محاسبه پارامترهاي چشمه و جنبش نيرومند زمين در زمين‌لرزه 21 آبان 1396 سرپل ذهاب، كرمانشاه، با استفاده از شتاب‌نگاشت‌هاي شبكه شتاب‌نگاري زلزله ايران

Source and Strong Ground Motion Parameters Estimation of the November 12, 2017, Sarpol-e Zahab, Kermanshah Earthquake Using Recorded Accelerograms of Iran Earthquake Strong Motion Network

در تاريخ 21/08/1396 زمين‌لرزه بزرگي با بزرگاي محلي 3/7Mn در نزديكي ازگله - سرپل ذهاب در استان كرمانشاه روي داد. اين زمين‌لرزه را 113 ايستگاه شتاب‌نگاري شبكه شتاب‌نگاري زلزله ايران (ISMN) ثبت كردند. در اين تحقيق با استفاده از داده ­هاي شتاب‌نگاري، پارامترهاي چشمه زمين‌لرزه از جمله گشتاور لرزه‌اي (M0)، بزرگاي گشتاوري زلزله (M) و فركانس گوشه (fc) محاسبه شده است. اين پارامترها با استفاده از روش اندروز (1986) و روش وارون­سازي طيف جابه‌جايي بر اساس مدل چشمه برون در حوزه فركانس تعيين شده‌اند. همچنين ضريب كيفيت مستقل از فركانس (Q)، شعاع چشمه فرضي دايره‌اي (r) و ميزان افت تنش (∆σ) در اين زمين‌لرزه نيز تعيين شده است. براي محاسبه پارامترهاي چشمه از دو پنجره زماني موج S و كل نگاشت استفاده شده است. مقادير به‌دست‌آمده براي بزرگاي گشتاوري 3/7 و 4/7 است كه همخواني بسيار خوبي با مقادير گزارش ­شده از طرف مراكز معتبر بين‌المللي نشان مي­دهد. علاوه ­بر پارامترهاي چشمه، پارامترهاي جنبش نيرومند زمين نيز بررسي و مقادير بيشينه شتاب، سرعت، جابه‌جايي و طيف پاسخ اين زمين‌لرزه محاسبه و ارائه شده است. مقايسه مقادير مشاهده ­شده شتاب و سرعت با روابط تخمين پارامترهاي جنبش شديد زمين بور و همكاران (2014)، حاكي از مناسب بودن اين روابط براي منطقه مورد نظر با درنظرگرفتن بزرگاي اين رويداد است.

On Sunday, November 12, 2017, a strong earthquake with local magnitude of Mn7.3 occurred in the border region between Iran and Iraq in vicinity of the Sarpol-e Zahab town. Unfortunately, this catastrophic event caused many causalities, thousands of injured and vast amounts of damage to the buildings, houses and infrastructures in the epicentral area. The earthquake epicenter was located about 10 km south of Ezgeleh in the Zagros seismotectonic zone. This earthquake was recorded by 113 stations of Iran Strong Motion Network (ISMN) across the country. The nearest station to the epicenter of the earthquake was Sarpol-e Zahab (SPZ) station at a distance of about 39 km. In this research, the source parameters of the earthquake including the seismic moment (M0), the moment magnitude (M) and the corner frequency (fc) were estimated using two methods of Andrews (1986) and inversion of the displacement spectra based on the Brune source model in the frequency domain using ISMN strong motion data. Additionally, the frequency independent quality factor (Q), the source radius (r) (considering a hypothetical circular source), and the stress drop of the earthquake (∆σ) were calculated. The above mentioned values were calculated based on two time windows: the S-wave window and the whole trace window for both methods. The moment magnitude of the event is M7.4 based on the displacement spectra inversion for the whole trace window and M7.3 based on the S-wave window. The estimated value for both windows using the Andrews (1986) methodis M7.3. The estimated M values are in excellent agreement with the reported values. The calculated corner frequencies are from 0.13 to 0.17 Hz and the estimated source radii of the event using both methods are in the 9-11.5 km range. Moreover, the estimated stress drops are in the range of 376 to 572 bar. Ultimately, the strong ground motion parameters were investigated. Using a band-pass filter, all recordings were filtered and their acceleration, velocity and displacement time histories along with their response spectra were extracted, then the peak ground acceleration (PGA), velocity, and displacement of this earthquake were estimated. The PGA recorded at SPZ station on horizontal and vertical components (uncorrected data) are about 684, 554 and 385 cm/s2, respectively. Interestingly, there are two clear packets of energy in the recorded time histories of the Sarpol-e-Zahab, Kerend and some other stations that possibly indicate two sequential failures or two simultaneous seismic events. Considering the magnitude of the event, comparison of the observed values of maximum ground acceleration and velocity with the predicted values by ground motion predication equations presented by Boore et al. (2014), suggests that these equations are appropriate for the region.