تحليل زمين ساخت فعال حوضه ي بجنورد با كمك شاخص هاي مورفوتكتونيكي

Active tectonic analysis of Bojnurd subcatchment by using morphotectonic indices

شاخص هاي مورفوتكتونيكي براي شناسايي خصوصيات خاص يك ناحيه و براي مثال براي تعيين سطح فعاليت هاي تكتونيكي مفيد هستند. از اين اطلاعات براي بدست آوردن جزييات زمين ساخت فعال در يك منطقه ي خاص استفاده مي شود. منطقه ي مورد مطالعه در البرز شرقي- بينالود قرار دارد. در اين مطالعه شاخص هاي مختلف شامل عدم تقارن حوضه زهكشي (AF)، شكل حوضه (Bs)، منحني هيپسومتري (Hc)و انتگرال هيپسومتري (Hi)، سينوسيتي (S) و طول- شيب رودخانه (SL) محاسبه گرديد. نتايج اين تجزيه و تحليل ها با هم جمع گرديد و با شاخصي تحت عنوان زمين ساخت فعال نسبي (Iat) بيان گرديد. در اين مطالعه براي محاسبه ي شاخص هاي مورفومتريك از نقشه ي توپوگرافي و DEM منطقه در محيط نرم افزار GIS استفاده گرديد. مقدار شاخص AF در حوضه ي اصلي 40 مي باشد كه نشان دهنده ي كج شدگي حوضه ي اصلي به سمت غرب مي باشد. مقدار شاخص شكل حوضه (Bs) نشان مي دهد كه قسمت شرقي منطقه ي مورد مطالعه فعال ترين بخش آن مي باشد. محاسبه ي شاخص منحني هيپسومتري (Hc)و انتگرال هيپسومتري (Hi) نشان‌دهنده‌ي مرحله ي جواني در قسمت شرقي منطقه ي مورد مطالعه مي باشد. شاخص سينوسيتي (S) در حوضه ي اصلي 1.35 مي باشد، كه نشان دهنده ي بالا آمدگي تكتونيكي منطقه ي مورد مطالعه مي باشد. منطقه ي مورد مطالعه (حوضه ي بجنورد) به چهار ناحيه ي تكتونيكي با فعاليت خيلي بالا، بالا، متوسط و پايين تقسيم-بندي شد و براي هر كدام از زير حوضه‌هاي منطقه مقادير شاخص زمين ساخت فعال نسبي (Iat) محاسبه شد و همچنين شدت فعاليت تكتونيكي مشخص گرديد. محاسبه شاخص زمين-ساخت فعال نسبي (Iat) نشان مي دهد كه بيشترين مقدار اين شاخص مربوط به قسمت شرقي حوضه ي بجنورد مي باشد. نقشه ي پهنه بندي نشان مي دهد كه بيشترين فعاليت تكتونيكي و پرخطرترين بخش منطقه ي مورد مطالعه قسمت شرقي حوضه ي بجنورد مي باشد.

Morphotectonic indices are useful for identifying a particular characteristic of an area, for example its level of tectonic activity. This information is used to obtain detailed information about active tectonics. Study area is located in the East Alborz- Binalud. AF index of main basin is 40 which are indicating high tilting of this subcatchment toward West. Computing of Bs index show that East side of studied drainage basin is active part of the study area. Calculating of hypsometric curve (Hc) and hypsometric integral (Hi) show that East side of drainage basin is in young stage. Sinuosity (S) index of main river is 1.35 that show uplifting of study area. Highest amount of relative active tectonics (Iat) is related to East side of Bojnurd subcatchment. Computing of various geomorphic indices show that tectonic activity at the East side of study area is high. Methodology In this study for calculating morphometric indices we utilize topographic map and DEM in GIS software. In this study some indices including drainage basin asymmetry index (AF), drainage basin shape (Bs), hypsometric curve (Hc) and hypsometric integral (Hi), Sinuosity (S) and stream length-gradient index (SL) were calculated. Results from this analysis are accumulated and expressed as an index of relative active tectonics (Iat). AF index is sensitive to tilting vertical to the trend of the trunk stream. High values of Bs are associated with elongated basins, generally associated with relatively higher tectonic activity. The hypsometric integral is an index that describes the distribution of elevation of a given area of a landscape. Any tectonic deformation that changes the slope of a river valley results in a corresponding change in sinuosity to maintain the equilibrium channel slope. The SL index will increase in value as rivers and streams flow over active uplifts. Several index combined to provide information of relative degree of tectonic activity. Result and Discussion Quantitative assessment of surface features and calculation of morphotectonic indices of rivers, river canals are the best method for investigation of active tectonics. The results of these investigations are useful in long term prediction of earthquake and earthquake hazard assessment of and analysis in the region. The studied area is located in NE Iran (East Alborz). This research has analyzed the six indexes drainage basin asymmetry index (AF), drainage basin shape (Bs), hypsometric curve (Hc) and hypsometric integral (Hi), Sinuosity (S) and stream length-gradient index (SL) were calculated. Results from this analysis are accumulated and expressed as an index of relative active tectonics (Iat). AF index of main basin was obtained 40 which are indicating high tilting of this subcatchment toward west. The results show that, the value of AF index in the east side of the main basin is high. Comparing west and east subbasins shows that eastern subbasin is more tilted than western subbasins. Amount of Bs index at the main basin is 1.05, eastern subbasins is more elongated than southern subbasins that is related to shape of main basin. High SL index is related to eastern part of study area high resistance rock in this part of study area. Tectonic activity map was classified as very active, active, relatively active and nonactive after calculating of noted indices and combination of information layers. Result of this research show that eastern part of study area is active part. Conclusion Geomorphic indices can be estimated for identify active part of each basin and determining level of tectonic activity in each area. In this research for separating high tectonic activity areas from low tectonic activity areas geomorphic indices were calculated. Highest amount of Af index is associated with eastern subbasin, the values of Af index is related to tectonic tilting. Calculating Af index shows that highest tilting is associated with eastern subbasin. Value of Hi index were found to be high in eastern subbasin, Computing of Hi index shows that rate of uplifting in eastern subbasins is more than other subbasins. Calculating SL index showed that, high tectonic activity of study area is related to eastern subbasins and offset of streams in this area. Relative tectonic activity (Iat) calculated by averaging from all indices. Class 1 of Iat, indicating that most active tectonics occurs in the eastern part of the study area along high resistance rock and high accumulated lineament. Class 4 of Iat mainly takes place in the tectonically inactive southern parts of the study area.