ساختار دوبعدی سرعت امواج برشی در پوسته و گوشتۀ بالایی البرز شرقی

نویسندگان

1 دانشجوی دکتری، گروه فیزیک زمین، موسسه ژئوفیزیک دانشگاه تهران، ایران

2 استادیار، گروه فیزیک زمین، موسسه ژئوفیزیک دانشگاه تهران، ایران

3 دانشیار، پژوهشگاه بین‌المللی زلزله‌شناسی و مهندسی زلزله، تهران، ایران

چکیده

کمربند کوهستانی البرز واقع در شمال ایران، به عنوان یک ناحیۀ فعال زمین‌ساختی و لرزه‌خیز شناخته می‌شود که تعیین ساختار سرعت این ناحیه به منظور تفسیر فعالیت‌های زمین‌ساختی حائز اهمیت است. در این پژوهش با استفاده از 12 ایستگاه لرزه‌نگاری در البرز شرقی، براساس روش وارون‌سازی همزمان تابع گیرندۀ موج P و منحنی‌های پاشش امواج ریلی، ساختار یک‌بعدی سرعت موج برشی در محدودۀ هر ایستگاه و نیز ساختار دو بعدی آن در طول دو پروفایل (یکی در راستای روند شرق البرز و دیگری عمود بر این روند) تعیین می‌شود. طبق نتایج به‌دست‌آمده، عمق موهو و مرز لیتوسفر-استنوسفر در این ناحیه به ترتیب 2±47 و 6±86 کیلومتر است. همچنین طبق ساختارهای دوبعدی سرعت موج برشی، یک لایۀ آنومالی پرسرعت در گسترۀ عمقی 120 تا 180 کیلومتر مطابق با زیرراندگی خزر به زیر البرز مشاهده می‌شود. با توجه به توپوگرافی سطحی البرز شرقی، ضخامت پوسته در این ناحیه جبران‌کنندۀ ارتفاعات کوهستانی در مقیاس پریودبلند است و در عین حال وجود لایۀ آنومالی سرعت بالا در زیر لیتوسفر، توجیه‌کنندۀ توپوگرافی سطحی در مقیاس پریودکوتاه است.

کلیدواژه‌ها

موضوعات

عنوان مقاله [English]

2D shear Wave Velocity Structure beneath Crust and upper Mantel in Eastern Alborz

نویسندگان [English]

  • Mehdi Rastgoo 1
  • Habib Rahimi 2
  • Hossein Hamzehloo 3
1 Ph.D. Student, Department of Earth Physics, Institute of Geophysics, University of Tehran, Iran
2 Assistant Professor, Department of Earth Physics, Institute of Geophysics, University of Tehran, Iran
3 Associate Professor, International Institute of Earthquake Engineering and Seismology, Tehran, Iran
چکیده [English]

Alborz mountain belt in the North of Iran is known as a tectonically and seismically active region. Determination of shear wave velocity structure is important to interpret the tectonic activities. In this study, we determine 1D shear wave velocity structure beneath 12 seismic stations in the Eastern part of Alborz and also 2D shear wave velocity structure along to two profiles (one is along to the trend of Eastern part of Alborz and another one is perpendicular to its trend), based on the joint inversion of P-wave receiver function (PRF) and dispersion curves of Rayleigh waves. To obtain the PRFs of each seismic station, we lonsider three-component body wave seismograms of 177 teleseismic earthquake events with magnitude Mw>5.2 and epicentral distance range 30° to 95°, related to the study region. Also the dispersion curves of Rayleigh waves in the vicinity of each station are extracted from surface wave tomographic study reported by Rahimi et al. (2014). Then these two group data are regarded as the input data for the joint inversion process using “joint96” program (Herrmann and Ammon, 2007). ). In this study, the initial models are taken from shear wave velocity models reported by Rahimi et al. (2014), based on tomographic inversion of Rayleigh wave dispersion for various tectonic region of Iran. We regard the maximum depth of investigation about 300 km (upper mantle) in this joint inversion process based on sensitivity kernels of the dispersion curves of the Rayleigh wave fundamental mode with respect to the shear wave velocity at different periods (Rahimi et al., 2014). To find the most robust final velocity model for each station, we regard two stability tests: first, searching for the optimal parameterization for the joint inversion process; second, simplify of the representative solution of the joint inversion process (Motaghi et al., 2015). According to the obtained results, the depth of Moho boundary beneath the eastern part of Alborz mountain range is relatively uniform and following 47±2 km. By attention to the absolute shear wave velocity structure along the two profiles, depth of lithosphere-asthenosphere boundary beneath covered area is roughly constant and mainly varies around 86±6 km. Also there are high velocity anomalies in depth range 120-180 km. These high velocity anomalies in the upper mantle are consistent with the presence of under thrusting of Caspian lithosphere beneath Alborz. This observation is reported previously by Jackson et al., 2002. These observations may support the remaining question about higher surface topography in the study region without enough supporting crustal thickness. Maggi et al. (2000), using the admittance between topography and gravity in frequency domain mentioned that the only very short period topography could be supported by the flexure of the layer, whilst any longer period topography must be supported by an isostatic response. This result supports our observations, which shows an isostatic compensation for much of the long period topography. On the other hand, for short period topography, the mechanism of elastic flexure layer beneath Alborz, allowing high topographies to be supported by thin crust. We observed almost well correlation between the thickness of high velocity under thrusted layer and surface topography and also our observation could support higher surface topography in study region without enough supporting crustal thickness.

کلیدواژه‌ها [English]

  • Eastern Part of Alborz
  • Shear Wave Velocity Structure
  • Crust and Lithosphere
  • P-wave Receiver Function
  • Dispersion curves of Rayleigh Waves

مراجع

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فیزیک زمین و فضا
دوره 43، شماره 2
مرداد 1396
صفحه 309-322

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