Authors: ESİN FİRUZAN
Abstract: In order to apply a statistical earthquake frequency analysis to western Anatolia, the area has been restricted within coordinates 36.50º-40º North and 26.00º-30.00º East. Data used in the analysis belong to the instrumental period from 1.1.1900 to 10.11.2006 (Ms ≥ 4.0). The earthquake record was assumed to be representative of longer periods and the possibility of clusters of seismicity or of extreme magnitude events were ignored. The study area has been divided into eight zones with different seismotectonic properties. All computations have been performed for these zones separately. The earthquake magnitude series is termed as the instantaneous peak over the threshold (POT) and the series of the largest earthquake magnitude per year in a zone as the annual maximum (AM). POT and AM series of the surface wave magnitude (Ms) are obtained from the catalog of the Kandilli Observatory and Earthquake Research Institute (KOERI) of Bosphorus University. Exceedance probabilities and return periods of earthquakes of various magnitudes have been calculated for each zone. This paper discusses the comparative assessment of four advanced distributions to provide accurate and reliable earthquake estimates for all sub-regions in western Anatolia. The models compared are the Exponential (EXP), Extreme value distribution Type 1 (Gumbel) (GUM), Log Pearson Type 3 (LP3), and Generalised Pareto (GP). Model parameters are estimated using the method of moments and the quantitative assessment of each model is based on the Anderson Darling Test results and quantile estimation of each model. On the basis of these comparisons, it is concluded that the GP and GUM distributions are most appropriate for describing the peaks over the threshold earthquake series and annual maximum earthquake data in western Anatolia. The results show that b values in the computed magnitude-frequency relations range from 0.40-0.76. These results are supported by previous studies of the Aegean and surrounding area (Papazachos 1990; Öncel & Wilson 2004; Sayıl & Osmanşahin 2007). The highest b value has been obtained for Zone 7 (Gökova Gulf). As a result of the GUM, EXP and GP models, Zone 8 has the lowest risk for magnitude 7.0 and longest return periods; however, LP3 model results show that Zone 7 has the lowest risk for magnitude 7.0 and the longest return period.
Keywords: extreme value distribution, Western Anatolia, zonation, seismic assessment, earthquake
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