تاثیر گسلش بر احتمال وقوع زمین‌لغزش‌ها، مطالعۀ موردی: زمین‌لغزش‌های منطقۀ نرگس‌چال

زمین لغزش ها به‌عنوان مخاطرات طبیعی، موجب خسارات فراوانی می شوند، در بسیاری از موارد می توان با شناخت زمین شناسی مناطق مختلف، وقوع زمین لغزش ها را پیشگویی و با راهکارهای مناسب از وقوع آن جلوگیری کرد. یکی از مناطق مستعد و پرخطر از نظر زمین‌لغزش در البرز شرقی، منطقه نرگس چال است. در این پژوهش، به‌کمک روش های سنجش از دور، gis و هم‌چنین مشاهدات و اندازه گیری های صحرایی اقدام به بررسی عوامل موثر بر زمین لغزش ها در منطقه نرگس چال شد. در این راستا هفت لایه اطلاعاتی برداری از عوامل موثر بر زمین لغزش تهیه شده و به مقادیر موجود در هر لایه اطلاعاتی وزن داده شده و کلاس بندی انجام شد. در نهایت با محیط نرم افزاری gis نقشۀ پهنه بندی خطر زمین لغزش تهیه شد. نتایج حاصل از تهیه نقشۀ پهنه بندی خطر زمین لغزش نشان می دهد که الگوی فضایی بیش‌ترین خطرات زمین لغزش به‌صورت نواری پهن با امتداد شمال شرق جنوب غربی قابل مشاهده است. نتایج حاصل از بررسی اثر شکستگی ها (به‌عنوان عوامل ساختاری) بر پهنه بندی خطر زمین لغزش نشان می دهد که عوامل ساختاری نظیر گسل های اصلی (و سایر شکستگی های مرتبط با آنها) به‌طور غیرمستقیم، با ایجاد شیب های تند، ایجاد مناطق خردشده و نامقاوم، ایجاد مناطق چشمه سار و ایجاد زلزله ها، تعیین کننده الگوی پراکنش بیش‌ترین خطر زمین لغزش بوده است و تاثیر زیادی بر رخ داد این بلای طبیعی داشته اند.

Influences of Faulting on Landslide Occurrence Probability-A Case Study: Landslides of Nargeschal Area

IntroductionThe landslides, as a natural hazard, caused to numerous damages in residential area and financial loss. In many cases, we can forecast the occurrence probability of this natural phenomenon with using of detail geological and Geomorphological studies. This seems that one of the most effective parameters in landsliding phenomenon is structural parameters, especially faulting in rocky outcrops. For verifying this hypothesis, the Nargeschal area, as high potential of hazardous area, is selected as case study for investigation on influences of faulting on landslide occurrence probability. Many large composite landslides were happened in 2016 and hence, this area is enumerated an active zone of landsliding. This area with geographic attitude 55 ° 09chr('39') 06 to 55 ° 27chr('39') 21 Eastern Longitude and 36 ° 54chr('39') 23 to 37 ° 05chr('39') 15 Northern Latitude located in south of Azad shahr (in Golestan Provinces) placed in Northeastern of Iran.Geological studies indicate that this area located in northern limb of Alborz fold belt (as a young foldthrust belt with 900 km length) which formed in late Alpine orogenic events by convergence AfroArabian and Eurasian plates. In this zone, the structures have main NESW trends with main active faults such as Khazar and North Alborz faults, as reverse faults with northward movements. The remnant part of Paleotethyan rocks (which transported from collision zone toward southern part by low angle thrusts) located between these faults and formed the mountainplain boundary hills.Material and MethodsIn this research, we investigated on effective parameters in landslide occurrence probability of Nargeschal area with using of remote sensing techniques, GIS environment abilities and complementary field investigations. Therefore, we have prepared the seven data layers of geological and morphological effective parameters which are affected on landslide probabilities. These data layers consist of: lithology of outcropped rocks, faulting condition, topographic slopes categorizes cultivation circumstances, seismicity condition, spring population (ground water condition) and surveyed occurred landslides. Then, the content of each data layer is weighted and classified into five classes in GIS environment. Finally, the content of each pixels in all of 7 layers are algebraically summed and recorded as an attributed table. Hence, the landslide hazard zonation map was prepared by drawing the isopotential surface map on the basis of quantities of attributed table by using of GIS functions in Arc view 3.2 software.Results and DiscussionThe results of this research illustrate that a high risk zone is located in central part of area as an oblique broadstripe zone with NESW trend [6]. This zone is correlatable with high density of fractures zone and high population of springs and earthquake focus and also, taken place in Shemshak formation with shale, marl and siltstone rocky outcrops (upper Triassic Jurassic in age). Also, the results of investigations on influences of structural parameters (especially faulting) in landslide hazard demonstrated that faults are indirectly impressed on this hazard probabilities via formed the high slope topography, poor strength faulted rocks, locating of spring presences and creation of seismicity, and hence, defined the spatial pattern of landslides.ConclusionNargeschal area in Northern limb of Eastern Alborz is selected as case study for investigation on temporal relationship between Faulting and Landslides. The following conclusions were drawn from this research. It seems that the fault surface plays the role of rupture planes for landsliding. The structural factors also increased the ground slope and then, the close relationship is formed between landslides and faults. The results demonstrate the genetically relationships between landslides and faults in macroscopic scale in Nargeschal area.