Site attenuation and source parameters on the North Anatolian Fault zone, eastern Turkey estimated from the aftershocks of 13 March 1992 Erzincan earthquake

Site attenuation and source characteristics of 45 aftershocks of the 13 Mar ch 1992 Erzincan, eastern Turkey earthquake have been determined from SH-wa ve spectra using a least-squares best-fit method. Although the most of the seismograms were recorded on the ophiolitic rock sites and the average regi onal attenuation correction of Q(s)(f) = 35 f(0.83) was applied, the high f all-off rates gamma ranging between 3.0 and 4.5 beyond the corner frequenci es of 8-13 Hz were observed on the SH spectra. The site attenuation value, kappa, has been calculated from the slope of the high frequency part of the SH spectra. It was found that kappa varied in the range of 0.0124-0.0364 s and the average was 0.0246 s with a standard deviation of 0.0047 s. The hi gh fall-off rates of observed spectra have decreased considerably as a resu lt of site attenuation corrections and converged to an average value of 2.3 . We concluded that the high fall-off rates of SH wave spectra are mainly c ontrolled by highly deformed Miocene ophiolitic formations which covers a w ide area in the North Anatolian Fault zone in the vicinity of Erzincan regi on. Using the spectra that were corrected for regional and site attenuation , and assuming a Brune's source model; seismic moment, source radius, and s tress-drops of the aftershocks were computed. We found that stress-drops fo r some of the aftershocks in Erzincan area have slightly decreased after re moval of site attenuation over SH wave observed spectra.