TEXTUAL RESEARCH ON THE TIANSHUI M 7 EARTHQUAKE IN 734 AD AND ANALYSIS OF ITS CAUSATIVE STRUCTURE

Abstract

Tianshui area locates at the northeastern margin of Qinghai-Tibetan plateau and the middle segment of the north margin of western Qinling tectonic zone.In history,several strong destructive earthquakes happened in the area.Because of the very long history and that the historical records are not quite clear,it is very difficult to study them in nowadays,and most of them became historical knotty problems.The Tianshui earthquake in 734 AD is one of the large earthquakes.On March 23,734 AD,a large earthquake happened in Qinzhou of Tang Dynasty,now the vicinity of Tianshui City,causing serious seismic disasters as “the earth ruptured and closed again,nearly all the houses damaged,about 4000 people dead,hills changed into valleys,and towns covered by landslip,and so on”.According to the detail textual research of the historical earthquake records,the meizoseismal area of the 734 AD Tianshui earthquake is in the Qinzhou and Maiji area,now,the Qincheng,Beidao and Maijishan area of Tianshui City.The epicenter intensity is about Ⅹ,the magnitude is about M 71/2.The direction of long axis of isoseismal line is NW,consistent with the strike of the eastern segment of Wushan-Gangu Fault of the northern margin of western Qinling active fault zone.The meizoseismal area is just in the step-over zone between the left-lateral strike-slip faults,i.e.the Tianshui-Baoji Fault and the Gangu-Wushan Fault,an area prone to strong earthquake.Comprehensive analysis indicates that the causative structure of the 734 M71/2 Tianshui earthquake is the east segment of the Gangu-Wushan Fault of the northern margin of western Qinling active fault zone.