Xiangjiaba Reservoir is currently China's third largest reservoir and began impounding water at the end of 2012. After the impoundment, the water level rose to 71m, while seismic activity near the dam was not significantly increased. At the end of June 2013, the reservoir began impounding water again, the water level continued to rise and flooded the tail region of the reservoir. In the reservoir area and the Xiluodu reservoir area in the upstream, a reservoir seismic network including 35 seismic stations was set up which can roundly record earthquakes in this area. According to the records of the reservoir seismic network from September 2007 to June 2013, only 38 earthquakes with ML≥1.0 occurred, 0.66 times a month on average, while in July-September 2013, 186 earthquakes with ML≥1.0 occurred, with an average of 62 events a month, nearly 100 times of that in the past. So, most of the earthquakes are induced earthquakes. At the same time 553 earthquakes with ML≤1.0 were also recorded in this area. A large number of small earthquakes occurring in the strong earthquake background area have caused a big stir. The source location of these earthquakes are rechecked based on 3D velocity model, 94% of the rechecked focal depth is less than 5km. Based on observations of the reservoir seismic network and vertical P- and S-wave's maximum amplitude ratio method, we inversed 9 focal mechanisms before the impoundment and 69 focal mechanisms after the impoundment in the tail region of the reservoir. Using these focal mechanisms, the stress field of the northern part and southern part of the study area is calculated in order to analyze the characteristic and cause of the induced earthquakes. The results indicate that most of the 69 focal mechanisms are strike-slip type, there is more transitional type, and less normal type and thrust type. The focal mechanisms spatial orientation is complex, fracture types are diverse, which may indicate that the stress state is uneven and the control of regional stress field to small earthquakes is weak. The stress field in the south and north is quite different and not consistent with regional stress field. The north shows compressive stress state while the south shows a state of weak extension. The Yaziba Fault, which passes through the tail region of reservoir, is an active fault, but does not control the induced seismicity, which may indicate that the reservoir storage inhibits the reverse fault activity. Carbonate rocks, limestone and karst cave are developed in the tail region. Analysis believes that reservoir water flows into the caves, penetrates into cracks and joints, leading to increased pore pressure, reducing the frictional strength and fracture strength and increasing reservoir water load which cause elastic deformation, so, it is believed that the combined action of all the above factors is the cause for the induced earthquakes.
