Estimation of the kinematic source parameters and frequency dependent shear wave attenuation coefficient of the 18th June, 2007 Kahak-Qom earthquake in north central Iran



In this study, analysis is presented in two steps. In the first step, the theoretical S-wave displacement spectra, conditioned by frequency-independent Q, was fitted with the observed displacement spectra. Therefore corner frequency, moment magnitude and frequency-independent Q for each record were estimated simultaneously and the estimate of error is given in the root-mean-square sense over all the frequencies. In the second step, the corrected observed displacement from source effect was fitted with path term of Brune’s source model to estimate frequency dependent shear wave Quality factor. For comparison, Frequency dependent shear wave quality factor also is estimated from spectral decay method. The earthquake in Qom, 18 June 2007 (Ml=5.7), was the largest earthquake in the south of Tehran that recorded on strong motion acceleration stations. The data represented more than 40 accelerograms recorded from Kahak-Qom earthquake in the hypocentral distance range from 18 to 170 km. The source term obtained from inversion was analyzed to estimate various source parameters. Thereby, we estimated seismic moment (1.86*1024 dyne-cm), corner frequency (0.72 Hz ), source radius (2.33 Km), fault slip (38 cm), source duration (1.5 sec), stress drop (12.3 bars) and moment magnitude (5.4), which are found to be consistent with the corresponding values reported in published studies. The path average value of Q is in the range Q=161 to 1652. The anelastic attenuation coefficient for the region as a whole is estimated in step 2 is Qs= 47f 0.71 in frequency range of 1 to 32 Hz. The frequency-independence attenuation for the study region shows that, in general, a Q value is significantly similar to the entire frequency range used than those found in other tectonic areas.