بررسي عملكرد نئوتكتونيك برمخروط افكنه‏ها با تاكيد بر توان لرزه‏خيزي گسل‏ها (منطقه مورد مطالعه: دامنه‏هاي شما¬لغرب سهند)

Neotectonic Performance Review on Alluvial Fans, with Emphasis on Seismic Power Fault (Case Study: Northwest slopes of Sahand Mountain)

مخروطه¬افكنه¬ها یكی از اشكال ژئومورفولوژی پایكوهی در مناطق خشك ¬و نیمه‌خشك می¬باشند كه با وجود موقعیت مناسب برای اجرای برنامه¬های عمران و توسعه سكونتگاهی، نسبت به فرایند¬های مخاطره¬آمیز ژئومورفیك ازجمله رخداد زمین¬لرزه حساس هستند. در پژوهش حاضر سعی بر این است كه با بررسی مخروطه¬افكنه¬های واقع در دامنه¬های شمالغرب سهند، توان لرزه¬زایی گسل¬ها و پهنه¬بندی خطرات ناشی از آن را روی مخروطه¬افكنه¬ها به‌خوبی نشان داده شود. مخروطه¬افكنه¬های منطقه یكی از مراكز مهم تأمین‌كننده نیازهای زراعی و كشاورزی سكونتگاه¬های اطراف به شمار می¬آیند. نتایج حاصل از بررسی مخروطه¬افكنه¬های منطقه مورد مطالعه حاكی از آن است كه این مناطق به‌دلایلی‌چند ازجمله نزدیكی به گسل¬ها و دارا بودن سازند¬های سست و متخلخل (آبرفت‌های كواترنر)، در معرض خطرات ناشی از زلزله هستند. توان لرزه¬زایی گسل¬ها و پهنه¬بندی خطر آن‌ها بر روی مخروط¬افكنه¬ها با استفاده از روش تحلیل فضایی بربریان می¬باشد. ابتدا با استفاده از شاخص¬های ژئومورفیك (VF، B، SMF، FCI) میزان فعالیت تكتونیك در منطقه مورد بررسی قرار گرفت. سپس توان لرزه¬زایی گسل¬های موجود در سطح و مجاورت مخروط¬افكنه¬ها با روش¬های زارع، مهاجر- اشجعی، ملویل و اسمیت محاسبه گردید، درنهایت با استفاده از روش تحلیل فضایی بربریان میزان خطر گسل بر روی مخروط-افكنه¬ها و كاربری¬های مختلف انسانی موجود بر سطح آن‌ها و درصد تراكم كاربری¬ها در حریم گسل محاسبه گردید، و همچنین نتایج حاصل از پهنه¬بندی خطر گسل بر روی مخروط¬افكنه¬ها نیز بیانگر آن است كه 58 /٠مناطق مسكونی، 69/٠ زمین زراعی، 61/٠ چشمه¬ها و 39/٠خطوط ارتباطی در حریم ۱٠٠٠متری گسل (محدوده با خطر بالا) قراردارند و قابلیت لرزه زایی گسل‌ها با طول گسل ارتباط مستقیم وجود دارد.

1. Introduction The Earth 's crust is composed of various landforms. These forms are constantly changing. In this change, both of internal and external factors have a decisive role. The Internal factors led to the formation of the primary structure and external factors that cause erosion, deformation and destruction of these forms. Areas that have been affected by these activities confronted by various geomorphological forms, the alluvial fans were formed by different causes in this highlands. This study is an attempt to study the influence of tectonic on the formation and development of alluvial fans in the northwestern slopes of Sahand, calculate the seismic generating power of faults, and evaluate the seismic zoning of the risk arising from fault position on the alluvial fans and human activities. Sahand Mountain is located in the northwestern Iran with a geographical coordination of 37°07 'to 38°02 ' North and 45°53 ' to 47°00 ' East. The total area of alluvial fans is 29.874297 square kilometers. This massive mountainous acreage covers an area of 8,000 square kilometers. 2. Material and Methods In this study, 1: 25,000 maps of National Cartographic Center were used as the basic maps to identify tophographical units and location of alluvial fans. The geological map of 1: 100,000 were used to identify faults and geological formations of the region. Aerial photos of 1: 50,000, 1: 40000.1: 20000, from this area as well as satellite images were used in the completion of studies. After identifying alluvial fans and their initial review of the documents, the seismic data of the area was collected. Fault zones were detected by studying available geological maps, and their most important ones were drawn to the field of satellite imagery. Then, the seismic characteristics of the region were analyzed and a list of earthquakes were collected in radial expansion of 300 kilometers .The seismic potential of active faults from the initial parameters related to earthquake risk assessment were calculated using zarei, Ashjaei and Navrozi 's formulas. 3. Results and Discussion The impact of tectonic activity in the form of alluvial fans were analyzed by these indicators: the effects of tectonic on alluvial fans using the index Vf ; calculation of Vf on the alluvial fans of the area shows that only two alluvial fans (6 and 7) of the index are between 1-2 and other alluvial fans index are smaller than 1. The effects of tectonics on alluvial fans using the curvature index: the results of the study show that the alluvial fans of numbers 2, 3, 5, 9 have the smallest curvature coefficient and alluvial fans 6, 7 and 10 have gained the largest amount of curvature coefficient. 4. Conclusion According to the results of this study that indicates tectonic activity status (fault) on the surface of the alluvial fans, high seismic potential of faults represent a high risk arising activity of faults in the surface of alluvial fans. Hence, any activity in the alluvial fans (build settlements, communication lines, construction projects, etc.) needs preparation and special considerations so that the losses of the operation can be reduced to a minimum. Values obtained indicate that the role of tectonic activity in the region has been higher on the alluvial fan because of the proximity of the large faults in the studied area. 2 faults in the region have the highest longitude. The risk of these faults are threatening Tabriz and Khosrowshahr cities with widespread devastation.