STUDY ON THE SEISMOGENIC TECTONIC OF THE M_S 5.0 EARTHQUAKE IN WENXIAN,GANSU ON JUNE 21,2006

No moderate and strong earthquake occurred for many years in Wenxian since the M 8.0 great earthquake occurring in 1879 A.D. The June 21,2006 Wenxian earthquake with M 5.0 occurred in an area where the tectonic setting is complex. Seismic activity level in Wenxian would enhance after the occurrence of this earthquake(M 5.0). Since the earthquake did not produce surface rupture zone,it is very difficulty to study the seismogenic fault. However,we can still study the seismogenic fault based on geological map(1∶200 000),remote sensing interpretation,inversion of focal mechanism and seismic sequence precise location method. In order to determine the future seismic risk of the region,the paper intends to analyze the earthquake causative structure by jointly using the remote sensing interpretation,various methods of inversion of focal mechanism,and double-difference earthquake location algorithm. The geological map(1:200 000)shows there are many faults in this area,and the interpretation of remote sensing reveals that only one fault named Shifang-Linjing Fault is active. Focal mechanisms derived from the two methods show that the earthquake is of left-lateral strike-slip and thrust,and the principal compressive stress is in the direction of N60°E. The results of double-difference earthquake locations also support this result. The distribution of aftershocks is related to the thrust faulting. Results of combined analyses show that the Shifang-Linjiang Fault is the seismogenic fault of this earthquake,and the direction of the principal compressive stress is N60°E.