Estimation of quality factor of coda waves in Makran region, southeast of Iran

Summary: Southeastern Iran، in Makran region، due to movement of the Arabian plate toward Eurasia، the oceanic crust is subducted beneath the continental crust forming a subductionzone. This movement and subduction has given a special situation to seismicity and geology of this zone. Covering the lake of seismological studies and the effect of geological structure on seismic wave propagation in this region، we have studied effects of the medium on propagated seismic waves from the source to the receiver using recorded data on stations of International Institute of Earthquake Engineering and Seismology from 2005 to 2011. The coda quality factor، Qc، in addition to engineering applications، could be used and provide remarkable information for seismicity studies and analyzing how the subduction of oceanic crust is treating. Regarding the extent of the studying area، it has been divided into three subareas of Southeastern of Central Iran،Southeastern of Zagros and Makran، then for each، the variation of quality factor with depth is evaluated. The “Single Back-Scattering Method” is used for estimate. The frequency dependence of coda wave quality factor for events up to 100 km epicentral distance at each three subareas is determined. Also، the lateral and depth variation of Qc are computed and discussed. In this study، the Qc values are calculated for 12 lapse times (5 to 60s with a step of 5s) for three regions. The frequency dependent relationships of Qc، for SE Zagros varies from Q  (12 1. 1) f (1. 260. 02) c at 5s to Q  (1251. 1) f (1. 070. 014) c at 60s lapse time windows. Similarly، for SE Central Iran، the relationship varies from Q (17 1. 3) f (1. 130. 051) c at 5s to Q  (147 1. 3) f (0. 890. 038) c at 60s lapse time windows; and for Makran region varies from Q  (12 1. 1) f (1. 30. 015) c at 5s to Q  (137 1. 1) f (1. 030. 018) c at 60s lapse time windows. In all regions، the value of Q is less than 200، which implies، beside a highly tectonically and seismically behavior، a highly heterogeneous medium. The results show an increase in Qc value with increasing lapse time window. Among three studied regions، SE Zagros has the minimum Qc and so is more tectonically active compared with the two others. The North Makran and SE Central Iran are places in thenext ranks، respectively. Estimated Qc at bigger lapse time window indicates less attenuation at bigger depth. In Makran region at a depth of ~97 km the variation rate of Q suddenly increases. Based on available geological cross sections، for the eastern and western Makran، depth of about 100km of oceanic crust is in the northern region of the western Makran، which shows good agreement with our observation that oceanic crust has higher velocity، so attenuation is less than continental crust.