مطالعه ساختار سنگ‌کره در منطقه برخوردی زاگرس شمالی با استفاده از وارون‌سازی هم‌زمان توابع گیرنده و منحنی‌های پاشش امواج سطحی

نوع مقاله: مقاله تحقیقی‌ (پژوهشی‌)

نویسندگان

1 دانشگاه تحصیلات تکمیلی علوم پایه زنجان

2 دانشگاه تحصیلات تکمیلی در علوم پایه زنجان

چکیده

مطالعه ساختار سرعتی هر منطقه کمک شایانی به شناخت وضعیت لرزه زمین‌ساختی آن می‌کند. تعیین پارامترهایی چون  ضخامت پوسته و سنگ‌کره، مکان‌یابی دقیق‌ زمین‌لرزه‌ها و پیش‌بینی تحولات آینده زمین‌ساختی هر منطقه در گرو داشتن مدل سرعتی موثق از منطقه است. فلات ایران گستره وسیعی بین دو صفحه عربی و اوراسیا است و دگرشکلی آن ناشی از همگرایی این دو صفحه است. در این میان، کمربند کوهزایی زاگرس یکی از فعال‌ترین و جوان‌ترین کوهزادهای موجود در مرز‌های همگرایی قاره‌ای در جهان است که شناخت ویژگی‌های ساختاری این منطقه می‌تواند به فهم نحوه تغییر شکل در مرحله اول برخورد قاره‌ای و کوهزایی کمک کند. در این تحقیق ساختار سنگ‌کره در پهنه برخوردی زاگرس شمالی، با استفاده از وارون‌سازی هم‌زمان توابع گیرنده و منحنی‌های پاشش امواج سطحی مورد مطالعه قرار می‌گیرد. منحنی‌های پاشش از مطالعه‌ای که در سال 2014 از طریق بُرش‌نگاری (توموگرافی) رحیمی و همکاران (2014) انجام دادند، برگرفته و توابع گیرنده از 161 دورلرزه با فاصله رومرکزی  °30 تا °95 و بزرگای بیش از 5 محاسبه شده است. دورلرزه‌ها را 38 ایستگاه لرزه‌نگاری موقت در امتداد پروفایلی به طول 400 ~ کیلومتر در حد فاصل شهرهای ایلام تا قم ثبت کرده‌اند. در این مطالعه با استفاده از فرایند وارون‌سازی هم‌زمان توابع گیرنده و منحنی‌های پاشش، مدل‌های سرعتی یک‌بُعدی موج برشی در زیر هر ایستگاه به‌دست می‌آید. سپس، از کنار هم قرار دادن این مدل‌های یک‌بُعدی، یک مدل دوبعدی سرعت برای سنگ‌کره زیرین ایستگاه‌های شبکه حاصل می‌شود. در این مدل یک زبانه کم‌سرعت در درون پوسته‌ در مدل سرعتی قابل تشخیص است که از محل گسل اصلی عهد حاضر شروع می‌شود و با فاصله گرفتن از این گسل در راستای شمال شرقی به عمق‌های بیشتر می‌رود. این بی‌هنجاری در فاصله 200 کیلومتری شمال شرق گسل در عمق 35 ~ کیلومتر قابل مشاهده است. مدل سرعتی نشان می‌دهد که مقدار ضخامت پوسته از ابتدای خط اندازه‌گیری در بخش‌های جنوبی زاگرس (محدوده ایلام)، 43 کیلومتر است که با حرکت به سمت شمال شرق به میزان 57 کیلومتر در زیر گسل اصلی عهد حاضر می‌رسد. در سنندج-سیرجان و ارومیه-دختر به‌تدریج  بر این ضخامت افزوده شده و در مرز این دو ناحیه به بیشینه مقدار 62 کیلومتر می‌رسد. در زیر ایران مرکزی میزان ضخامت پوسته کاهش یافته و در انتهای خط اندازه‌گیری در زیر ایران مرکزی این ضخامت به 42 کیلومتر می‌رسد. مدل سرعتی به‌دست آمده، همچنین، اطلاعات خوبی از بخش سنگ‌کره‌ای گوشته ارائه می‌دهد. این مدل نشان می‌دهد که سنگ‌کره پُرسرعت زیرین زاگرس شمالی در زیر سنندج-سیرجان و ارومیه‌دختر و لبه‌ جنوبی ایران مرکزی گسترش یافته است. چنین مشاهده‌ای در تطابق با زیرراندگی مشاهده شده در پوسته است و می‌تواند شاهدی بر زیرراندگی بلوک عربی به زیر ایران مرکزی باشد.

کلیدواژه‌ها


عنوان مقاله [English]

Lithosphere structure in the north Zagros collision revealed by joint inversion of P receiver function and surface wave dispersion

نویسندگان [English]

  • Forough Kalvandi 1
  • Khalil Motaghi 2
  • Esmaeil Shabanian 1
1 Institute of Advanced Studies in Basic Sciences
2 Institute for Advanced Studies in Basic Sciences
چکیده [English]

The Zagros mountain belt, situated on the northern margin of the Arabian plate, is one of the youngest continental collision belts. This belt was formed by a collision between the Arabian plate and the Central Iranian micro-continent. In this study, we used data from 38 temporary seismological stations installed on a 400 km long profile from May to November 2003.The trend of the profile is N58°E across northern Zagros and part of the Central Iran. The stations are part of Zagros03 profile (Paul et al., 2010) operated by the International Institute of Earthquake Engineering and Seismology (IIEES) of Iran in collaboration with CNRS - Université Joseph Fourier, France. We examine the structure of the lithosphere, across the profile by analysis of P-wave receiver functions and Rayleigh wave fundamental mode phase velocity dispersion curves. Joint inversion of Rayleigh wave phase velocity dispersion and receiver functions have been used to estimate the velocity structure beneath 28 seismic stations. Receiver functions are time-series computed from the three-component body-wave seismograms and are sensitive to the earth structure near the receiver station. They are composed of P- to S-wave conversions in discontinuities under the stations. These converted waves are isolated by deconvolving the vertical component of a teleseismic P-wave record from its radial component. For each event, a 120s time-window centered at the direct P arrival is selected and used for the calculation of the receiver function. The deconvolution used is the iterative deconvolution method of Ligorria and Ammon (1999). Surface waves arise from the presence (boundary conditions) of the stress-free surface of the Earth, and in the presence of layering, they are dispersed. They provide valuable information on the absolute S-wave velocity, but they are relatively insensitive to sharp velocity contrasts. On the other hand, receiver functions are sensitive to S-wave velocity contrasts, which give rise to converted phases, but allow for a substantial trade-off between the depth and velocity above an impedance change. Combining them in a joint inversion process bridges the resolution gaps associated with each data set. We jointly inverted the stacked receiver function and surface wave dispersion data. We employ the program joint96 which is available in the software package “Computer Program in Seismology” (Herrmann and Ammon, 2003). In this study, we try to calculate the Moho depth and velocity structure in the north Zagros collision zone using the joint inversion of receiver function and surface wave dispersions. Receiver functions are calculated using teleseismic events of magnitude greater than 5.1, located between 30and 95epicentral distances. The fundamental mode Rayleigh-wave group velocities are extracted from thetomographic study conducted by Rahimi et al. (2014). The 1D velocity models resolved by joint inversion are juxtaposed, and a 2D velocity model is obtained. Results obtained from the 2D model reveal that the thickness of the sediments beneath the Zagros is 12 km, the Moho depth beneath this region of Zagros is 43-57 km, which increases towards Sanandaj–Sirjan zone and Urumieh–Dokhtar magmatic arc and reaches an expanse of 62 km and then decreases in the central Iran with a depth of 42 km. The velocity model confirms the presence of a crustal root and a thick high-velocity lithosphere beneath and north of the suture. These evidences imply that the Arabian plate continues to underthrust beneath the Central Iran.
The Zagros mountain belt, situated on the northern margin of the Arabian plate, is one of the youngest continental collision belts. This belt was formed by a collision between the Arabian plate and the Central Iranian micro-continent. In this study, we used data from 38 temporary seismological stations installed on a 400 km long profile from May to November 2003.The trend of the profile is N58°E across northern Zagros and part of the Central Iran. The stations are part of Zagros03 profile (Paul et al., 2010) operated by the International Institute of Earthquake Engineering and Seismology (IIEES) of Iran in collaboration with CNRS - Université Joseph Fourier, France. We examine the structure of the lithosphere, across the profile by analysis of P-wave receiver functions and Rayleigh wave fundamental mode phase velocity dispersion curves. Joint inversion of Rayleigh wave phase velocity dispersion and receiver functions have been used to estimate the velocity structure beneath 28 seismic stations. Receiver functions are time-series computed from the three-component body-wave seismograms and are sensitive to the earth structure near the receiver station. They are composed of P- to S-wave conversions in discontinuities under the stations. These converted waves are isolated by deconvolving the vertical component of a teleseismic P-wave record from its radial component. For each event, a 120s time-window centered at the direct P arrival is selected and used for the calculation of the receiver function. The deconvolution used is the iterative deconvolution method of Ligorria and Ammon (1999). Surface waves arise from the presence (boundary conditions) of the stress-free surface of the Earth, and in the presence of layering, they are dispersed. They provide valuable information on the absolute S-wave velocity, but they are relatively insensitive to sharp velocity contrasts. On the other hand, receiver functions are sensitive to S-wave velocity contrasts, which give rise to converted phases, but allow for a substantial trade-off between the depth and velocity above an impedance change. Combining them in a joint inversion process bridges the resolution gaps associated with each data set. We jointly inverted the stacked receiver function and surface wave dispersion data. We employ the program joint96 which is available in the software package “Computer Program in Seismology” (Herrmann and Ammon, 2003). In this study, we try to calculate the Moho depth and velocity structure in the north Zagros collision zone using the joint inversion of receiver function and surface wave dispersions. Receiver functions are calculated using teleseismic events of magnitude greater than 5.1, located between 30and 95epicentral distances. The fundamental mode Rayleigh-wave group velocities are extracted from thetomographic study conducted by Rahimi et al. (2014). The 1D velocity models resolved by joint inversion are juxtaposed, and a 2D velocity model is obtained. Results obtained from the 2D model reveal that the thickness of the sediments beneath the Zagros is 12 km, the Moho depth beneath this region of Zagros is 43-57 km, which increases towards Sanandaj–Sirjan zone and Urumieh–Dokhtar magmatic arc and reaches an expanse of 62 km and then decreases in the central Iran with a depth of 42 km. The velocity model confirms the presence of a crustal root and a thick high-velocity lithosphere beneath and north of the suture. These evidences imply that the Arabian plate continues to underthrust beneath the Central Iran.
 
Keywords: receiver functions, surface wave dispersion, joint inversion, velocity structure, north Zagroscollision

کلیدواژه‌ها [English]

  • receiver functions
  • surface wave dispersion
  • Joint inversion
  • velocity structure
  • north Zagros collision
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