Probabilistic seismic hazard mapping of the Marmara Region
Erol Kalkan, Polat Gülkan, Nazan Yılmaz, Mehmet Çelebi
Sayı: 08-01, 2008/01
In 1999, the eastern coastline of the Marmara region witnessed increased seismic activity on the North Anatolian Fault (NAF) system with two damaging earthquakes (M7.4 Kocaeli and M7.2 Düzce) that occurred two months apart. These events have resulted in major stress-drops 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 to understand the seismotectonic environment of the Marmara basin that has remained a perplexing tectonic domain. Based on collected new data, seismic hazard of the Marmara region is re-assessed using a probabilistic approach. Two different earthquake source models ( smoothed-gridded seismicity model and  fault model), and alternate magnitude-frequency relations ( Gutenberg-Richter and  characteristic) were used with local and imported ground motion prediction models. 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 sec, and 1.0 sec on uniform firm rock site condition (universally accepted as 760 m/sec average shear wave velocity in the upper 30 m). These acceleration levels were computed for maximum credible earthquake for 2% and 10% probabilities of being exceeded in 50 years corresponding to return periods of about 2475 and 475 years, respectively. The maximum PGA computed (at rock site) is 1.5g along the fault segments of the NAF zone extending into the Sea of Marmara. The new maps generally show 10% to 15% increase for PGA, 0.2 sec, and 1.0 sec spectral acceleration 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.
Earthquake Engineering Research Center
Middle East Technical University, Department of Civil Engineering
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