مطالعه تغييرات زماني الگوي لرزه خيزي كمربند چين خورده -رانده زاگرس با استفاده از روش شرايدر

Study of the Temporal Variations of Seismicity Pattern in the Zagros Fold and Thrust Belt Using Schreider Algorithm

در اين پژوهش براي بررسي تغييرات زماني الگوي لرزه خيـزي در كمربند چين خورده- رانده زاگرس از الگوريتم شرايدر استفاده شد. اين الگوريتم براي بررسي آرامش لرزه اي در مناطق مختلف به كار مي رود. به همين منظور چهار زمين لرزه با بزرگاي 6Mw≥ كه اخيراً در زاگرس به وقوع پيوسته اند مورد مطالعه قرار گرفت. به منظور بررسي تغييرات زماني لرزه خيزي علاوه بر نمودار پيچش زماني T(k)))، نمودارهاي بزرگي - زمان، فراواني زمين لرزه ها - زمان و مكان - زمان نيز ترسيم گرديد. نتايج نشان مي دهد كه پيش از وقوع زمين لرزه هاي 2006 سيلاخور و 2014 مورموري كه در بخش شمال غربي زاگرس رخ داده اند، الگوي پيش نشانگر دونات ديده مي شود. قبل از زمين لرزه هاي 2013 دشتي و 2005 قشم كه هر دو در بخش جنوبي زاگرس به وقوع پيوسته اند، تا چندين سال آرامش لرزه اي مشاهده مي شود كه با وقوع ناگهاني اين زمين لرزه ها خاتمه مي يابد. نتايج تجزيه وتحليل حساسيت نشان داد كه پارامترهاي هموارسازي الگوريتم شرايدر بر روي خروجي الگوريتم تاثير بسزايي داشته و بايد با دقت انتخاب شوند. نتايج اين تحقيق نشان داد كه الگوريتم شرايدر به دليل استفاده هوشمندانه از پارامتر زمان (t) توانايي نمايش پيش نشانگر آرامش-لرزه اي پيش از وقوع زمين لرزه هاي بزرگ را دارد.

Seismicity pattern studies are one of the effective tools in the interpretation of variation in seismic sequence. The study of variations of seismicity parameters as a function of time indicates that the temporal distribution of events is not uniform, and these parameters can give quantitative information about the seismic patterns of different regions. In this research, to investigate temporal variations of seismicity pattern in the Zagros fold and thrust belt, the Schreider algorithm is applied. This algorithm that introduced by Schreider (1990) to detect seismic quiescence has been used in different parts of the world. For this purpose, four earthquakes with Mw≥6 that have recently been occurred in Zagros have been studied. At first, a complete catalogue from the period of 2000 to 2017 within a circular area has been selected. Then, the catalogues are homogenized to ML and the Minimum magnitude of completeness are computed (Mc=3.4). To perform Schreider algorithm, the time between consecutive earthquakes ( ) should be calculated. The smoothness procedure is used to evaluate a convolution function of . The smoothness of this function is done by Gaussian function. In the R radius, smoothness parameter (s) has controlled the extent of surrounding earthquakes to detect smooth values. The kth seismic event is related with the temporal convolution T(k) that decrease and increase indicate seismic activity or low seismic activity, respectively. The numbers of earthquakes that are located in the nearest distance to main shock determine the l parameter. The value of l is determined when the function f(n,s) is approximately zero. Therefore, the function T(k) depends on the s and l parameters. To investigate temporal variations of seismicity, in addition to the temporal convolution (T(k)) plot the magnitude-time, number-time and space-time plots have drawn. The results show that before the 2006 Silakhor and 2014 Mormori earthquakes, both of which occurred in the north part of Zagros, the precursory doughnut pattern is seen. Several years before the 2013 Dashti and 2005 Qeshm earthquakes, both of which occurred in the south part of Zagros, the seismic quiescence is seen for which ended with the sudden occurrence of these earthquakes. The result of sensitivity analysis showed that smoothing parameters of Schreider algorithm have a significant influence on the algorithm outcomes, and these parameters should be selected with more accuracy. The results of this paper show that Schreider algorithm can demonstrate precursory seismic quiescence before the occurrence of large earthquakes due to the intelligent usage of t parameter.