برآورد توموگرافی دوبُعدی سرعت‌های فاز، گروه و ساختارسرعت امواج بُرشی در پهنه البرز

نوع مقاله: مقاله تحقیقی‌ (پژوهشی‌)


مؤسسه ژئوفیزیک دانشگاه تهران، ایران


در این تحقیق نقشه‌‌های توموگرافی دوبُعدی سرعت‌های فاز و گروه، اولین‌‌بار به روش معکوس‌سازی خطی تهیه شده و ساختار سرعت امواج بُرشی میانگین برای پوسته و گوشته بالایی در پهنه البرز به روش معکوس‌سازی غیرخطی به‌دست آمده است. برای دستیابی به هدف‌های پیش‌گفته، تحقیق حاضر در دو بخش اساسی صورت گرفت. در اولین بخش، منحنی‌های پراکندگی محلی با استفاده از داد‌‌ه‌‌‌های مناسب ثبت شده در ایستگا‌‌ه‌‌‌های نوار پهن شبکه‌ لرزه نگاری پزوهشگاه بین المللی زلزله برآورد شد. از این منحنی‌ها با استفاده از روش معکوس‌سازی خطی برای برآورد نقشه‌‌‌های توموگرافی دوبُعدی سرعت‌های فاز و گروه استفاده شد. نقشه‌‌‌های توموگرافی دوبُعدی به‌دست آمده در این تحقیق، همخوانی خوبی با ناهمگنی‌های جانبی در پهنه البرز نشان داد. تناوب‌‌های کم با رسوبات و ساختارهای پوسته بالایی و تناوب‌‌های متوسط و زیاد با ساختارهای پوسته پایینی و گوشته بالایی همخوانی خوبی دارند. در مرحله دوم، منحنی‌های میانگین مشاهده‌ای به‌دست آمده در مرحله قبل با استفاده از روش معکوس‌سازی غیرخطی هجهاگ برای برآورد ساختار سرعت امواج برشی به‌کار برده شدند. ساختار سرعتی به‌دست آمده در این تحقیق علاوه بر اینکه در تحقیقات زلزله‌شناسی و برآورد پارامترهای جنبش نیرومند زمین و برآورد واقع‌گرایانه خطر زلزله دارای اهمیت ویژه‌ای است، با فرایندهای زمین‌ساختی و ژئودینامیکی مطرح شده در ناحیه البرز، سازگاری خوبی نشان می‌دهد. عمق موهو در ناحیه البرز، با استفاده از معکوس‌سازی هم‌زمان منحنی‌‌های سرعت گروه و فاز 46 کیلومتر برآورد شد. در این ناحیه، دو لایه به‌دست آمده برای پوسته دارای ضخامتهای 15 و 25 کیلومتری زیر یک لایه رسوبی 6 کیلومتری به‌دست آمدند که دارای سرعت‌‌های موج بُرشی 1/3 و7/3 کیلومتر بر ثانیه هستند. دراین پهنه سه لایه گوشته بالایی با ضخامت‌‌های 20، 60 و80 کیلومتر دارای سرعت‌‌های 3/4، 6/4 و 5/4 کیلومتر بر ثانیه به‌دست آمدند که لایه به‌نسبت کم‌سرعت با ضخامت 80 کیلومتر در عمق 126 تا 206 کیلومتری واقع شده است.


عنوان مقاله [English]

Estimation of the two-dimensional tomography of phase and group velocities and shear wave structure for Alborz region

نویسنده [English]

  • Habib Rahimi
چکیده [English]

The delineation of the elastic, or velocity, structure of the Earth has long been a goal of the world's seismologists. For the first few decades of seismological research, the investigation on velocity structure was restricted to the determination of one-dimensional models of the solid Earth and of various regions within it. Seismologists are currently obtaining three dimensional velocity models and are working to resolve finer and finer features in the Earth. The knowledge of seismic velocity structure of the crust and the upper mantle is important for several reasons: these include accurate location of earthquakes, determination of the composition and origin of the outer layers of the Earth, improvement of our ability to discriminate nuclear explosions from earthquakes, interpretation of large-scale tectonics and reliable assessment of earthquake hazard. In this study, we first prepared the two-dimensional phase, group velocity images and also the shear wave velocity structure of the lithosphere and asthenosphere of the Alborz region. To achieve these goals, in the first step, we conducted a tomographic inversion of Rayleigh wave dispersion to obtain the two-dimensional (2-D) phase and group velocity tomographic images in a period range from 10 s to 100 s for the Alborz region. For this purpose, the fundamental mode of Rayleigh waves, recorded along paths by broad-band stations, has been identified by applying the frequency time analysis (FTAN) to each epicenter–station path which, at the same time, satisfies the two-station method conditions. The fundamental modes, identified by FTAN, are used to determine the inter-station path average phase and group velocities at selected periods. With this procedure, group and phase velocity dispersion curves have been processed to obtain tomographic maps by applying the Yanovskaya–Ditmar formulation, for periods in the range between 10 and 100 s. Each tomographic map has been discretized with a grid of 0.5° of latitude per 0.5° of longitude.
    Our results demonstrated that the Alborz region is characterized by low crustal and uppermost-mantle group and phase velocities. Tomographic maps at high frequencies are well correlated with the upper crust structure and especially with sediment layer thicknesses. In the second step, we used fully non-linear inversion procedure, commonly known as hedgehog (Valyus et al., 1969; Valyus, 1972; Knopoff, 1972; Panza, 1981; Panza et al., 2007) to derive tomographic images of the elastic  structure of the lithosphere and asthenosphere of the Alborz region.An estimated shear wave structure can be useful to estimate the strong ground motion as well as the realistic seismic hazard assessment. On the other hand, the derived tomographic phase, group images and the shear wave velocity structure are well correlated with major tectonic and geological features of the Alborz region. The Moho depth in this region is derived around 46 km in which the shear wave velocity varies between 3.7 and 4.3 km/sec when passing from the crust to the mantel. The thickness of the two resolved crustal layers are 15 and 25 km located beneath of a sediment layer with 6 km. In this region, three upper mantel layers are resolved with thicknesses of 20, 60 and 80 km having velocities of 4.3, 4.6 and 4.5 km/sec, respectively.

کلیدواژه‌ها [English]

  • Crust and upper mantel shear wave velocity structure
  • surface wave
  • Alborz
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