lithospheric features of the Zagros region, SW Iran,based on attenuation of sesmic waves

We have used over 2800 earthquake to estimate coda wave quality factor, Qc, in the Zagros fold-and-thrust belt (ZFTB) and the Sanandaj-Sirjan metamorphic zone (SSZ) in southern Iran. We also investigated lateral and depth variations of Qc in the study region. We observe that in the ZFTB there is no significant lateral variation in attenuation, except in the SE Zagros (the Bandar-Abbas region) which shows lower attenuation. The average frequency relations for SSZ, Bandar- Abbas region and ZFTB were obtained as: = ( ± ) . ± . , = ( ± ) . ± and = ( ± ) . ± , respectively. To investigate the depth-dependence of attenuation, Qc values were calculated for 18 lapse-times (5, 10, 15, . . . 90s) for epicentral distance range of R < 100 km. We observed that in general, Q0 (quality factor at 1 Hz) is an increasing function of lapse-time, while the n value (the frequency dependence factor) is a decreasing function of lapsetime. Beneath SSZ and at the depth range of 50 to 80 km, there isa high correlation between the previously reported low velocity medium and the observed sharp changes in the trend of the Q0 and n curves. In the reported velocity structure in mind, unlike SSZ, Under the crust of the Zagros, our results in different parts of the mountain range show no sharp changes in the trends of Q0 and n. On the other hand, we observe a significant correlation between the reported lithosphere thicknesses and the trends of Q0 and n in longer lapse-times (i.e. larger depths). The NW Zagros has the thickest lithosphere. In this region the Q0 curve shows anincreasing slope up to a lapse-time of 85 s. Other parts of Zagros with thinner lithosphere show a constant or gradually increasing trend of Q0 and n at large lapse-times.

Keywords:

Quality factor , Attenuation , Seismicity , Coda , Zagros