Planar Seismic Source Characterization Models Developed for Probabilistic Seismic Hazard Assessment of Istanbul

This contribution provides an updated planar seismic source characterization (SSC) model to be used in the probabilistic seismic hazard assessment (PSHA) for Istanbul. It defines planar rupture systems for the four main segments of the North Anatolian fault zone (NAFZ) that are critical for the PSHA of Istanbul: segments covering the rupture zones of the 1999 Kocaeli and Düzce earthquakes, central Marmara, and Ganos/Saros segments. In each rupture system, the source geometry is defined in terms of fault length, fault width, fault plane attitude, and segmentation points. Activity rates and the magnitude recurrence models for each rupture system are established by considering geological and geodetic constraints and are tested based on the observed seismicity that is associated with the rupture system. Uncertainty in the SSC model parameters (e.g., b value, maximum magnitude, slip rate, weights of the rupture scenarios) is considered, whereas the uncertainty in the fault geometry is not included in the logic tree. To acknowledge the effect of earthquakes that are not associated with the defined rupture systems on the hazard, a background zone is introduced and the seismicity rates in the background zone are calculated using smoothed-seismicity approach. The state-of-the-art SSC model presented here is the first fully documented and ready-to-use fault-based SSC model developed for the PSHA of Istanbul

Kuzey Anadolu fay hattı Marmara bölümü için fay bazlı düzlemsel sismik kaynak modeli ile sismik tehlike analizi

Eastern and central segments of NAFZ were ruptured progressively by large and destructive earthquakes in between 1939-1999. From the west of the 1999 Kocaeli Earthquake rupture zone, the northern strand of NAFZ is submerged beneath the Marmara Sea, bringing in major uncertainties related to fault’s geometry, segmentation model and earthquake recurrence parameters. Building a proper seismic source characterization model for the segments of the NAF beneath Marmara Sea that takes these uncertainties into account have great importance in the Probabilistic Seismic Hazard Analysis (PSHA) results for Istanbul. The objective of this study is to build a seismic source characterization model for Istanbul by considering the uncertainties related to fault segments and rupture models. Previous PSHA studies in Marmara Sea Region are evaluated and compared for this purpose. The planar fault geometry is preferred and 4 main segments (length, depth and segmentation points) are defined with the help of available fault maps. The composite magnitude distribution model (Youngs and Coppersmith, 1985) is applied to accurately define the characteristic behavior of NAF. Fault segments, rupture sources, rupture scenarios and fault-rupture model are defined using WGCEP-2003 terminology and multi-segment rupture scenarios are considered systematically. The events in the instrumental earthquake catalog (KOERI, 2011) are combines with the defined seismic sources and the rupture scenario weights are determined by balancing the accumulated and released seismic energy. The uncertainties of the described model parameters (e.g. annual slip rate, fault depth, characteristic earthquake magnitude, b-value, and scenario weights) are combined by the logic tree method. Kuzey Anadolu Fayı’nın (KAF) kuzey kolunun 1999 Kocaeli depreminin kırılma bölgesinin batı ucundan itibaren Marmara Denizi’nin altında devam etmesi, sismik kaynak modellemesinde kullanılacak fay bölümlerinin lokasyon, devamlılık ve deprem tekrarlanma parametrelerinde belirsizliklere neden olmaktadır. KAF’ın Marmara Denizi içerisinde ve etrafında yer alan bölümlerinin sismik kaynak modellemesinin doğru bir biçimde yapılması İstanbul’da yapılacak Olasılıksal Sismik Tehlike Analiz’leri (OSTA) için büyük önem taşımaktadır. Bu çalışmada, belirsizlikler taşıyan fay bölümlerinin beraber ve/veya tek kırılma olasılıkları mantık ağacı çerçevesinde modellenerek düzlemsel faylara dayalı bir sismik kaynak modeli oluşturulmuştur. Geliştirilen sismik kaynak modeli kullanılarak, İstanbul Kaynarca Merkez – Pendik – Tuzla Tersane ve Çekmeköy-Sancaktepe-Sultanbeyli metro hatlarının uygulamaya esas kesin projelerinde yer alan (yaklaşık) istasyon lokasyonlarında OSTA yapılmış ve istasyon kotu altında (mühendislik ana kayası koşullarında) maksimum yer ivmesi için sismik tehlike eğrileri elde edilmiştir. Bu sismik tehlike eğrilerindeki en büyük yer ivmesi değerleri 475-yıllık geri dönüş periyodu için 0.24g ile 0.38g arasında değişmektedir, ancak 2475-yıllık geri dönüş periyodu için 0.55g mertebesine ulaşmaktadır