Abstract

We selected 136 earthquakes recorded at stations of the Institute of Geophysics of the University of Tehran 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 western Asia. The selected events provide 1260 records of high signal‐to‐noise ratio with relatively good spatial coverage. We graphically find the distances at which the nature of geometrical spreading attenuation (Rb) changes significantly using a locally weighted scatter‐plot 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.

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