اثر تغییرات تنش کولمب ناشی از زمین‌لرزه 31 اوت 1968 دشت بیاض در چکانش رخدادهای آتی

نوع مقاله : Articles

نویسندگان

1 دانشگاه بیرجند

2 پژوهشگاه بین المللی زلزله شناسی و مهندسی زلزله

چکیده

مدل تنش کولمب بر اساس محاسبه آستانه شروع لغزش در سطوح گسلی تحت تنش با فرمول معیار گسیختگی کولمب و  محاسبه تنش حاصل از جنبش بلوک‎ها‎ی دو سوی یک گسیختگی سرچشمه بنا شده است. دستاوردهای این مدل برهم‌کنش زمین‌لرزه‌ها‎ و پراکندگی مکانی پس‌لرزه‌ها‎ را در بر می‌گیرد و با احتمالات زمین‌لرزه‌ای و تغییرات نرخ لرزه‌خیزی همخوانی زیادی دارد. پهنه گسلی دشت بیاض در شمال بلوک لوت، به‌عنوان یک منطقه فعال زمین‎ساختی در شرق ایران شناخته شده است. بررسی تغییرات تنش کولمب نشان می‎دهد که زمین‌لرزه اصلی دشت بیاض در سال 1968 بخش‎ها‎یی از گسل رانده فردوس (زمین‌لرزه 1968 فردوس) و شرق گسل راستالغز چپ‌بر دشت بیاض شرقی(زمین‌لرزه 1979 کولی- بنیاباد) را برانگیخته است اما بر روی گسل آوش (زلزله 1976 وندیک) با افت تنش و باربرداری همراه بوده است. شواهد زمین‎ریخت‎شناسی از جمله جابه‌جایی قنات‌ها، تپه‎ها‎ی باستانی تخریب شده، جابه‌جایی آبراهه‎ها‎ و افراز‎ها‎ی فرسایش‌یافته نشان‌دهنده‌ی تجدید فعالیت این گسل در طی تاریخ می‎باشند. به نظر می‎رسد که علاوه بر امتداد گسل دشت بیاض که طی زمین‌لرزه‎ها‎ی 1968 فردوس و 1979 کولی- بنیاباد فعال شده‌اند، بخش‎ها‎ی جنوبی گسل دشت بیاض به سمت قائنات نیز از مناطق پر خطر در آینده باشد.The Effect of Coulomb Static Stress Changes on Triggering the Future Events after the 31 August 1968 Dasht-e Bayaz Earthquake (Mw = 7.1) Saeed Zarei, Mohamad Mehdi Khatib, Mehdi Zare, Seyed Morteza MoussaviSeveral large earthquakes have influenced the settlements in the Lut block in the past, causing heavy damages in the region. The Dasht-e Bayaz area is located in the north of the active right-lateral faults of the Sistan suture zone, which may accommodate a large proportion of the about 20 mmyr-1 right-lateral shear between Iran and Afghanistan. The numerous Dasht-e Bayaz destructive earthquakes were outstanding clusters of interactive earthquakes that include earthquakes on left-lateral, right-lateral, and reverse faults.New studies have been discussed recently by several authors in which a strong correlation between Coulomb stress changes and the spatial distribution of earthquakes have been observed. The stress change maps would be useful for earthquake hazard to foresee the most likely locations of the upcoming aftershocks. Generally, the changes in Coulomb stress range from 0.1 to 1 bar, which are considered enough to trigger the future earthquakes.  It is perceived that the successive events on the Dasht-e Bayaz areas have characteristic spatial distribution patterns, and the seismicity after the main shocks is consistent with the Coulomb stress change. For all the computations, we used the shear modulus (G) of 3.2 × 105 bars, in a uniform elastic half-space with Poisson’s ratio (ν) of 0.25, Young modulus (E) 8 × 105 bar and an effective coefficient of friction that includes the effect of pore fluid pressure and has been postulated to vary in the range 0.2 – 0.8 with an average value of μ'= 0.4 often used. Coulomb 3.3 software is used to calculate Coulomb failure stress changes. The rupture parameters (strike, dip, rake and depth) and source faults of the earthquakes after the 1968 west Dasht-e Bayaz earthquakes are defined from the published studies. The empirical relationships proposed by Leonard have been used to calculate the width, length and slip of the source faults and assuming homogeneous fault slip. The calculated Coulomb stress change due to the 1968 W; Dasht-e Bayaz event indicates that this main shock can "trigger" the second one or move the Ferdows thrust fault (1968.09.01, Ferdows earthquake) because it was located on the  high stress zone of Coulomb stress changes pattern. Besides, the cumulative effects of Coulomb stress changes due to the 1968  Dasht-e Bayaz and Ferdows events on the Avash fault as a receiver (1976 Vondic event) revealed that the central section of Dasht-e Bayaz and hypocenter of Vondic earthquake is located in the negative lobe of Coulomb stress changes with 0.2 – 0.5 bar.The influence of the earlier earthquakes (1968-1976) on the 1979 Koli-Buniabad earthquakes indicates that the eastern section of the Dasht-e Bayaz segment prompted (1979.11.27, Koli-Buniabad earthquake) into the failure regime by showing a positive increase (0.2 – 0.4 bar) in coulomb stress.To validate the Coulomb stress calculations, we plotted the earthquakes after the main shocks on each stress changes map, and good correlations between the main shock stress changes and the spatial pattern of the seismicity were observed at the terminate part of strike slip faults. Moreover, geomorphic evidences such as, eroded earthquake scarps, displacement of channel, castle trace on ancient hills and displacement of Qanate show several seismic activities of these faults.Keyword: Earthquake Sequence, Triggering Effect, Coulomb Stress, Dasht-e Bayaz Fault, Lut Block. 

کلیدواژه‌ها


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