A 3-D velocity model of the southeast Gansu Province region,including the 3-D grid searching algorithm and double difference method,was used to examine the relocation the Minxian-Zhangxian MS6.6 earthquake occurring on July 22,2013,and its aftershocks from July 22to July 25.The grid search method was used to determine earthquake location and the S-P arrival time of 400seismic events,which agreed with data detected by at least three stations.Of these events,336were used to calculate double-difference location,and 261were used to obtain the relocation results.After relocation,the average estimation errors of the source location were 1498m in the E-W direction,1385.2min the N-S direction,and 2 492.3min the vertical direction.On the basis of these calculation results,the seismogenic structure was compared with the data of geological structure,and the results showed that the mainshock was relocated to 34.54°N,104.189°E,with a focal depth of 13.5km.The aftershocks were distributed mainly between the Lintan-Tanchang fault and the northern edge of the West Qinling fault;several were distributed in the southwestern region of the Lintan fault.The distribution of the aftershocks was in the NW or NWW direction,which is essentially consistent with the strike of Lintan-Tanchang fault,although the dominant distribution was not obvious.Therefore,we examined the A-A′profile perpendicular to the direction to research the relationship between the distribution of earthquakes and fracture structure and determined that this profile crosscuts the Lintan-Tanchang fault.We obtained similar results using the 3-D grid searching algorithm and the double difference method, which revealed that the focus depths are mainly distributed in 0~10km before relocation,and the average depth was 6km.In contrast,the focus depths of the aftershocks were distributed mainly in 5~20km after relocation,which indicates that the aftershock sequence occurred mainly in the upper and lower crust.The distribution of the earthquake was in the SW direction,thus,we speculate that the dip direction of the seismogenic fault was SW,and the deep distribution is deeper.The focus depths determined by the 3-D grid-searching algorithm can reach to 25km,whereas those determined by the double difference method were deeper at a maximum of 30km.The Lintan-Tanchang fault,with a NW-NWW strike and a SW dip of 50°-70°,is in closest proximity to the earthquake swarm and is located between the East Kunlun fault and the northern edge of West Qinling fault.The transition fracture of the structural change and the geologic structure of the transition zone are complex and provide the regional geological background.The seismic profile A-A′shows that the basic characteristics of the seismogenic fault,with a SW dip direction,differ from those of the Lintan-Tanchang fault such that their exposures at the surface are 20km apart.Therefore,we believe that Fault F3is a seismogenic fault.However,other characteristics of Fault F3are unclear and need to be developed through active fault detection research.