برآورد ساختار سرعت موج برشی و تغییرات عمق موهو در جنوب‏شرق ایران، با استفاده از وارون‏سازی هم‌زمان پاشش امواج سطحی و تابع گیرنده موج P

نوع مقاله: مقاله تحقیقی‌ (پژوهشی‌)

نویسندگان

1 مؤسسه ژئوفیزیک دانشگاه تهران

2 پژوهشگاه بین‏المللی زلزله‏شناسی و مهندسی زلزله

3 تهران، ایران

چکیده

در ناحیه جنوب‏شرق ایران که شامل بخش غربی زون فرورانش فعال مکران است به دلایل مختلفی همچون لرزه‏خیزی اندک و نبود تراکم ایستگاه‏های لرزه‏نگاری دائم، مطالعات اندکی در رابطه با ساختار سرعت موج برشی انجام شده است. هدف از این پژوهش، برآورد ساختار سرعت موج برشی و تغییرات عمق موهو در زیر چهار ایستگاه لرزه‏نگاری نوارپهن ZHSF، KRBR، CHBR و BNDS واقع در محدوده جنوب‏شرق ایران است. برای این منظور از روش وارون‏سازی هم‌زمان منحنی‏های پاشش امواج سطحی حاصل از زمین‏لرزه‏های محلی ناحیه ریگان، به همراه تابع گیرنده موج P حاصل از لرزه‏نگاشت‏های زمین‏لرزه‏های دورلرز ثبت شده در چهار ایستگاه لرزه‏نگاری ذکر شده، استفاده می‏شود. عمق موهو به‌دست آمده از این روش برای چهار ایستگاه ZHSF، KRBR، CHBR و BNDS به‌ترتیب برابر است با 4±38، 6±46، 2±26 و 5±56 کیلومتر. کم‌عمق بودن موهو و نیز بالاتر بودن مقادیر سرعت موج برشی به‌دست آمده در زیر ایستگاه CHBR با فرورانش کم‌عمق لایه اقیانوسی سرعت بالای صفحه عربی به زیر حاشیه جنوبی مکران مطابقت دارد. در عین حال ضخامت زیاد پوسته در محدوده ایستگاه BNDS، در تطابق با توپوگرافی مکان این ایستگاه و نیز  برخورد پوسته ضخیم قاره‏ای صفحه عربی با زاگرس در این ناحیه می‏باشد.
 

کلیدواژه‌ها


عنوان مقاله [English]

Estimation of shear wave velocity structure and Moho depth variations in the southeast of Iran, based on joint inversion of surface waves dispersion and P-wave receiver function

نویسندگان [English]

  • Mehdi Rastgoo 1
  • Habib Rahimi 1
  • Hosein Hamzehloo 2
  • Vahid Gholami 3
1 Institute of Geophysics, University of Tehran
2 International Institute of Earthquake Engineering and Seismology
3 Tehran, Iran
چکیده [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.

کلیدواژه‌ها [English]

  • Southeast of Iran
  • Shear wave velocity
  • Moho Depth
  • surface waves dispersion
  • P-Wave Receiver Function

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