The Extraction Of Terrace And Geomorphology Index In The South Longmenshan Fault And Its Insight Into Tectonic Implications

The Longmen Shan range,located in the east of Himalayan,defines the margin of Himalayan and Sichuan Basin,where is a easy place to occur earthquake。The Lushan Ms 7.0 earthquake on 2013 in the south segment of Longmen shan fault is another destructive earthquake after the 2008 Wenchuan earthquake。Relative to the wenchuan earthquake and longmenshan fault zone in the middle of the Longmen Shan fault, the researches of Southwest section of longmenshan fault zone are very few。This article attempts to study the tectonic activity of southwest longmenshan fault zone and the seismogenic structure of the earthquake from the perspective of tectonic geomorphology。The research object is the basin of qingyi river, which is a big local river across the southwest section of longmenshan fault. This Paper mainly involves two aspects: the first mission is the automatic extraction of river terraces from high resolution DEM based on GIS, the other task is to obtain some geomorphic parameters for analysising the tectonic activity in a large scale landscape.Combining the results of the two aspects, we analysis the difference of tectonic activity in the southwest section of longmenshan fault and its revelation with the seismogenic structure of Lushan earthquake.Some key results of this dissertation listed as follows:(1)the automatic extraction method of River terracesAccording to the spatial relations of basins,we determine the effective extraction range of river terrace.By making use of characteristics of the slope and curvature of terraces, we get some polygons, then intersect each other and project it to the river line, finally we use hierarchical clustering and K ‐ means clustering method to divide the levels of river terraces.Using this method we try to obtain more residual geomorphic surfaces quickly, establishing a relatively complete river terraces profile. we make our experiments about terrace extraction between XiaoGuanZi ‐ Feixian village, the field measurement of multiple terraces data have a hight similarity with computer automatic extraction.we also extract two terraces profiles in tianquan ‐ Yang town area and Laochang village ‐ xinhuaxiang.(2)Large scale landscape factor analysis in the drainage of qingyi riverUsing ASTER GDEM data and we extract the basin terrain relief index, hypsometric integral(HI), channel steepnes index(Ksn), valley floor width to valley height ratio index(VF) and transverse topographic symmetry index(T ‐ factor).When breaking through river drainage ranking 3, its relief、HI value become higher than other basins in the same leel in sub‐basin, showing that the faults control of the basin landform. Between yanjing‐wulong fault and longdong‐gengda fault, the Vf index is low, exhibiting v shaped valley, from XiaoGuanZi fault to dachuan‐shuangshi fault Vf shows high value, displaying u‐shaped valley, but the east and west do not consistant with each other,the west is lower, Vf become complex in lushan basin, downstream from yaan city basically belongs to Vf high value area.Stream steepness is very high in Lushan basin and west of yanjing‐wulong fault, because rivers are the most sensitive,so the two regions are affected more seriousness by the recent tectonic activity. Values of transverse topographic symmetry index of drainage ranking 3 has three advantages directions, WNW, NE, SE, indicating the tilt of drainges are influnced by faults,but T value is generally small, much less than 0.2 respectively, so the activity of those faults are week.Geomorphology index clearly show that Dachuan‐Shuangshi fault is weeker that Yanjin‐Wulong fault in the thrust region, and Dachuan‐Shuangshi fault may be in an old stage.In the front mountain,there exists a higher activity region between Dachuan‐Shuangshi fault.(3)River terraces, geomorphology index`relationship with seismogenic structure of Lushan earthquakeComparing the profile of qingyi river and minjiang river,we find the form and uplift rate in both upstream and downstream of Front‐range Faults, showing the segmentation in longmenshan fault, also indicating the difference of seismogenic structure.In all, although Dachuan‐Shuangshi fault is active in Holocene, it has a weeker activity, all those evidences show Longmenshan fault has extend into Sichuan basin, the deformtion pattern is forming fold.