Coda wave attenuation's dependency on lapse time and frequency in west of Iran plateau using local earthquakes

Abstract

In this study, the attenuation of coda waves has been estimated in the western part of Iran plateau using single backscattering method. Coda wave attenuation was estimated at seven central frequencies (fc = 1.5, 3, 4.5, 6, 9, 12, 18 Hz) and eight lapse times (tl = 10, 20, 30, 40, 50, 60, 70, 80s). More than 9100 local earthquakes with magnitude range 2-5.5 and epicentral distance less than 200 km have been used in this study. The frequency dependency relation of attenuation has been determined by using Q0fn where the Q0 and n are varying from 51 to 225 and from 0.71 to 1.12 for lapse times from 10 to 80 s. The obtained low Q0 and high n values indicate that the study region is characterized by high seismic attenuation and, therefore, it is comparable with other seismically active areas. In order to investigate the attenuation at different depths, two datasets with epicentral distance less than 100 km and between 100 and 200 km were separately used. In the next step by using ellipsoidal sampled region, the variation of coda wave attenuation with depth are explored form the first dataset (epicentral distance less than 100 km). A slight decrease in the value of estimated attenuation was observed at depth of ~74 km in Western Alborz (WA) which may be related to high dissipating medium. Also an attenuation anomaly was found at Zagros and Sanandaj-Siran Zone (SSZ) at depth of ~136 and ~145 km, respectively, which may be related to the low-velocity zone caused by partial melting of the remnant of the fossil Neo-Tethyan. Using the second dataset (epicentral distance between 100 and 200 km), a clear decrease in the value of Q0 observed at depth of 178 km in SSZ, which may coincide with the bottom of Arabian lithosphere.