اثر تغييرات تنش كولمب ناشي از زمين لرزه 31 اوت 1968 دشت بياض در چكانش رخدادهاي آتي

The Effect of Coulomb Static Stress Changes on Triggering the Future Events after the 31 August 1968 Dasht-e Bayaz Earthquake (Mw = 7.1)

مدل تنش كولمب بر اساس محاسبه آستانه شروع لغزش در سطوح گسلي تحت تنش با فرمول معيار گسيختگي كولمب و محاسبه تنش حاصل از جنبش بلوك ها ي دو سوي يك گسيختگي سرچشمه بنا شده است. دستاوردهاي اين مدل برهم كنش زمين لرزه ها و پراكندگي مكاني پس لرزه ها را در بر مي گيرد و با احتمالات زمين لرزه اي و تغييرات نرخ لرزه خيزي همخواني زيادي دارد. پهنه گسلي دشت بياض در شمال بلوك لوت، به عنوان يك منطقه فعال زمين ساختي در شرق ايران شناخته شده است. بررسي تغييرات تنش كولمب نشان مي دهد كه زمين لرزه اصلي دشت بياض در سال 1968 بخش ها يي از گسل رانده فردوس (زمين لرزه 1968 فردوس) و شرق گسل راستالغز چپ بر دشت بياض شرقي (زمين لرزه 1979 كولي - بنياباد) را برانگيخته است اما بر روي گسل آوش (زلزله 1976 ونديك) با افت تنش و باربرداري همراه بوده است. شواهد زمين ريخت شناسي از جمله جابه جايي قنات ها، تپه ها ي باستاني تخريب شده، جابه جايي آبراهه ها و افراز ها ي فرسايش يافته نشان دهنده ي تجديد فعاليت اين گسل در طي تاريخ مي باشند. به نظر مي رسد كه علاوه بر امتداد گسل دشت بياض كه طي زمين لرزه ها ي 1968 فردوس و 1979 كولي - بنياباد فعال شده اند، بخش هاي جنوبي گسل دشت بياض به سمت قائنات نيز از مناطق پر خطر در آينده باشد.

Several 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.