تعیین مدل سرعتی برای ناحیۀ بوشهر و تحلیل زمین‌لرزۀ Mw 6.3، 2013 کاکی بوشهر

بررسی ساختار سرعتی پوسته بر اساس انتشار امواج لرزه‌ای در سال‌های اخیر در مناطق مختلف ایران انجام‌گرفته است که استفاده از نتایج، در تحلیل زون‌های لرزه‌زا و تعیین موقعیت زمین‌لرزه‌ها با دقت مناسب موثر بوده است. مدل ساختار سرعتی پوسته هم در مکان‌یابی زمین‌لرزه‌ها و هـم در شـناخت سـاختمان پوسته از اهمیت بسیاری برخوردار است. در ایـن تحقیق جهت کاهش ماکزیمم گپ آزیموتی و تعیین مکان زمین‌لرزه‌ها با دقت بیشتر، از داده‌های ثبت‌شده در ایستگاه‌های شبکۀ لرزه‌نگاری کشور عربستان استفاده شد. 137 پس‌لرزه که مکان‌یابی آن‌ها قابل‌اعتماد بود، برای محاسبۀ مدل ساختار سرعتی پوسته به روش برگردان یک‌بعدی به کار گرفته شد و ضخامت و سرعت موج p در لایه‌ها از سطح زمین تا ژرفا برای پوستۀ منطقه تعیین گردید. این تحقیق نشان می‌دهد که پوستۀ منطقه متشکل از 5 لایه است. یک ‌لایۀ 4 کیلومتری با سرعت vp=5.75 km/s که روی لایه‌ای به ضخامت 11 کیلومتر و vp=5.95 km/s قرار دارد. لایۀ سوم با ضخامت 7 کیلومتر و vp=6.30 km/s روی لایۀ چهارم با ضخامت 12 کیلومتر و vp=6.60 km/s قرار دارد و در نهایت لایۀ پنجم با سرعت vp=7.25 km/s به ضخامت 9 کیلومتر روی نیم فضا با سرعت vp=8.00 km/s قرار دارد. همچنین عمق موهو در منطقه 43 کیلومتر تعیین گردید. مکان‌یابی مجدد پس‌لرزه‌ها و مطالعۀ توزیع مکانی رومرکز پس‌لرزه‌های این زمین‌لرزه با مدل سرعتی جدید موید این است که گسل پیشانی کوهستان (mff) عامل این زمین‌لرزه است.

Crustal velocity in the Busher region and analyses of 2013 Mw 6.3 Kaki-Busher Earthquake

Investigation of velocity structure based on seismic wave propagation have been performed in recent years in different parts of Iran that results obtained is effective in the analysis of seismic zones and locate earthquakes with reasonable accuracy. On a broad scale, the seismotectonics of southern Iran are controlled by active convergence between the Arabian and Eurasian tectonic plates that at the latitude of the event, convergence rates between Arabia and Eurasia are approximately 30 mm/yr. The April 9, 2013 Mw 6.3 earthquake in southern Iran occurred as result of northeastsouthwest oriented thrusttype motion in the shallow crust of the Arabian plate. The depth and style of faulting in this event are consistent with shortening of the shallow Arabian crust within the Zagros Mountains in response to active convergence between the Arabian and Eurasian plates. To determine crustal velocity for Busher region we used the aftershock sequence of the 2013, Mw 6.3 KakiBusher earthquake that widely felt in Bahrain, Iran, Kuwait, Qatar, Saudi Arabia and United Arab Emirates. Velocity structure is significant in locating and also understanding of the structure of crust. In this study, to reduce the maximum gap azimuth and locate earthquakes with more accurately were used recorded data in the Saudi Seismic Network stations. 137 aftershocks that were a reliable locating were employed to calculating the onedimensional velocity structure with inversion method and thickness and P wave velocity in the layers was measured from the ground to the depth for crust region. This study shows that the crust is consists of five layers, a layer with a thickness of 4 km with Vp = 5.75 km/s over the layer with thickness of 11 km and Vp = 5.95 km/s and the third layer with a thickness of 7 km and Vp = 6.30 km/s is over the fourth layer with a thickness of 12 km and Vp = 6.60 km/s. Finally the fifth layer with 9 km thickness and Vp=7.25 km/s located on the halfspace with velocity of Vp=8.00 km/s. In this study Moho depth of area was determined 43 km, The ability of the method to detect the velocity anomaly, is visible in well resolution the final model and the acceptable results and To comparing these models (the model used in this study and of the Institute of Geophysics), we relocated aftershocks with new model that the locating errors overall is reduced in comparison with Geophysical Institute model but under same conditions. Look at the previous seismic activity in the region indicates that this earthquake occurred in a region of Iran which not has been occurred any earthquake greater than 5 from radius of 25 km and earthquakes above 4 in area, indicating the active faults in the region. Study of spatial distribution of epicenters of aftershocks of this earthquake that relocate with new velocity model is indicated that the causative fault for 2013, Mw 6.3 KakiBusher earthquake is mountain front fault (MFF). The normal profiles to aftershocks trend shows that the causative faults dip is towards northeast. It was observed that the depth of aftershocks is between 15 and 20 km in sections that can be indicate the seismogenic zone depth in area.