Velocity and Qvalue tomography,in addition to precise positioning methods of small earthquakes,were used to obtain the deep structure of seismic source area and spatial distribution images of aftershocks.The data of velocity and Qvalue imaging consist of two parts.The first includes the observation data from a high-density portable seismic array consisting of 150 stations with a reporting period of November 2009 to November 2011 in the area of 32.2-36°N,102.8-107.1°E.The second includes data from a fixed network during the period of 1970-2010.From these datasets,we selected 6 334seismic events recorded by at least 8stations and conducted velocity and Qvalue inversion calculation.We applied the three-dimensional grid search method to aftershock relocation,in which 746seismic events met with S-P arrival time differences detected by at least three stations.Finally,we obtained the relocation results of 400seismic events,and we applied the same method to relocation of the Minxian MS5.2earthquake occurring on November 13,2003,and the Minxian-Zhuoni MS5.0earthquake occurring on September 7,2004.We set the P1profile perpendicular to the direction to research in the deep distribution of the seismogenic fault.The profile P1crosscut the Lintan-Tanchang fault and the Minxian-Zhangxian seismic source area.We obtained the velocity profile,Q value profile,and aftershock distribution,which were combined with the focal mechanism solutions of these three earthquakes and the distribution of smaller earthquakes recorded since instrumental record have been kept.The results show that the seismogenic fault of Minxian-Zhangxian MS6.6earthquake is a buried fault with a NW strike direction,SW dip direction,and a dip angle steeper in the shallow region.In addition,the fault is thrust and sinistral strike-slip,and its exposure at the surface is 20km from Minxian section of the Lintan-Tanchang fault.The seismogenic fault and the Minxian section of the Lintan-Tanchang fault may have met at a deep level to form a fault zone to the south with a NW dip direction.The seismogenic faults of the Minxian MS5.2,Minxian-Zhuoni MS5.0,and Minxian-Zhangxian MS6.6earthquakes may belong to the same fault.The seismogenic fault of the Minxian-Zhangxian MS6.6earthquake is located in the suture parts of two secondary blocks.The east block is relatively stable,and the west block moves eastward and is blocked by the east block to form the extrusion area.The Qvalue of the extrusion area is lower and shows velocity uplift.In the Earth＇s crust of the west block,the Qvalue is lower,which provides eastward movement of west block with more favorable conditions.The occurrence region for earthquakes of MS≥5has included three events since 2003with seismogenic faults at least 20km from the Lintan-Tanchang fault;the dip direction is contrary.The seismogenic fault is located in the southwestern edge of Zhangxian＇s southern block,which is a secondary block(triangle)with a stable internal region.Three strong earthquakes occurred in western boundary of this block,which is conducive to the strain energy accumulation of the eastern region and the northern edge,which is the north rim fracture of western Qinling.The eastern region and northern boundary will likely be main regions of seismic activity of the southeast area of Gansu for the next several years.