Estimation of the Frequency-Dependent Shear Wave Attenuation from Acceleration Spectra of the Ahar-Varzeghan Earthquake 2012

Two destructive earthquakes occurred on the 11th of August 2012 in the regions of Ahar and Varzaghan, NW Iran. High-frequency strong-motion data of these earthquakes have been analyzed to determine the Qβ (f) and source parameters by inversion of the recorded data. The data from a local network of 21 acceleration records, for the first main shock and 19 records for the second main shock was used to estimate Q-relationship in 30 stations. The seismic hazard map for this region illustrates that most of the area in this province is located within high relative risk and characterized by a large number of heterogeneities. For frequency band of 1 to 20 HZ, the frequency-dependent attenuation for this region found to be Qβ = (94±29) f (0.82±0.05). The authenticity of achieved Qβ (f) relation is checked by comparing the source spectra in various stations with the theoretical spectra. Low values of the coefficient (Q 200) in the Qβ (f) relation suggest that the region is seismically and tectonically active. The present inversion of strong-motion data gives the source parameters. The corner frequencies and stress drops are calculated as fc1= 0.08[HZ],fc2= 0.11[HZ], Δσ1=77.14[bar] and Δσ2=33.06[bar], respectively.