Variation of Coda wave attenuation in two different Tectonic Areas in Iranian plateau, Alborz and NW Zagros

Abstract

The attenuation of coda wave in Alborz and NW Zagros has been estimated using the single backscattering (SBS) and the single isotropic scattering (SIS) methods at 9 frequency bands with central frequencies of 1, 1.5, 2, 3, 4, 6, 8, 12 and 16 Hz. The database consisted of 677 local earthquakes (418 events in the Alborz region and 259 events in the NW Zagros region) with the MN magnitude range from 3.0 to 5.7. A total of 8,717 seismograms with signal-to-noise ratios of greater than 3 and epicentral distances less than 200 km were used. To investigate the depth variation of attenuation in this study, the coda quality factor Qc was estimated in each area at epicentral distance range of R<100 km and 100<R<200 km, through 11 coda window lengths between 10 and 60 s. The estimated average frequency-dependent relation of Qc = Q0f n at coda window length of 10 to 60 s varies from 66 f 1.04 to 164 f 0.72 in Alborz and 66f 0.99 to 157 f 0.76 in NW Zagros. In this study, the value of the frequency-dependent parameter decreases with increasing the coda window lengths, and suggests that the lithosphere becomes more homogenous with increasing depth. Furthermore, the coda wave quality factor was determined from all three components.We found that there is no significant difference in obtained results by using the vertical and horizontal components. This indicates that the seismic waves encounter similar heterogeneities and attenuation in the vertical and horizontal directions in the study area.The average frequency- dependent relations of coda waves have been derived Qc=109f 0.87and Qc= 108f 0.86, for the Alborz and NW Zagros regions, respectively, usingthe SBS model at the coda window length of 25s. Comparison of estimated coda-Q values show that, the obtained Qc values using the SIS method are slightly higher than that those obtained using the SBS method.