Seismic hazard in Istanbul following the 17 August 1999 Izmit and 12 November 1999 Duzce earthquakes

Two recent destructive earthquakes that occurred along the western part of the North Anatolian Fault Zone (NAFZ), the 17 August 1999 (M-w 7.4) Izmit and 12 November 1999 Duzce (M-w 7.2) earthquakes, have caused major concern about future earthquake occurrences and their possible consequences in the Istanbul area. Probabilistic seismic hazard analyses are performed for the larger Istanbul area including the Gulf of Izmit and the Marmara Sea region. Hazard computations were done assuming different combinations of four attenuation relations and three alternative source models. The three models used are (1) Standard Poissonian earthquake occurrence with area sources, (2) renewal model (assuming characteristic earthquakes) with area and fault sources, and (3) renewal model (assuming characteristic earthquakes) with refined area and fault sources. Results are presented in twelve different maps of peak ground acceleration (PGA) with a 10% chance of exceedance in 50 yr. Among the earthquake recurrence models, the results assuming model 3, yield the highest PGA values, reaching greater than 0.3 g at the western end of the Gulf of Izmit. In general, PGA values decrease toward north and are reduced down to less than 0.2 g in central Istanbul in the Bosphorous area. The four attenuation relations examined display significant variations, and their effects become especially critical for distances less than 50 kin. In addition, spectral hazard levels (corresponding to 475-yr return period) are computed and presented as uniform hazard response spectra for 5% damping. Two sites are selected, one in hard rock and the other in soft sediments. In general, these results correlate well with the Turkish Seismic Design code (TSDC) recommendations for the two site conditions. The estimates of the future earthquake hazard potential of the Istanbul area are sensitive to our present-day understanding of the behavior of the fault segments in the Marmara Sea region.