Research on Stress Field Characteristics and Normal Fault Small Earthquakes of Southeastern Qinghai

Digital seismic waves recorded by Qinghai and Gansu digital networks from January 2008 to August 2013 are used to obtain 92 focal mechanism solutions of 4.9≥ ML≥2.5 moderate and small earthquakes in southeastern Qinghai by using the methods of P-wave and S-wave maximum amplitude ratio and cut and paste(CAP).The focal mechanism solutions of 3.9≥ ML≥2.5 are retrieved by the former method,and the other larger earthquakes are retrieved by the latter.Moreover,analysis of these focal mechanism solutions by means of statistics and systematic cluster analysis reveals that the predominant distributions of the plunge of T-and P-axes are less than 45°and that a few have a high plunge of P.These results indicate that the moderate and small earthquakes in this region are mostly strike-slip and that a certain number of normal fault earthquakes are present;however,small thrust fault earthquakes are rarely seen.The distributions of all of the azimuths of the P-axis are widely scattered,which may be attributed to the fact that the small earthquakes are more happenstance than tectonic earthquakes;however,the distributions of the P-axis azimuth of ML≥3.0 are so regular that it is easy to see the predominant distributions.By using focal mechanism solutions of ML≥3.0 by the grid test method,the mean regional stress field is obtained.The tectonic stress field is primarily represented by NE-SW horizontal compressive stress and NW-SW horizontal tensile stress in the southeastern Qinghai province,and the mean contradictory ratio is 0.32.These results are the same as those in previous research.Furthermore,it is determined that the small normal fault earthquakes are mainly distributed in the eastern part of the Kunlun active fault(Maqin-Maqu)and in the southern part of the Elashan Mountain fault.This paper analyses the mechanism in which small normal fault earthquakes are produced.The southeastern region of Qinghai province still belongs to the inner Qinghai-Tibet block,and the significant differences in elevation could create many small pull-apart basins in some area,such as the eastern part of the Kunlun active fault(Maqin-Maqu)In this region,the small normal fault earthquakes have a certain proportion;therefore,it is suggested that these small normal fault earthquakes may be attributed to gravity slumping because of the high altitude.