برگردان سرعت گروه امواج ریلی به ساختار سرعت موج برشی برای منطقه شمال غرب ایران

نویسندگان

1 دانشگاه تحصیلات تکمیلی علوم پایه زنجان

2 موسسه ژئوفیزیک، دانشگاه تهران

چکیده

در این مطالعه تغییرات سرعت موج برشی با استفاده از داده های ثبت شده در 23 ایستگاه باند پهن شبکه موقت دانشگاه تحصیلات تکمیلی علوم پایه زنجان، در منطقه شمال غرب ایران مورد بررسی قرار می گیرد. به همین منظور با استفاده از 230 رخداد منطقه ای و دورلرز منحنی های پاشش سرعت گروه امواج ریلی در مد پایه برای 20 مسیربین ایستگاهی محاسبه می شود. سپس جهت محاسبه ساختار سرعت موج برشی در هر مسیر، برگردان خطی و غیرخطی منحنی های پاشش به ساختار سرعت موج برشی انجام می شود. منحنی های پاشش بین ایستگاهی به روش دو ایستگاهی و ساختارهای سرعت نهایی به روش جستجوی محاسباتی- تصادفی "هجهاگ" محاسبه می شوند. با توجه به بازه دوره تناوبی در منحنی های پاشش محاسبه شده در این مطالعه (بین 5 تا 48 ثانیه) تنها پارامترهای سرعتی در پوسته و گوشته بالایی قابل محاسبه است. ساختارهای سرعت موج برشی نشان دهنده یک ساختار ناهمگن با ضخامت متغیر پوسته در امتدادپروفیل لرزه نگاری است. عمق مرز موهو بین 40 تا 56 کیلومتر و عمق مرز میان پوسته بالایی و پایینی نیز بین 12 تا 28 کیلومتر به دست آمده است. در بازه عمقی 12 تا 22کیلومتر در شمال آتشفشان سهند یک توده کم سرعت و احتمالا گرم مشاهده می شود.همچنین اثر عبور مسیر 20 رخداد منطقه ای بر میرایی امواج ریلی با دوره های زمانی بیش از 32 ثانیه نیز نشان داده می شود و منشاء این میرایی غیر عادی به اثر رسوبات ضخیم حوضه خزر جنوبی نسبت داده می شود.

کلیدواژه‌ها

موضوعات


عنوان مقاله [English]

Inversion of Rayleigh waves group velocity to shear wave velocity structure in the NW Iran

نویسندگان [English]

  • Reza Davoudian 1
  • Farhad Sobouti 1
  • Habib Rahimi 2
  • Abdolreza Ghods 1
1
2
چکیده [English]

We determined inter station shear wave structure using data from a temporary network of 23 broadband stations in the north west of Iran. Waveforms were used from 230 tele-seismic and regional earthquakes to obtain inter station dispersion curves of the group velocity of the Rayleigh waves. Events in the epicentral distance range of 250 to 3000 km with magnitudes 3.0 ≤ Mw ≤ 7 were used. The individual dispersion curves of the group velocity of Rayleigh waves for each source-station path have been calculated; Then via double-station method we calculated 20 dispersion curves for inter station paths. The group velocities are available in the range of 6-48 sec; in general it is only possible to resolve the parameters of upper mantle and crust. We divided study area to 5 regions, and then we calculated the average dispersion curve in each region. These curves have been used to determine shear wave structure in each region via non-linear Hedgehog inversion method. We need to initial velocity model to start non-linear inversion process, therefore initial model are calculated via linear inversion method.
In additional, the obtained velocity models show that crustal thickness in these 5 regions varies between 40 and 56 km. Also the boundary between Upper and Lower crust changes between 12 and 28 km. The results from the non-linear Hedgehog inversion as applied to derived dispersion curves show a crustal thickness of approximately 40 km in the west part of studied area, 56 km in the middle of studied area and 43 km in the western coast of Caspian Sea.
Based on obtained results the Moho depth varies from 56 km to 40 km when you move from the middle of study area to western coast of the Caspian Sea. We propose that under thrusting of Caspian Sea basement beneath the Talesh Mountains impresses Moho depth in Talesh zone. But no geological observation prove the under thrusting of Caspian Sea basement beneath the Talesh Mountains, thereforewe cannot be certain about this propose. In other hand, Talesh zone is located in passive continental margin of Caspian Sea; these kinds of margins have complicated structure. We can assume that observed results in Talesh zone have been created by passive continental margin of Caspian Sea. Also we observed a low velocity and warm (probably) anomaly in range of depth 12-22 km beneath the Sahand volcano.
We derived attenuation effects of south Caspian basin when periods bigger than 32 seconds of fundamental mode Rayleigh waves propagate across the south Caspian Basin. We used 20 events in along the Apsheron Sill and calculated dispersion curves of these events at our stations. We collected 172 waveforms from used events; we found only 31 fundamental mode waveforms of Rayleigh waves. In other waveforms energy of fundamental mode was diffused and we cannot specify any trend for dispersion. The South Caspian Basin contains one of the thickest sedimentary deposits in the world. In the South Caspian Basin, based on Priestley et.al. (2001), attenuation of surface waves is largely controlled by sediments in the basin. Therefore we guess that our observations about attenuation of the Rayleigh waves are related to sediments in this basin.

کلیدواژه‌ها [English]

  • Crust
  • Rayleigh Waves
  • Non-linear Inversion
  • Dispersion curve
  • Hedgehog
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