Attenuation of Fourier spectra for 2012 Ahar–Varzaghan earthquakes, Northwestern Iran

In this research, we have used 102 strong motion recordings from 2012 AharVarzaghan earthquakes (Mw=6.5 and Mw=6.3) to study the form of attenuation of shear wave Fourier litude spectra of those two events. The analysis is carried out in a broadband frequency range from 0.1 to 20 Hz. A bilinear shape for geometrical spreading is assumed based on nonparametric regression of the data. The hinge point of the bilinear shape is around 60 km away from the earthquake source the geometric spreading forms for the first and second segments are R0.9 and R0.5, respectively. The results of this study show that there is considerable dependency of the rate of geometrical spreading on frequency. If only frequencies above 1 Hz are considered, the first segment of geometrical spreading will have a slope steeper than R1. In contract, for lower frequencies it has a gentle slope. The associated quality factor for the assumed shape of geometrical spreading (appropriate for frequencies logarithmically spaced between 0.1 to 20 Hz) is Q(f)=148 f 0.62. The estimated Q(f) in this study agrees well with the other estimated shear wave quality factors in the region however, if the whole attenuation model (consisted of geometrical spreading and quality factor) is considered, there will be conspicuous differences between different models.