In the paper,we relocate the small earthquakes occurring in Linfen region from 1981 to 2010 by using the double-difference earthquake location algorithm. Our result shows that the distribution of relocated earthquakes is characterized by cluster activities,and most of the them occurred at the depths of 5～11km and 15～27km,which suggests that the two main seismogenic layers in this region are located in the upper and mid-upper crust. Earthquake frequency is high along both sides of the Subu Fault zone in the northern Linfen Basin,and most of the earthquakes in the north of Subu Fault zone are deeper than that in the south of Subu Fault. To some extent,this result indicates that the regional tectonic setting plays a major control role in the distribution of earthquakes. Analysis on the distribution of earthquake focal depths reveals that there exist deep faults which cut through the crust into the mantle in the middle of the Linfen Basin and connect with the active faults around the basin in the upper crust. The dislocations along the active faults on both sides of the basin are a reflex on the surface of faulting of the deep faults beneath the central basin.

Gas explosions in karst caves might be a cause for reservoir induced seismicity.The occurrence condition and mechanism of gas explosion are discussed and the possible magnitude of induced earthquake is estimated in this paper.Phenomena of gas explosion have been found at the reservoirs in karst area in China. But the induced earthquakes at Wujiangdu reservoir which have been considered as typical gas explosion are not caused by the gas explosion.