بررسي اثر پالس جهت‌پذيري در ركوردهاي حوزه‌ي نزديك ايران با استفاده از روش كاتوره‌‌‌يي گسل محدود

Investigation into the Pulse of Directivity in Near field Records of Iran using Stochastic Finite Fault Simulation

در فواصل نزديك گسل با توجه به اينكه تعداد ركوردهاي واقعي اندك است و زلزله‌هاي حوزه‌ي نزديك، ويژگي‌هاي متفاوتي نسبت به زلزله‌هاي حوزه‌ي دور دارند، استفاده از روش‌هاي شبيه‌سازي ركورد مي‌تواند مفيد باشد. يكي از روش‌هاي شبيه‌سازي لرزش‌هاي بسامد بالا، روش شبيه‌سازي احتمالي است. اين روش با فرض چشمه‌ي نقطه‌يي و براساس گسل محدود ارايه شده و از آنجا كه روش گسل محدود، هندسه‌ي شكست و اثر جهت‌پذيري را در نظر مي‌گيرد؛ در اين پژوهش براي چند زلزله‌ي ايران كه ركورد حوزه‌ي نزديك داشته‌اند، مورد استفاده قرار گرفته است. پس از شبيه‌سازي زلزله‌ها و محاسبه‌ي پارامتر افت تنش ،كه عدم قطعيت زيادي دارد، به‌منظور بررسي اثر پالس جهت‌پذيري به بررسي پارامترهاي تاثيرگذار در ركوردهاي حوزه‌ي نزديك پرداخته شده است.

There are few actual records of near fault ruptures with features different from those of far fields. These features are mostly affected by forward directivity in near field earthquakes. So, investigation into this field of study by using methods such to simulate records would be essential Until now, different methods have been used by researchers to simulate strong ground motion. Stochastic simulation is a method widely used for simulating high frequency ground motion in recent years. This method, which considers a point source, was presented by Boore (1983). The seismic source is considered to be a rectangular fault plane divided by some sub-faults in its longitudinal and traversal directions. Bersneve and Atkinson (1998) have introduced earthquake stochastic simulation based on finite fault modeling. In such simulations, each subfault is considered a point source, using the source model presented by Brune, with a corner frequency and a constant stress drop. The target accelerogram is obtained by summation of accelerograms generated by each subfault and by considering their corresponding delay times. This new modeling considers rupture geometry and the directivity effect; therefore, its results will be more appropriate. The stress drop parameter is one of the most important parameters in stochastic simulation that has a high uncertainty. This parameter is studied here, based on both stochastic point source and finite fault modeling. For this purpose, the stress drop is calculated for 7 Iranian earthquakes with at least one near field record. Then, these earthquakes have been simulated by using the results of the stress drops. Finally, several parameters, such as nu, t0, gamma, and the impulse peak, which affect near field records and directivity pulses, have been investigated.