مطالعه دگرشكلي پسالرزه‌اي زمين‌لرزه 2010 ريگان 6.5 Mw ، جنوب‌شرق ايران با تداخل‌سنجي راداري

Studying post-seismic deformation 2010 Mw 6.5 Rigan earthquake in SW Iran using InSAR

در 20 دسامبر 2010، زمين‌ لرزه‌اي با بزرگاي 6/5 Mw در منطقه ريگان واقع در جنوب‌ شهر بم رخ­داد. 37 روز بعد، در 27 ژانويه 2011 پس لرزه‌ بزرگي (با بزرگاي 2/6Mw) در 20 كيلومتري زمين‌ لرزه اول اتفاق افتاد. براي مطالعه دگرشكلي پسا لرزه‌اي پس از وقوع زمين‌ لرزه‌هاي فوق، تصاوير ماهواره COSMO-SkyMed (از آژانس فضايي ايتاليا) در بازه زماني 27 ژانويه 2011 و 15 جولاي 2011 تهيه و تحليل سري زماني روي تصاوير با استفاده از روش كوتاه ­ترين خط‌ مبنا (SBAS) انجام شد. نقشه ميانگين سرعت به‌ دست‌ آمده از تحليل سري زماني كوتاه‌ترين خط‌ مبنا، بيانگر وجود فاز پسالرزه‌اي با سا زو كار راست‌بر در منطقه ريگان است. سري زماني جا‌به‌جا‌يي دو طرف گسل مسبب زلزله اصلي، نشان­ دهنده 8 ميلي­متر جا‌به‌جا‌يي در مدت 5 ماه پس از زلزله است. پس از اين مدت، فاز پسالرزه‌اي به اتمام مي ­رسد، گسل قفل مي­ شود و فاز ميان‌ لرزه‌اي با انباشت آرام تنش روي صفحه گسلي شروع مي‌شود. تعيين پارامترهاي چشمه فاز پسالرزه‌اي با استفاده از مدل اكادا (1985)، نشان­مي­دهد كه يك توزيع لغزش با بيشينه جابه‌جايي 1 متر، در زير گسيختگي زلزله اصلي (در عمق 10 كيلومتري) و همچنين در لبه­ هاي گسل متمركز شده است. نتيجه مدل‌سازي نشان ­مي­ دهدكه فاز پسالرزه‌اي مشاهده شده صرفا به ­دليل وقوع پس لرزه‌ها در بخش‌هاي مجاور گسيختگي اصلي، روي صفحه مسبب زلزله اول است. سازوكار اين دگرشكلي، پس‌ لغزش است.

On 20 December 2010, an earthquake with Mw 6.5 occurred in Rigan, a small town in the desert south of Bam city. The earthquake epicenter was in a low population area so, luckily, it caused only few casualties. Five days later 76 aftershocks reported by Iranian Seismological Center (ISC). On 27 January 2011, another earthquake (Mw 6.2) stroke an area at ~ 20 km southwest of the first earthquake. Bam earthquake Mw 6.6 occurred in 2003 with 40,000 victims is one of the deadliest earthquakes in Iran which is located in shear zones at southeast Iran. Considering the active faults distribution of the region and aftershocks of the 2010 Rigan earthquake encouraged us to better investigate and model the post-seismic deformation related to the 2010 earthquake. Post-seismic syudying provides information about rheology of the surrounding region and improves our knowledge about the strain release after the earthquake. In this study, COSMO-SkyMed (from Italian Space Agency, ASI) images spanning the temporal interval between 27 January 2011 and 15 July 2011 are used to investigate the post-seismic deformation following both earthquakes. We applied the Small Baseline Subset (SBAS) algorithm for images to obtain the post-seismic mean velocity map and the relative deformation time series. 109 interferograms, post-seismic mean velocity map and the relative deformation time series obtained Mean velocity map shows that displacements of post-seismic phase are right lateral strike slip same as co-seismic mechanism. Time series analysis reveals a clear post-seismic signal exponentially increasing with time until reaching the rate of more than 8 mm/year which indicates the end of post-seismic phase and following inter-seismic phase, starts with steady stress accumulation. Later, we modeled the post seismic signal considering a dislocation on a finite fault in an elastic and homogeneous half-space that are the assumptions for the Okada (1985) model. Post-seismic results modeled by adopting a two-step approach: (1) a non-linear inversion performed to constrain the fault geometry parameters and considering a uniform slip, then (2) a linear inversion performed to retrieve the slip distribution on the fault plane previously obtained. The fault plane is split into 1×1 km patches along strike and down-dip. Determining fault parameters and slip distribution by Okada model, indicates that the slip is concentrated in downdip of the coseismic depth with 1.2 m slip and also at the edges of the coseismic asperity. This slip distribution indicates that “afterslip” is the mechanism for post-seismic deformation of the Rigan earthquake.