سازوکار گسله پیشانی کوهستان (MFF) زاگرس در طول جغرافیایی 46 تا 5/48 درجه شرقی

نوع مقاله: مقاله تحقیقی‌ (پژوهشی‌)

نویسندگان

پژوهشگاه بین المللی زلزله شناسی و مهندسی زلزله

چکیده

زمین‌ساخت زاگرس به خاطر پوشش لایه‌های نمک در پوسته به‌گونه‌ای است که حتی زمین‌لرزه‌های بزرگ نیز به‌ندرت به‌صورت گسیختگی به سطح می‌رسند و رخنمون دارند. از این‌رو عوارض گسله‌های فعال به‌آسانی امکان‌پذیر نیست. مناسب‌ترین روش مطالعه گسله‌ها در این‌گونه موارد استفاده از تکنیک‌های زلزله‌شناسی است. از اهداف مهم مشترک زلزله‌شناسان و زمین‌شناسان پی بردن به سازوکار گسله‌ها و شناخت گسله‌ مسبّب زمین‌لرزه است. از این‌رو حل سازوکار کانونی به روش‌های مختلفی در مناطق گوناگون انجام می‌گیرد. در این مطالعه با روش شبیه‌سازی شکل موج اقدام به حل سازوکارهای کانونی زمین‌لرزه‌ها در پهنه گسله پیشانی کوهستان شده است و به دنبال آن با روش برگردان حل‌های صورت گرفته به حل و بررسی وضعیت تنش و شناخت صفحات گسلی منطقه پرداخته شده است. برای تعیین میزان صحت و درستی سازوکارهای کانونی به‌دست‌آمده از روش پلاریته (اولین رسید موج) نیز استفاده شده است. روندهای غالبی که به‌عنوان صفحه گسلی همخوان با تانسور تنش انتخاب شده است به دو دسته خاوری- باختری و شمال باختری تقسیم‌بندی شده‌اند. آن دسته از روندهای خاوری- باختری با شیب رو به شمال همخوان با گسله پیشانی کوهستان هستند. همین روندها با شیب رو به جنوب در این منطقه تاکنون به نقشه در نیامده است. روندهای خاوری– باختری خود به دو دسته پر شیب دارای مؤلفه افقی بیشتر و کم‌شیب با مؤلفه شاقولی بیشتر تقسیم شده‌اند.

کلیدواژه‌ها


عنوان مقاله [English]

Focal mechanism of Mountain front fault (MFF) at a longitude of 46 to 48.5 Degree

نویسندگان [English]

  • Sotodeh Mohammadnia
  • Mohammadreza Abbassi
  • Gholam Javan-Doloei
  • Mohsen Azqandi
International Institute of Earthquake Engineering and Seismology (IIEES)
چکیده [English]

One of the most seismically active parts of Iran is Zagros area. The basement-involved active fold-thrust belt of the Zagros in southwest Iran is underlain by numerous seismogenic blind basements thrust covered by the folded Phanerozoic sedimentary rocks. The present morphology of the Zagros active fold-thrust belt is the result of its structural evolution and depositional history: a platform phase during the Paleozoic; rifting in the Permian Triassic; passive continental margin (with sea-floor spreading to the northeast) in the Jurassic-Early Cretaceous; subduction to the north­east and ophiolite-radiolarite emplacement in the Late Cretaceous; and collision-shortening during the Neogene.Besides, there are a lot of different faults in Zagros, for example, the Main Zagros Reverse Fault (MZRF), the Main Recent Fault (MRF), the High Zagros thrust belt, the High Zagros Fault (HZF), and Mountain Front Fault (MFF). This study is focused on the last-mentioned one. The MFF flexure is introduced for the first time by Falcon (1961) and then is presented as the mountain front fault by Berberian and Tchalenko (1976)], which delimits the Zagros simple fold belt and the Eocene-Oligocene Asmari limestone outcrops to the south and southwest. The Mountain front fault (MFF), is a segmented master blind thrust fault with important structural topographic, geomorphic and seismotectonic characteristics. Therefore, the study and recognition of seismic parts of Iran are important. The aim of this study is to determine the focal mechanisms of Mountain Front Fault (MFF) at a latitude of 46 to 48.5 degree in Zagros. Due to the salt layers, large earthquakes rarely reach the surface. In such cases, the seismic method is an appropriate tool to understand the faulting mechanisms. By means of focal mechanisms, it is possible to gain information about the fault geometry and its related mechanism. The data used in this study are from International Institute of Earthquake Engineering, and Seismology (IIEES) and Institute of Geophysics of the University of Tehran (IGUT). Because of some wrong relocation, during this study relocated them to reach a well-relocated data base and better results. Getting the focal mechanism of an earthquake can occur in various ways. In this study, first, the waveform modeling by Isola software was used to find the focal mechanisms. To determine the accuracy of focal mechanism solutions obtained by waveform modeling, the polarity method was used to solve focal mechanisms. Besides, some of these earthquakes have also been reported by CMT. After determining focal mechanism solutions with the stated method, they were compared with CMT, and all the focal mechanism were mapped in the area so that the trend of this part of MFF can be recognized better. Because there are many earthquakes in this area, a reliable decision can be made. By looking at the maps, it is easily understandable that the trend in this part is obviously EW. Finally, the prevailing trend that obtained in the study area is found. Most of these earthquakes are trending EW. The study of 31 focal mechanisms in the area has permitted to constrain the faulting mechanism of MFF.

کلیدواژه‌ها [English]

  • focal mechanism
  • mountain front faulting (MFF)
  • state of stress
  • waveform modeling
  • Zagros

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