Characteristics Research of Tectonic Stress in Southern Ningxia and its Adjacent Areas by Focal Mechanisms of Small Earthquakes

Firstly,we calculated the focal mechanism solutions of 40 small and moderate earthquakes in southern Ningxia and its adjacent areas from Jan. 2003 to Oct. 2009 by the amplitude ratio method of Pg and Sg waves in vertical direction and did the system clustering of the calculated focal mechanism solutions to analyze the stress field. Secondly,we studied the average focal mechanism of Lingw u-Wuzhong region( I) and southern Ningxia region( II) by grid testing method. The results show that it is easy to develop dextral strike slip fault with stretching component in the study area under the compressive stress in NEE. The direction of principal compressive stress of tectonic stress field in I and II region is mainly horizontal. The structure deformations in the focal region caused by earthquake are compression in NE and expansion in NW in I region,and are compression in NEE and expansion in NNW in II region.