TEXTUAL RESEARCH OF LUQU EARTHQUAKE IN 842 AD IN GANSU PROVINCE AND ANALYSIS OF ITS CAUSATIVE STRUCTURE

An earthquake with MS6. 6 occurred near the border between Minxian and Zhangxian counties in southeastern Gansu Province on July 22,2013. This earthquake caused serious personnel casualties and property damages. According to the field investigation,the intensity of the epicenter area is aboutⅧ,the causative structure is a branch fault of the eastern segment of Lintan-Dangchang active fault.The southeastern region of Gansu Province is located at the eastern boundary of Tibetan active block. A series of strike-slip faults with thrust components are developed and their combination is complicated and a series of strong or even large earthquakes occurred in this area in the history and present-days,and one of them is the Luqu earthquake occurring in 842 AD at the boundary of Han and Tibet area( now the southeastern area of Gansu Province). The earthquake caused seismic rupture,spring gushing,landslip in the Minshan Mountains and countercurrent of the Taohe River for three days. According to the detail textual research of historical references and field investigation,the epicenter area of this earthquake locates at the Guanggaishan-Dieshan mountain area,at the border area between Luqu County,Zhuoni County and Diebu County in the Gannan Tibetan Autonomous Prefecture. The date of the Luqu earthquake is possibly on the 24 thday of the twelfth month of the second year of Huichang Reign in Tang Dynasty,that is,on January 31 or 27,843 AD,and the magnitude of this earthquake is about 7~71/2,the intensity near the epicenter area is about nine to ten. There are three late Quaternary active fault zones of thrust with left-lateral components,namely,Lintan-Dangcang Fault,Guanggaishan-Dieshan Fault and Diebu-Bailongjiang Fault. According to the comparative analysis of the field investigation of active faults in recent years and present seismic activity,we think that Luqu earthquake is the result of new activity of Guanggaishan-Dieshan Fault,the causative fault of this earthquake. This fault is an important branch fault of the eastern segment of northern boundary faults of Bayan Har block( Eastern Kunlun Fault zone),a main activity area of large earthquakes with magnitude larger than 7 in Chinese continent in the recent 10 years,and has the tectonic condition to generate M≥7 large earthquakes.