بررسی پیچیدگی توزیع زمانی لرزه‌خیزی در گستره زاگرس با استفاده از آنالیز فرکتالی

در این مطالعه سعی شده است تا میزان پیچیدگی توزیع زمانی لرزه‌خیزی در پهنه زمین‌ساختی زاگرس مورد بررسی و ارزیابی قرار گیرد. برای این منظور از راه‌کار تجزیه و تحلیل فرکتالی مبتنی بر تبدیل موجک استفاده شده است. در این خصوص سری‌های زمانی لرزه‌خیزی برای تمامی زیرپهنه‌های گستره زاگرس تهیه شده و سپس مورد تجزیه و تحلیل فرکتالی قرار گرفته‌اند. نتایج به‌دست آمده نشان‌دهنده ماهیت چندفرکتالی و مستقل از مقیاس توزیع زمانی لرزه‌خیزی در تمامی قسمت‌های پهنه مورد مطالعه است. با این وجود ویژگی‌های فرکتالی زیرپهنه‌های مختلف یکسان نبوده و هر یک از آنها دارای سرشتی متفاوت هستند. در نواحی ایذه و زاگرس مرتفع که دارای میزان تمرکز تنش به نسبت بالایی هستند، توزیع زمانی لرزه‌خیزی دارای ماهیتی پادهمبسته و تا حدی ساده‌ است. این در حالی است که در زیرپهنه‌های لرستان، فروبار دزفول و کمان فارس، توزیع مذکور به نسبت پیچیده‌‌ بوده و ماهیتی نسبتاً همبسته و گاهاً تصادفی دارد. همچنین یافته‌های این تحقیق نشان می‌دهد که در گستره زاگرس لرزه‌خیزی مستقل نسبت به لرزه‌خیزی وابسته دارای توزیع زمانی ناهمگن‌تری است. هر چند که این ناهمگنی در بخش‌های مختلف پهنه مذکور یکسان نبوده و متغییر است.

Complexity investigation of seismicity temporal distribution in the Zagros region by using fractal analysis

In this research it is tried to examine the fractal complexity of seismicity temporal dispersion in the Zagros Mountain range. The Wavelet Transform Modoulos Maxima (WTMM) as an innovative strang attractor formalism has been utilized for the multifractal investigation. Earthquakes that occurred from December 2003 to May 2016 have been collected from the master catalog of the International Institute of Earthquake Engineering and Seismology (IIEES). As all events in the master list are reported based on the local magnitude (ML), the achieved catalog is already homogeneous. ML is saturated for the earthquakes with magnitude greater than 5.5, so they are converted to the moment magnitude (MW) using some empirical relations. For a reliable and comprehensive seismicity examination, the Gutenberg–Richter analysis is performed over the cumulative distribution of events, and the minimum magnitude of completeness (MC) has been obtained. For MC calculation, the maximum curvature method is used and an overall Mc=3.1 is computed for the attained earthquake catalog. To complete the catalog, all events with MW<MC have been removed from the earthquakes list. As the occurrence time is the most reliable seismicity parameter, the timeseries are prepared as interevent times between the consecutive earthquakes for the different subzones of the Zagros region. The WTMM technique has been applied to each of the timeseries and their fractal characteristics are gaind from the attributes of the related scaling and singularity spectrums. The obtained results revealed that the seismicity is scale invariant; however, its multifractal nature is not constant. There are some differences among the fractal aspects of seismicity temporal changes in the different portions of the belt. Chronological distribution of earthquakes in the simplyfolded belt and Dezful embayment are remarkably more complex than the other portions of the Zagros Mountain range. Dezful embayment as an indenter plays an important role on deformation style in the Zagros Mountain. It causes crust materials to escape from the frontal regions toward the FarsArc and Lorestan sidesalients. Our findings indicate a relatively complex and heterogeneous temporal variation of earthquakes in the salients and Dezful indenter with respect to those in highZagros and Izeh frontal subzones. Abadan plain is the quietest subzone seismically and it shows the least amuont of temporal complexity. From the dependency point of view, the seismicity of highZagros, Izeh, and Abadan plain has an anticorrelate sharing. On the contrary, FarsArc and Lorestan salients have correlated seismic activities and in Dezful embayment the seismicity behaves in a random (stochastic) manner. These findings reveal that the seismicity offers relatively inconsistent configuration in regions with a highstress concentration and in contrary, earthquakes work dependably in other calm areas. Generally, in the Zagros region independent (scattered) earthquakes are more heterogeneous with respect to the dependent (clustered) seismicity. In other words, the Zagros tectonic setting is such that the independent earthquakes have more intricate temporal spreading with respect to the affiliated temblors. The results of this study are in agreement with KolahiAzar and Golriz (2018) examination. In the mentioned work topography complexity has been measured for the different subzones of the Zagros region. Assuming the topography is affected by the superficial tectonic processes; they concluded shallow tectonic processes that act more intricately in Dezful embayment, FarsArc, and Lorestan sidesalients. Similarly, our results show the more intricate temporal distribution of seismicity for the same regions. The fractal study of seismicity temporal distribution is a useful tool for the better understanding of the geodynamic conditions in a region. This approach reveals new seismotectonic aspects of the Zagros region which has not been addressed from this point of view.