1, Zone 2 and Zone 3 are the polygons used to calculate the b values. (d) Slip distribution model for the Çınarcık segment. Right bending of the northern Çınarcık segment is 28◦ with respect to the central Marmara and Hersek–Gölcük segments. This results in a 17 mmyr−1 slip along the northern Çınarcık segment (NÇF) and 9 mmyr−1 normal slip transverse to the fault. This 9 mmyr−1 slip is the total slip on the northern and southern Çınarcık faults (SÇF). (e) Simplified geometries of the Çınarcık faults delimiting the Çınarcık Basin based on seismic profile of Laigle et al. (2008) almost passing through the line XY . cık Basin or on mapped normal faults in the southern parts forecast. Even though multi-segment ruptures were consid- of the Marmara Sea. These events were not allocated to the ered, the relative probabilities of the multi-segment ruptures other segments in order not to violate the inter-event time cal- vs. single-segment ruptures were not systematically defined culations, although they could have occurred on the northern in Erdik et al. (2004). This seismic source model was updated strand of the NAFZ. for the Earthquake Hazard Assessment for Istanbul project The fault segmentation model proposed by Erdik by OYO (2007). The fundamental differences between the et al. (2004) was similar to the segmentation model proposed Erdik et al. (2004) and OYO-2007 models are (1) small seg- by Parsons (2004) in terms of the fault geometry; however, ments around Marmara Sea used in the Erdik et al. (2004) smaller segments were preferred. Erdik et al. (2004) noted model were combined to form bigger segments in the OYO- that “the Main Marmara Fault cuts through Çınarcık, cen- 2007 model, (2) fault segments that represent the floating tral and Tekirdag˘ basins, follows the northern margin of the earthquakes were defined. The segmentation model used in basin when going through the Çınarcık trough in the north- OYO-2007 source characterization is very similar to the seg- westerly direction, makes a westwards kink around south of mentation model proposed by Parsons (2004). Ye¸silkoy until it reaches the 1912 Murefte–¸Sarköyrupture”. The fault segmentation model used by Kalkan et al. (2009) All of these fault lines were interpreted as separate fault seg- includes significant differences in terms of the fault geom- ments in the segmentation model. Erdik et al. (2004) con- etry with the Erdik et al. (2004) model, even though both sidered multi-segment ruptures by assigning lower probabil- studies used the active fault maps of ¸Saroglu˘ et al. (1992) for ities to “cascading ruptures”. Based on the rupture zones of inland faults and the fault segmentation model from Le Pi- previous large-magnitude events, multi-segment ruptures in- chon et al. (2003) and Armijo et al. (2005) for the segments volving the segments in connection with the 1999 Kocaeli beneath the Sea of Marmara. On the other hand, the mag- earthquake and 1509 earthquake were included in the rupture nitude recurrence models used by Erdik et al. (2004), in the Nat. Hazards Earth Syst. Sci., 17, 2365–2381, 2017 www.nat-hazards-earth-syst-sci.net/17/2365/2017/ Z. Gülerce et al.: Planar seismic source characterization models for Istanbul 2367 OYO-2007 model, and by Kalkan et al. (2009) were rather and the fault systems that are available in these databases similar. In all of these studies, linear fault segments were and in the current scientific literature are used in combination modeled (fully or partially) by the characteristic model pro- with the segmentation models proposed by Gülerce and Ocak posed by Schwartz and Coppersmith (1984); therefore, only (2013) and Murru et al. (2016) to define the rupture systems. large-magnitude events were associated with the fault seg- Fault segments, rupture sources, rupture scenarios, and fault ments. Additionally, a background source representing the rupture models are determined using the terminology given small-to-moderate magnitude earthquakes (earthquakes be- in the Working Group of California Earthquake Probabilities tween 5 and 6.5–7 depending on the study) were added to (WGCEP-2003) report and multi-segment rupture scenarios the source model and the earthquake recurrence of the back- are considered in a systematic manner.