Seismic risk in Turkey, the Aegean and the eastern Mediterranean: the occurrence of large magnitude earthquakes

Abstract

Summary. The file of Turkish seismicity developed by Kandilli Observatory, Istanbul, for earthquakes to 1970 is extended here up to 1978 using 1SC and PDE data. Entries into this file are maintained on the surface wave magnitude scale M,, and conversion of body wave magnitude mb to M, has been carried out where necessary using a formula derived for Turkish earthquakes. Completeness analysis suggests that magnitudes M,> 4.5 may be used for statistical evaluation of seismic risk. This file is analysed by a range of methods to provide a suite of risk forecasts. Forecasting results from least squares and maximum likelihood estimates of the whole process Gutenberg-Richter cumulative frequency law of earthquake magnitude occurrence, and from the part process of Gumbel’s first extreme value distribution, all show small systematic differences in forecasts, but all three methods lead to magnitude forecasts which fall well within the range of standard deviation. However, these forecasts are obtained by subdividing Turkish seismicity in a cellular manner, and many of these cells show curvature of the earthquake frequency magnitude distribution curve concave with respect to the origin: Gumbel’s third asymptotic distribution of extreme values is chosen as an appropriate statistical description. Approximate upper bounds o to earthquake surface wave magnitude occurrence are evaluated and estimates of largest magnitudes expected over an interval of 75yr are forecast with uncertainties. Values of w are asymptotic, uncertain, and theoretically correspond to infinite return periods. Strain energy release diagrams are then invoked to estimate empirically the large magnitude M, which is equivalent to the total strain energy which may be accumulated in a region. This equivalent magnitude M, is consistently less than w and there is a finite ‘waiting time’, typically ranging from about 15 to 70 yr, during which the energy equivalent to M3 may be accumulated.