Activity Of Main Faults And Related Geohazard Effects In Southeast Of Weihe Basin

Weihe Basin is not only one importart Cenozoic rift basin, but also one significant seismically active belt belonged to Fenhe-Weihe seismic zone in central China. Xi’an city is located on Weihe basin, and it is the center of political, economic and cultural of western China. There are many active faults and earthquakes in Weihe basin. Huaxian county 8.0 earthquake in AD 1556 is a key earthquake and occurred in the high angle normal faults of Weihe basin. The seismogenic structure of Huaxian earthquake is still controversial. Huashan piedmont fault is one high angle normal faults and maybe related to Huaxian earthquake. Thus, Huashan piedmont fault is a typical example of study relationship of normal fault and its disaster activities. This paper provide scientific basis for the stability evaluation of Xi’an city to study the relationship between fault activity and its earthquake and others disaster.This study is based on detailed geological survey, and combine many means and methods, such as profile measurement, large scale mapping, outcrop observation, OSL dating, 14C isotope dating, ESR dating, indoor mapping. Using Geophysical data and surface survey, the author analysis the relation and the characteristics of geometry and kinematics of those faults of the Weihe Basin southeastern, and to explore the Weihe basin formation mechanism and the relationship between activity faults and related geological disasters. In the end, the following conclusions and understanding can be got.(1) Active faults in the Weihe Basin are made up of EW trending fault system, NE～NNE strike fault system, NW trending fault system. Among of those faults, except Tieluzi fault is a left lateral strike-slip fault, the rest of the faults showe normal fault.(2)By means of DEM geomorphological analysis, the fault outcrop survey and combined with the crustal deformation data, and the result shows that activity since the late Pleistocene of the northeastern section in Tongren Loess Plateau is the strongest in the Lintong-Chang’an fault zone, the central section in Bailu and Shaoling Loess Plateau is second and the southwestern section in Shenhe Loess Plateau and Fengyu area is the weakest. Author forecast there is earthquake risk in the future in Tongren Loess Plateau area in the northeastern section of Lintong-Chang’an fault zone.(3)According to outcrop survey and geological-age determination, the auother identify Holocene multiple paleo-seismic events in Huashan piedmont fault zone, namely twice between (8590～7570) a BP, (75.7 to 6180) a BP,5610a BP,2715a BP or (2500～3000a BP) and AD1556a etc. Huashan piedmont fault can be divided into three sections. The East and West sections had strong activities during the Pleistocene, but activity is not obvious during the Holocene. The centre section has a strong activity during both Pleistocene and Holocene and it is the earthquake faults of 1556 Huaxian Mw8 earthquake. Long-term dip-slip rate of Huashan piedmont fault is about 0.64±0.05mm/yr; the dip slip rate during Holocene is about 1.67～2.71mm/yr. The long-term dip-slip rate of North Margin Fault of the Weinan Loess Tableland is about 0.32～0.42mm/yr; the dip-slip rate Holocene is about 0.58mm/yr. North Margin Fault of the Weinan Loess Tableland is not seismic fault of 1556 Huaxian Mw8 earthquake.(4) In order to find out cause of mass accumulation Lianhuasi area in Huaxian country Shaanxi province, the auother have made a geological mapping with 1:5 million scale and combined with loess OSL age, identify Lianhuasi mass accumulation containing two ancient landslide and two collapse induced by earthquake. The accumulation under loess layer is not caused by AD 1072 or ADI 556 earthquake, and it is the oldest reported high-speed and long runout ancient landslide 18.7 million years ago. The inducement of ancient landslides may is one seismic activity on Huashan piedmont fault.(5) The Huashan piedmont fault was a highly active normal fault during the late Quaternary; however, the disasters that have occurred due to the fault have been distributed significantly differently between the hanging-wall and the footwall. Ground fissures parallel to the strike of the fault mainly occur on the northern (hanging) wall of the fault. The footwall of the Huashan piedmont fault is the mountainous area of the Huashan Mountains; disasters that occur in the footwall mainly include landslides, collapses and mudslides. Collapses mainly occur on steep slopes with gradients of more than 45° on the northern slope of the Huashan Mountains.(6) The formation of Weihe River Basin is not in only one structural stress field and is the product of the deep mantle uplift and shallow multi stage and multi direction under the action of tensile stress superposition. The mechanics model of formation is a simple shear model. Its deep dynamics related to mantle uplift and the deep fracture of the upper mantle and the Moho surface. Because Weihe basin is not only the mantle uplift but have the low velocity layer, and the main fault convergence in the low velocity and high conductive layer and the existence of Moho fault, there is an important area of strong earthquakes risk in the future.