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

In this research, we have used 102 strong motion recordings from 2012 Ahar-Varzaghan earthquakes (Mw=6.5 and Mw=6.3) to study the form of attenuation of shear wave Fourier amplitude spectra of those two events. The analysis is carried out in a broad-band 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 R-0.9 and R-0.5, 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 R-1. 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.