برآورد ساختارهاي سرعتي رسوبات سطحي در حوزه تهران با استفاده از پراكنش امواج سطحي لاو

Estimation of shallow sediment structure of Tehran Basin by using surface wave Love dispersion curves

در اين تحقيق داده‌هاي ثبت شده در ده ايستگاه شتاب نگاري سازمان پيشگيري و مديريت بحران شهر تهران، كه به مدت 8 ماه به‌طور پيوسته ثبت شده اند، مورد بررسي قرار گرفته و نوفه ثبت شده در اين مدت براي تعيين تابع‌هاي گرين تجربي استفاده شد. از طرفي ديگر داده‌‌هاي ثبت شده از زلزله‌‌هاي محلي رخ داده در اطراف تهران در ايستگاه‌هاي نصب شده شركت پارسيان براي برآورد منحني‌‌هاي پاشش تك‌ايستگاهي استفاده شد. همبستگي متقابل روي مولفه مماسي يك ساعته صورت گرفت و منحني هاي پاشش سرعت گروه مُد اساسي امواج لاو، در بُرد تناوبي 2 تا 5 ثانيه، محاسبه شد. در مرحله بعد، با استفاده از روش وارون سازي مدل دوبُعدي و منحني‌‌هاي پاشش برآورد شده از روش هاي تك‌ايستگاهي و نوفه، تغييرات جانبي سرعت گروه امواج لاو در تناوب‌‌هاي پيش‌گفته محاسبه شد. براي رسيدن به اين هدف، منطقه تهران به 88 ياخته با ابعاد o1/0×o1/0 تقسيم‌بندي و در هر ياخته مقادير سرعت گروه امواج لاو در تناوب‌‌هاي 2 تا 5 ثانيه برآورد شد. با توجه به پوشش پرتوها، كمينه ابعاد ناهمگني تفكيك پذير در اين تحقيق، شش كيلومتر به‌دست آمد. با توجه به نتايج به‌دست آمده، سرعت هاي كم با سنگ هاي كنگلومرا و رسوبات با ضخامت زياد هماهنگي مناسبي داشت و در مناطق گوناگون، جنس رسوبات با سرعت به‌دست آمده همخواني خوبي نشان داد.

The delineation of the elastic, or velocity, structures of the Earth has long been a goal of the world's seismologists. In the first few decades of seismological research, the investigation of velocity structures was restricted to the determination of onedimensional models of the solid Earth and various regions within it. Seismologists are currently working on threedimensional velocity models, and they are trying 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: It includes the accurate location of the earthquakes, it is used in the determination of the composition and origin of the outer layers of the Earth, it improves our ability to discriminate nuclear explosions from earthquakes, it helps to interpret the largescale tectonics as well as a reliable assessment of earthquake hazards. In this study, we used highfrequency dispersion curves to estimate the elastic properties of Tehran and the suburbs. Tehran city is located in the Alborz major seismic tectonic zone. The Alborz is an arcuate chain of mountains in the Northern Iran that wraps around the Southern side of the South Caspian basin; the boundary is roughly the present shoreline of the Caspian Sea. The range is actively deforming on rangeparallel thrusts and left lateral strikeslip faults. The thrusts dip inward toward the interior of the range from both its northern and southern sides, and the GPSderived shortening across the range is 5 ± 2 mm/yr at the longitude of Tehran (Vernant et al., 2004b). Most are parallel to the range and accommodate the presentday oblique convergence across the mountain belt. Recent large earthquakes occurring in this region suggest that the seismicity is connected with major faults of the recent age that cut across the regional Quaternary Lineaments. In this study, in order to estimate the upper crust elastic structure, we conducted a tomographic inversion of the Love wave dispersion to obtain twodimensional Love wave group velocity tomographic images in a period range from 2 s to 5 s for the city of Tehran and the suburbs. We used two databases to derive dispersion curves for different paths. In the first dataset, continuous ambient noise in ten stations located in and around the city of Tehran and installed by the Tehran Disaster Mitigation and Management Organization network was used to explore the interstation Green’s Functions. In the second database, forty seven earthquakes recoded by the Parsian stations were prepressed and rotated to be used as single station records to estimate the Love wave group velocity. In the next step, the interstation Green functions and singlestation records were used to estimate the Love wave dispersion curves by applying a multiple filtering technique. All dispersion curves were used to estimate the twodimensional Love wave group velocity models. For this purpose, Tehran region was divided into 0.1° × 0.1° cells. According to the ray coverage, the minimum dimension of distinct heterogeneity was 6 km. In our study, topographical features and nearsurface known geological structures were two main criteria to assess the credibility of the estimated Love wave group velocity variations. There was a strong correlation between the estimated group velocities and topographical features. The prominent surface geology units at the mountain range consisted of varying structures including sandstone, siltstone, claystone, and massive limestone.