Double-difference seismic tomography method, which makes use of both absolute and relative arrival times of seismic phases, is adopted to image the velocity structure beneath seismic stations and relocate aftershocks of the 1 Jun 1996 Tianzhu MW5.2 earthquake recorded by a surveillance network of 6 digital seismic stations. Depth extent of active faults is deduced from the velocity structure and relocated earthquakes. The imaged velocity distribution shows that in upper crust there are obvious heterogeneity both horizontally and vertically. It reveals as alternative high and low velocity zones near surface along a N-S AAprime;profile. The earthquakes took place mainly in high velocity zones and high to low velocity transition zones. High velocity body represented by hard rock is crucial for preparation and occurrence of large events. The occurrence of 1927 Gulang M8.0 earthquake was inevitably related to a high velocity zone under Qilianshan thrust.