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

مطالعه ساختار سرعتی هر منطقه کمک شایانی به شناخت وضعیت لرزه زمین ساختی آن می کند. تعیین پارامترهایی چون  ضخامت پوسته و سنگ کره، مکان یابی دقیق زمین لرزه ها و پیش بینی 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 فعال ترین و جوان ترین کوهزادهای موجود در مرز های همگرایی قاره ای در جهان است که شناخت ویژگی های ساختاری این منطقه می تواند به فهم نحوه تغییر شکل در مرحله اول برخورد قاره ای و کوهزایی کمک کند. در این تحقیق ساختار سنگ کره در پهنه برخوردی زاگرس شمالی، با استفاده از وارون سازی هم زمان توابع گیرنده و منحنی های پاشش امواج سطحی مورد مطالعه قرار می‌گیرد. منحنی های پاشش از مطالعه ای که در سال 2014 از طریق بُرش نگاری (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 کرده اند. در این مطالعه با استفاده از فرایند وارون‌سازی هم زمان توابع گیرنده و منحنی های پاشش، مدل های سرعتی یک بُعدی موج برشی در زیر هر ایستگاه به دست می آید. سپس، از کنار هم قرار دادن این مدل های یک بُعدی، یک مدل دوبعدی سرعت برای سنگ کره زیرین ایستگاه های شبکه حاصل می شود. در این مدل یک زبانه کم سرعت در درون پوسته در مدل سرعتی قابل تشخیص است که از محل گسل اصلی عهد حاضر شروع می شود و با فاصله گرفتن از این گسل در راستای شمال شرقی به عمق های بیشتر می رود. این بی هنجاری در فاصله 200 کیلومتری شمال شرق گسل در عمق 35 ~ کیلومتر قابل مشاهده است. مدل سرعتی نشان می دهد که مقدار ضخامت پوسته از ابتدای خط اندازه‌گیری در بخش های جنوبی زاگرس (محدوده ایلام)، 43 کیلومتر است که با حرکت به سمت شمال شرق به میزان 57 کیلومتر در زیر گسل اصلی عهد حاضر می رسد. در سنندجسیرجان و ارومیهدختر به تدریج  بر این ضخامت افزوده شده و در مرز این دو ناحیه به بیشینه مقدار 62 کیلومتر می رسد. در زیر ایران مرکزی میزان ضخامت پوسته کاهش یافته و در انتهای خط اندازه‌گیری در زیر ایران مرکزی این ضخامت به 42 کیلومتر می رسد. مدل سرعتی به‌دست آمده، همچنین، اطلاعات خوبی از بخش سنگ کره ای گوشته ارائه می دهد. این مدل نشان می دهد که سنگ کره پُرسرعت زیرین زاگرس شمالی در زیر سنندجسیرجان و ارومیه دختر و لبه جنوبی ایران مرکزی گسترش یافته است. چنین مشاهده ای در تطابق با زیرراندگی مشاهده شده در پوسته است و می تواند شاهدی بر زیرراندگی بلوک عربی به زیر ایران مرکزی باشد.

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

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 microcontinent. 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 Pwave 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 timeseries computed from the threecomponent bodywave seismograms and are sensitive to the earth structure near the receiver station. They are composed of P to Swave conversions in discontinuities under the stations. These converted waves are isolated by deconvolving the vertical component of a teleseismic Pwave record from its radial component. For each event, a 120s timewindow 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 stressfree surface of the Earth, and in the presence of layering, they are dispersed. They provide valuable information on the absolute Swave velocity, but they are relatively insensitive to sharp velocity contrasts. On the other hand, receiver functions are sensitive to Swave velocity contrasts, which give rise to converted phases, but allow for a substantial tradeoff 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 30◦ and 95◦ epicentral distances. The fundamental mode Rayleighwave 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 4357 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 highvelocity 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 microcontinent. 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 Pwave 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 timeseries computed from the threecomponent bodywave seismograms and are sensitive to the earth structure near the receiver station. They are composed of P to Swave conversions in discontinuities under the stations. These converted waves are isolated by deconvolving the vertical component of a teleseismic Pwave record from its radial component. For each event, a 120s timewindow 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 stressfree surface of the Earth, and in the presence of layering, they are dispersed. They provide valuable information on the absolute Swave velocity, but they are relatively insensitive to sharp velocity contrasts. On the other hand, receiver functions are sensitive to Swave velocity contrasts, which give rise to converted phases, but allow for a substantial tradeoff 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 30◦ and 95◦ epicentral distances. The fundamental mode Rayleighwave 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 4357 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 highvelocity 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