توموگرافي سه‌بُعدي منطقه‌‌هاي اصفهان و شهركرد با استفاده از زمين‌لرزه‌‌هاي محلي

3D Tomography of Isfahan and Sharekord regions using local earthquakes

يكي از هدف‌هاي مهم زلزله‌شناسي به‌دست آوردن مدل سرعتي مناسب براي هر منطقه است. هر چه اين فرايند دقيق‌تر صورت گيرد مي‌توان در پژوهش‌هاي ديگر در منطقه به نتايج بهتري دست يافت. براي اين منظور از روش‌هاي توموگرافي لرزه‌‌اي استفاده مي‌شود. استفاده از زلزله‌‌ها به‌منزله چشمه‌‌هاي طبيعي در توموگرافي بسيار مفيد است. در اين پژوهش يك مدل سرعت سه‌بُعدي براي دو منطقه اصفهان و شهركرد واقع در محدوده ?31 تا ?34 شمالي و ?50 تا ?53 شرقي ارايه مي‌شود. براي اين منظور از زمين‌لرزه‌‌هاي رخ‌داده در اين ناحيه بين سال‌هاي 2000 تا 2012 كه در 8 ايستگاه لرزه‌‌نگاري موسسه ژيو‌فيزيك دانشگاه تهران ثبت شده‌‌اند استفاده شده است. ابتدا لازم است يك مدل يك‌بُعدي متوسط بهينه براي منطقه به‌دست آورده شود. نتايج حاصل از وارون‌سازي هم‌زمان داده‌‌ها يك مدل 4 لايه‌‌اي پيشنهاد مي‌دهد كه در آن سرعت امواج P تا عمق 5 كيلومتري برابر با 4/5 كيلومتربرثانيه، 5 تا 20 كيلومتر، 0/6 كيلومتربرثانيه ، 20 تا32 كيلومتر، 2/6 كيلومتربرثانيه، 32 تا 47 كيلومتر، 9/6 كيلومتربرثانيه است. عمق موهو 47 كيلومتر و سرعت Pn، 9/7 كيلومتر بر ثانيه برآورد شده است. زمين‌لرزه‌‌هاي منطقه با استفاده از مدل بدست آمده تعيين‌محل مجدد شدند و نتايج حاصل در وارون‌سازي سه‌بُعدي مورد استفاده قرار گرفت. بعد از تعيين مدل سه‌بُعدي اوليه، زمان سير اولين فازهاي رسيده وارون‌سازي شدند كه براساس نتايج آن، مقاطع افقي از ساختار زيرين ناحيه موردنظر رسم شد. بي‌هنجاري‌‌هاي سرعتي مدل نهايي به‌دست آمده درعمق‌هاي گوناگون مطابقت خوبي با روندگسل‌هاي منطقه دارد. نتايج آزمايش مدل نهايي با بوردهاي شطرنجي، بيانگر وضوح خوبي براي عمق‌هاي گوناگون است.

One of the most important purposes in seismology is determination of crustal velocity using earthquakes data that have been recorded by regional and local seismic stations. The more precise the process is carried out, the better shall be the results reached in various studies in the area including earthquake locating, seismicity of area, seismic zone mapping or the determination of the plane of faults causing earthquakes Imaging velocity structure of Earthʹs interior using travel times inversion commonly called seismic tomography which is usually done two or three-dimensionally. . This method has extensively been used in recent decades by researchers. Since the seismic waves are associated with valuable information about direction of propagation and environmental properties, using earthquakes as natural seismic sources are very useful in seismic tomography as well as the artificial sources such as limited and controlled explosions, air guns and bore-hole sources.. The seismic tomography characterizes the size, geometry and extent of velocity anomalies. In this method subsurface structure is modeled initially by several parameters and improved by the inversion of seismic travel times data. In this study the crustal velocity structure was determined using three-dimensional inversion of local earthquakes travel times recorded by seismic networks of Institute of Geophysics University of Tehran (IGUT) occurred within the period from 2000 and 2012 in the study area. The study area is bounded within 31oE to 34oE and 50oN to 53oN. We used the VELEST software in one-dimensional modeling section. The procedure of study is minimizing the differences between observed and calculated travel time by applying the initial obtained model. This software simultaneously optimizes the earthquake locations, crustal velocity model and station corrections using the Joint-Hypocenter-Determination (JHD) method. The initial estimates for P waves velocity and crust thickness of the region are achieved using the travel-time curve of primary phases of all earthquakes occurred in the area. Then the larger relative earthquakes are selected and the best crustal one-dimensional model was derived by simultaneously inverse modeling method using this data set and VELEST algorithm. This method can be considered as one of the useful methods in study of 1D crust structure. The results proposed a 4 layers model of crust in which the P wave velocity is equal to 5.4 km/s for depths less than 5 km, 6.0 km/s for depths from 5 km to 20 km, 6.2 km/s for depths of 20 km to 32 km and finally 6.9 km/s for depths of 32 km to 47 km. The thickness of crust and Pn velocity are respectively obtained 47 km and 7.9 km/s. The aim of this work is obtaining an optimal crust model that can aid to improve seismic data and can be used to determine the next earthquake locating. Then the obtained crustal model is used as an initial model to study of three-dimensional inverse modeling of crust in the region by using FAST algorithm. All the earthquakes relocated using new obtained model. In this study a data set recorded by the 8 seismic stations of Isfahan and Shahrekord networks were used. The resolution of final solution of 3D model was investigated using synthetic dataset (checkerboard model) that shows fair resolution for various depths. The lateral variations of the main resolved structures in the model obtained are highly correlated with the faulting systems in the region.