Meghdad Samaei - Postdoctoral Researcher, Graduate School of Natural Science and Technology, Kanazawa University, Kanazawa, Japan
Masakatsu Miyajima - Professor, Graduate School of Natural Science and Technology, Kanazawa University, Kanazawa, Japan
Nobuoto Nojima - Professor, Department of Civil Engineering, Gifu University, Gifu, Japan

In this research, we have used ۱۰۲ strong motion recordings from ۲۰۱۲ Ahar-Varzaghan earthquakes (Mw=۶.۵ and Mw=۶.۳) 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 ۰.۱ to ۲۰ 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 ۶۰ km away from the earthquake source; the geometric spreading forms for the first and second segments are R-۰.۹ and R-۰.۵, respectively. The results of this study show that there is considerable dependency of the rate of geometrical spreading on frequency. If only frequencies above ۱ Hz are considered, the first segment of geometrical spreading will have a slope steeper than R-۱. 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 ۰.۱ to ۲۰ Hz) is Q(f)=۱۴۸ f ۰.۶۲. 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.

Ahar-Varzaghan earthquakes, Attenuation, Fourier spectra, Geometrical spreading, Northwestern Iran, quality factor