DISCUSSION ON SEISMOGENIC STRUCTURES AND SEISMIC ACTIVITY PROCESS BEFORE THE YUTIAN M_S7.3 EARTHQUAKE ON FEBRUARY 12,2014,XINJIANG

The Yutian MS7. 3 earthquake occurred on February 12,2014 in Xinjiang Uygur Autonomous Region,China and the epicenter is located in the western part of Altyn Tagh Fault. This is the second M≥7 earthquake following the March 21,2008 Yutian MS7. 3 earthquake in the south of the Tarim Basin. Aftershocks of this Yutian MS7. 3 earthquake are distributed mainly along the NE direction,and that in the southwest part of the aftershock area presents a near-NS distribution. Most of the aftershocks including foreshocks( accounting for 85% of the total aftershock sequence) are densely distributed in the southwest of the aftershock zone,the vast majority of strong aftershocks( all of the MS≥5 and 81% magnitude 4 earthquakes) are distributed in this area. Aftershocks of the first day are mainly distributed in this area and in a near-NS distribution. The aftershocks extended from west to east. The authors noted that there occurred several M5 ~ M6 earthquakes in 1982,2011 and 2012 along the near-NS direction of this Yutian earthquake and this Yutian earthquake filled up the empty section. Based on the regional tectonic environment, earthquake focal mechanism solutions and aftershocks distribution, etc., we analyzed the process of this earthquake and found that the earthquake occurred at the branch fault of Altyn Tagh Fault zone on the south margin of Xiaoerkule Basin. Affected by the tectonic stress of Altyn Tagh Fault zone,Xiaoerkule Basin suffers the local near east-west extension,earthquake rupture occurred first along the near-NS direction,the unlocking of this tectonic position promotes the left-lateral movement of the Altyn Tagh Fault,producing NEoriented rupture, and stress transferring to the east. This paper also discusses the seismogenic structures of historical earthquakes above MS7 occurring on the Altyn Tagh Fault zone and their impact on the fault zone.