Attenuation of Ground-Motion Spectral Amplitudes and Its Variations across the Central Alborz Mountains

Abstract

We selected 136 earthquakes recorded at stations of the Institute of Geo- physics of the University of Tehran (IGUT) during 1996-2007, in order to estimate the average attenuation parameters and variations of station corrections across the central Alborz mountains, the northern extension of the Alpine-Himalayan orogeny in wes- tern Asia. The selected events provide 1260 high signal-to-noise ratio (SNR) records with relatively good spatial coverage. We graphically find the distances at which the nature of geometrical spreading attenuation (R b ) changes significantly using a locally weighted scatterplot smoothing (LOWESS, local regression smoothing method) called robust LOWESS. A trilinear function with hinges at distances of about 80 and 160 km describes the geometric spreading attenuation with distance. By regressing to the hinged trilinear function, we found that b1 � −1:15 � 0:21, b2 � 0:09 � 0:31, and a fixed b3 � −0:5 minimize the average absolute value of the Fourier spectrum amplitude residuals. Using an anelastic attenuation coefficient at different frequencies, the direct quality factor Q in the central Alborz region is obtained as Q � 109 � 2f 0:64� 0:04 . The geographical distribution of station corrections at 1 Hz can be clearly separated along a dividing line connecting the North Tehran, Mosha, and Attari faults into two regions: the northern low-attenuating and the southern high attenuating regions. The stations with strong positive station corrections clearly align along the dividing line. This suggests the existence of a higher attenuation zone south of the dividing line. The presence of thick Tertiary-Quaternary sedimentary rocks with thick layers of salt in the southern region may explain the observed greater attenuation of seismic waves.