نوع مقاله : مقاله تحقیقی (پژوهشی)
1 مؤسسه ژئوفیزیک دانشگاه تهران
2 پژوهشگاه بینالمللی زلزلهشناسی و مهندسی زلزله
3 تهران، ایران
عنوان مقاله [English]
The southeast region of Iran includes the western part of Makran as an active subduction zone on the North side of the Oman Sea. The velocity structure of this region is not well understood because of its low-level seismicity and a small number of permanent seismic stations in this region. The main purpose of this study is the estimation of shear wave velocity structure and Moho depth variations in the southeast of Iran. To this end, we apply the joint inversion process of Rayleigh and Love waves dispersion curves and the “P-wave receiver function” (PRF) around four permanent broadband seismic stations of the region named ZHSF, KRBR, CHBR, and BNDS. To find the group velocities of surface waves in the southeast of Iran, the “frequency–time analysis” (FTAN) is applied to the waveforms of 40 local earthquakes, which occurred in Rigan region and recorded by the four permanent broadband seismic stations. These local earthquakes include two main shocks of 20 December 2010 (ML=6.4) and 27 January 2011 (ML=6.2) accompanied by their foreshocks and aftershocks in Rigan region. Therefore, the group velocities of fundamental modes of Rayleigh and Love waves are calculated in the period range from 5 to 60 sec in the paths among the four seismic stations and the sources of Rigan earthquakes. Also, to calculate P-wave receiver functions around the four permanent broadband seismic stations, 485 teleseismic earthquakes with suitable signal-to-noise ratios and epicentral distances between 30° and 95° related to the region, are selected. The radial PRFs are computed by deconvolving the vertical component from the radial component based on the iterative deconvolution method (Ligorria and Ammon, 1999). After preparing these two groups of data, we can determine the shear wave velocity structure and Moho depth vicinity of each seismic station by applying the joint inversion process to the dispersion data and the PRF data related to each seismic station (using the joint96 program; Herrmann and Ammon, 2007). Based on the results obtained in this study, the average group velocity of surface waves was estimated at less than 3.5 kms-1 in the period range from 5 to 60 sec. The lowest average group velocity of surface waves was obtained in the paths between the CHBR station and the sources of the Rigan earthquakes. Also, the Moho depths beneath the ZHSF, KRBR, CHBR, and BNDS stations were estimated 38±4, 46±6, 26±2 and 56±5 km, respectively. The minimum thickness of crust beneath CHBR station as well as the higher velocity of shear wave estimated beneath this station, are consistent with the shallow subduction of a high-velocity oceanic crust of Arabian plate beneath the south side of Makran. Furthermore, the thicker crust beneath the KRBR station and the lower velocity of shear wave estimated in this area, when compared with the area encompassing the CHBR station, is due to the existence of magmatic assemblage in the vicinity of the KRBR station. These results are consistent with the crust thickening from the south to the north of Makran. The maximum thickness beneath the BNDS is due to the location of this station being in the southeast of Zagros mountain belt, where the thick continental Arabian plate collides with Zagros. This collision leads to thickening of crust in Zagros.