شناسايي گسل مسبب زمينلرزه 21 ارديبهشت 1392 گوهران با استفاده از مكان‌يابي مجدد پس لرزه ها و سازوكار كانوني آن

Identifying causative fault of 11th May 2013 Goharan Earthquake using relocation of aftershocks and focal mechanisms

در ساعت 6:38 روز 21 ارديبشهت 1392 زمين لرزه اي به بزرگي 6/1 و به گزارش برخي مراكز ديگر 6/2در مقياس گشتاوري (Mw)، در نزديكي روستايي به نام گوهران روي داد.روستاي گوهران از توابع شهرستان بشاگرد واقع در استان هرمزگان مي‌باشد. اين زمين‌لرزه، پس‌لرزه‌هاي بزرگي را به دنبال داشت كه بزرگ ترين آنها با بزرگي 5/8 ثبت شده است. در فاصله سه ماه بعد از رويداد اصلي توسط ايستگاه‌هاي لرزه‌نگاري كشوري، 284 پس‌لرزه ثبت و مكان‌يابي شده اند. در اين پژوهش با استفاده از روش اختلاف زماني دوگانه، مجدداً زمين‌لرزه‌ اصلي و پس‌لرزه‌هاي آن مكان‌يابي شدند كه در طي اين فرايند خطا نسبت به قبل بهينه شده و با استفاده از تحليل مكاني و زماني پس لرزه‌ها، راستاي گسل مسبب اصلي اين زمين‌لرزه با امتداد تقريبي جنوب ‌غربشمال ‌شرق شناسايي شد. همچنين با استفاده از روش وارون سازي تانسور گشتاور، سازوكارهاي كانوني مربوط به اين زمين‌لرزه و پس‌لرزه هاي بزرگ آن به دست آمد كه با توجه به امتداد به دست آمده از توزيع پس‌لرزه‌ها، صفحه اصلي از صفحه كمكي تشخيص داده شد. در نهايت با تلفيق اين دو نتيجه گسل مسبب اصلي با مولفه امتدادلغز چپ‌گرد با شيب تقريبا قائم شناسايي شد. تطبيق نتايج زمين‌شناختي با نتايج اين پژوهش نشان مي‌دهد كه اين زمين‌لرزه تحت تأثير عملكرد گسل ناشناخته است كه تاكنون در نقشه‌ها مشاهده نشده و بعد از وقوع اين زمين‌لرزه آشكار شده است.

On the 11thMay 2013 at 2:08:08 GMT, a moderate earthquake (Mw=6.2) occurred in Hormozgan province, near the Goharan village (southeast Iran). The largest aftershock of magnitude 5.8 occurred on 12th May 2013 at 12:07:02 GMT near the mainshock, which produced more destruction. According to the reports, one person died and 17 people injured. This earthquake is the first noticeable recorded event in this area. By this time,the most important seismic activity had occurred in 1983 with magnitude Ms=5.7 (ISC). Aftershock sequences provide valuable information about the Earth’s crust and source properties of large earthquakes. This is because a large number of events occur during a short time in a small area. This earthquake had a lot of aftershocks, after 3 months Iranian seismological center (IRSC) recorded 284 events.     Generally, initial location of earthquakes produce inevitable error because of algorithms we use to locate earthquakes. But this error will be decreased by relocating. One of the most common methods used is doubledifference algorithm developed by Felix Waldhauser (2000). The doubledifference is one of the relative earthquake location methods in these methods, effects of errors produced by deviation from real structure velocity model can also be minimized. According to the theory of doubledifference, if the hypocentral separation between two earthquakes is small (compared to the event station distance and the scale length of the velocity heterogeneity), then the ray paths between the two sources and a common station are similar along the entire ray path. In this case, the difference in travel times for two events observed at one station can be related to the spatial offset between the events with high accuracy.     Previous studies have shown that the direction of causative fault (related to mainshock) can be obtained by the distribution of aftershocks, but it does not exactly assess motion on the fault occurred during the earthquake. So obtaining focal mechanism of the mainshock and largest aftershocks could help us to solve this important problaluablem. There are some methods to obtain focal mechanism and one of the most powerful methods is moment tensor inversion. This method has developed for a local earthquake by Zahradnik et al (2005).  This algorithm has been written in Fortran language programing named ISOLA. The ISOLA GUI has been created using the Matlab GUIDE tool.ISOLA is based on multiple pointsource representation and iterative deconvolution method, similar to what Kikuchi and Kanamori (1991) have done for teleseismic records, but here the full wavefield is considered, and Green's functions are calculated by the discrete wavenumber method of Bouchon (1981). Thus, the method is applicable to regional and local events.    In this study, we use doubledifference algorithm to relocate the mainshock and aftershocks of this earthquake. During this process, the error of location improved (compare to initial location). By using spatialtemporal analyse of aftershock sequence we derived that the causative fault of this earthquake is in an eastwest direction. All the aftershocks are distributed in the area with length 40 km and width 25 km, but the most of them are being in a cluster with length 15 km and width 10 km. We determined the focal mechanisms of this earthquake, and 15 other large aftershocks by moment tensor inversion method (Isola software) and the results show that all of the events have a predominatinely strikeslip mechanism.  According to the distribution of the aftershocks we determined, fault plane is distinguished from the nodal plane. The fault plane is a leftlateral strikeslip in the SWNE direction. According to the Geological evidence, this earthquake  is related to the unknown fault in this region, but topographic map shows that probably this earthquake is related to linear structure parallel to aftershocks distribution and the fault plane that obtained in this study using CMT(Centroid Moment Tensor) method.