Seismic image of the crustal structure in Kermanshah and Khorramabad region, northwest of Zagros, using teleseismic waves

Summary The Zagros fold and thrust belt (ZFTB) is resulted from the collision of the Arabian Plate with the continental crust of Central Iran and is considered as an example of a young continent–continent collision belt. In this study, we have used P receiver function technique to determine the crustal thickness and Vp/Vs ratio in northwest of Zagros. Our dataset includes teleseismic data (with magnitude Mb ≥ 5.5, epicentral distance  from 30◦ to 95◦) that have been recorded at 11 three component short-period and broadband stations of Kermanshah and Khoramabad telemetry seismic networks from 2010 to 2015. First, the differential travel time between the incident P wave and S converted wave (delay time) is used for computation of crustal thickness. Then, we have used the arrival times of crustal multiples (PpPs, PpSs PsPs) to determine crustal thickness (H) and Vp/Vs ratio using Zhu and Kanamori method. Applying this method, the average Moho depth is determined that is 44 km in northwest of Zagros and varies between 36 km and 55 km. In northern part of the region, the average thickness of the crust is about 38 km, and toward the center and south of the region is going to be thicker. The average Vp/Vs ratio in the crust of northwest of Zagros is about 1.74 and varies from 1.66 to 1.86.
Introduction Investigation of crust and upper mantle structures below the surface of the Earth is one of the important objectives of geophysics. Receiver functions are widely employed to detect P-to-S converted waves and are especially useful to image seismic discontinuities in the crust. This study intends to improve our knowledge on crustal thickness in northwest of Zagros. The region, which is referred as northwest of Zagros in Iran, includes the area located between 45°-49° longitude and 33°-36° latitude.
Methodology and Approaches Teleseismic events with relatively high signal-to-noise ratio (>4) have been carefully selected at each station. We have considered a time window of 120 s, starting 20 s before the P-onset arrival time. Firstly, to broaden the response of short-period and broad band instruments into a more useful teleseismic frequency band, the instrument response is denconvolved from the original records. The three components in the coordinate system ZNE are then rotated into the local ray coordinate system LQT using theoretical back azimuth and incidence angle. To isolate the P-to-S conversions on the Q component, the L component is deconvolved from the Q component. They are stacked after move out correction for reference slowness of 6.4 s/°. P-RFs are sorted by increasing back azimuth. Moho depths are obtained by using the model presented by Afsari et al. (2011) (Vp=6.4 km/s, Vp/Vs=1.78). Then, we obtain the depths using Zhu and Kanamori method, which performs a grid search through the H and Vp/Vs space, and searches for the largest amplitudes at the predicted times of direct conversions and multiples. In this regard, we have used the weight factors of 0.5 and 0.25 for the Moho conversion and multiples, respectively.
Results and Conclusions By applying Zhu and Kanamori method in northwest of Zagros reveals that the average Moho depth ia about 44 km, which is in good agreement with the results obtained by P receiver function analysis (~ 42 km). Our results also are in accordance with those obtained from other studies in ZFTB. Beneath the central part of Kermanshah region, a significant crustal thickening to a depth of approximately 51 km (station Kermanshah) has been observed. It may show a local effect beneath this part of region that has previously been reported in earlier studies. A local overthrusting system just beneath this station including dipping Moho boundary could be an alternative explanation for the observed feature. The results show that the Moho discontinuity in the study region is not flat.