برگردان سرعت گروه امواج ريلي به ساختار سرعت موج برشي براي منطقه شمال غرب ايران

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

در اين مطالعه تغييرات سرعت موج برشي با استفاده از داده هاي ثبت شده در 23 ايستگاه باند پهن شبكه موقت دانشگاه تحصيلات تكميلي علوم پايه زنجان، در منطقه شمال غرب ايران مورد بررسي قرار مي گيرد. به همين منظور با استفاده از 230 رخداد منطقه اي و دورلرز منحني هاي پاشش سرعت گروه امواج ريلي در مد پايه براي 20 مسيربين ايستگاهي محاسبه مي شود. سپس جهت محاسبه ساختار سرعت موج برشي در هر مسير، برگردان خطي و غيرخطي منحني هاي پاشش به ساختار سرعت موج برشي انجام مي شود. منحني هاي پاشش بين ايستگاهي به روش دو ايستگاهي و ساختارهاي سرعت نهايي به روش جستجوي محاسباتي- تصادفي "هجهاگ" محاسبه مي شوند. با توجه به بازه دوره تناوبي در منحني هاي پاشش محاسبه شده در اين مطالعه (بين 5 تا 48 ثانيه) تنها پارامترهاي سرعتي در پوسته و گوشته بالايي قابل محاسبه است. ساختارهاي سرعت موج برشي نشان دهنده يك ساختار ناهمگن با ضخامت متغير پوسته در امتدادپروفيل لرزه نگاري است. عمق مرز موهو بين 40 تا 56 كيلومتر و عمق مرز ميان پوسته بالايي و پاييني نيز بين 12 تا 28 كيلومتر به دست آمده است. در بازه عمقي 12 تا 22كيلومتر در شمال آتشفشان سهند يك توده كم سرعت و احتمالا گرم مشاهده مي شود.همچنين اثر عبور مسير 20 رخداد منطقه اي بر ميرايي امواج ريلي با دوره هاي زماني بيش از 32 ثانيه نيز نشان داده مي شود و منشاء اين ميرايي غير عادي به اثر رسوبات ضخيم حوضه خزر جنوبي نسبت داده مي شود.

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.