Abstract:The Minxian-Zhangxian M6.6 earthquake,with a focal depth of 20 km,occurred at the boundary of Minxian and Zhangxian counties （34.5°N,104.2°E）,Dingxi city,Gansu Province,at 7：45 CST on July 22,2013.By 8：00 on July 29,1034 aftershocks occurred.Among them,nine above M3.0 include one M5.0～5.9,one M4.0～4.9,and seven M3.0～3.9 earthquakes.The strongest aftershock,which occurred at 9：12 on the date of the mainshock,attained MS5.6.Catastrophic damages caused by this earthquake are mainly attributed to poor residential structures erected by the government during a weak economy.The affected area was more than 10 000 km2 and involved in 6 cities and 13 counties ineluding Dingxi,Baiyin,Tianshui,Longnan,and Gannan Zang and Linxia Hui autonomous prefectures.Residents in Baoji,Hanzhong,Xianyang,Xi＇ an,Chengdu,and Mianyang felt this earthquake.The earthquake occurred at southeastern region of Gansu province,which is covered by very thick loess layer and exhibits complex topography such as steep,highly sloped mountains and crisscrossed ravines.It killed 95 people and injured 1 366.Secondary disasters caused by this earthquake include loess landslides,collapses,and pitfalls.Immediately following the earthquake,the China Earthquake Administration quickly organized field survey teams for seismicity monitoring,intensity studies,earthquake damage assessment,and scientific research,through which the first intensity distribution map of the Minxian-Zhangxian M6.6 earthquake was developed for conditions at 14：00 on July 24.The macroscopic epicenter of this earthquake,with an intensity of Ⅷ degrees,was located at Yongguang and Yongxing villages of Meichuan and Lalu Village of Hetuo.The long axes of intensity circled with an NW strike,in accordance with regional structures.This study discusses the basic parameters of this earthquake such as basic intensity distribution,damage features and seismogenic structure.This earthquake occurred at the eastern margin of the Qinghai-Tibetan Plateau at the intersection of the N-S seismic zone and the Kunlun-Qinling fault zone,which is the tectonic transformation zone between the NWW east-striking Kunlun fault and the northern margin of the western Qinling fault zone.We determined that the Hetuo fault,active since the late Pleistocene,passes through the extreme seismic area.Surface rupture phenomena such as landslides,ground fissures,earthquake pitfalls,sandblasting,and water oozing all exhibited zonal distribution along the Hetuo fault.The focal mechanism of this earthquake is consistent with the Hetuo fault.The strike of the nodal plane Ⅱ is 320° and 301.3°,as reported respectively by the Institute of Geophysics and the Institute of Geology,China Earthquake Administration,which also corresponds to the strike of the Hetuo fault.Moreover,the NE-dipping slip plane reported by the two agencies,51° and 57.3°,respectively,corresponds with the dipping direction of the Hetuo fault.Further,we determined that the Anjia Shan landslide slips laterally along a fault valley striking NW that resembles a long tongue with length and width of 310 m and 30～35 m,respectively.The plunge angel of fault striations on the slip plane is 12°～23°,and the main slip plane corresponds with the fault strike.Therefore,the Hetuo fault is seismogenic fault of the Minxian-Zhangxian M6.6 earthquake,which is a secondary fault,belongs to the Lintan-Tanchang fault zone.Three strong earthquakes above M6.0 have occurred near the epicenter of the Minxian-Zhangxian M6.6 earthquake on the Lintan fault,and two moderate earthquakes have occurred at this site in recent years,which include the Minxian M5.2 on November 13,2003,and the Minxian-Zhuoni M5.0 on September 7,2004.This earthquake represents the most recent activities of the Lintan-Tanchang fault zone.
