Attenuation of High-Frequency Seismic Waves in NW Iran

Abstract

We analyzed 872 local earthquakes recorded by 18 seismic stations to estimate P-, S-, and coda-wave quality factors for NW Iran, an active seismic and geothermal region in the central part of the Alpine-Himalayan orogeny in western Asia. The calculated frequency-dependent attenuation relations are as follows: Q_p = 38 ± 2 f ^1.00±0.07, Q_s = 79 ± 2 f ^0.90±0.17, and Q_c = 64 ± 1 f ^0.86±0.18 (for a lapse time window length of 50 s). The depth variations of Q_c were investigated by estimating Q_c at five lapse time windows ranging from 30 to 70 s. The Analysis of Q_c in sub-regions surrounding Sahand volcano, Sabalan volcano, and Talesh Mountains indicates that Q_c is increasing with the increase of the lapse times. However, the Q_c values estimated in the areas surrounding the volcanoes are generally lower than those of the Talesh Mountains, confirming a lithospheric contrast between the warm and weak lithosphere of NW Iran and the more rigid South Caspian Basin lithosphere located beneath the Talesh Mountains. The average Q_s/Q_p ratio in NW Iran is > 1.6 at all frequencies, likely due to a high degree of heterogeneities and thermal activities in the crust.