青海东南部应力场特征及正断层小震现象的初步研究

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

利用2008—2013年青海东南部数字地震波资料，采用Pg、Sg最大振幅比及CAP方法反演得到92个％．2．5～4．9中小地震震源机制解。统计结果表明该区中小地震震源类型以走滑为主，应力轴接近水平，兼有一定量正断层类型，逆冲类型较少；利用格点尝试法求出研究区平均现代构造应力场是以处于近NE—SW方向的水平压应力和近NW—SE方向的水平张应力为特征；进一步分析了研究区正断层小震产生的背景，初步认为这些正断层类型小震可能主要源自高海拔地形产生的重力滑塌作用。

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 ≥ MI 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 distribu- tions of the P--axis azimuth of ML≥ 3.0 are so regular that it is easy to see the predominant dis- tributions. By using focal mechanism solutions of ML≥3.0 by the grid test method,the mean re- gional stress field is obtained. The tectonic stress field is primarily represented by NE--SW hori- zontal compressive stress and NW-SW horizontal tensile stress in the southeastern Qinghai prov- ince,and the mean contradictory ratio is 0.32. These results are the same as those in previous re-search. Furthermore, it is determined that the small normal fault earthquakes are mainly distribu- ted 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 earth- quakes are produced. The southeastern region of Qinghai province still belongs to the inner Qing- hai--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.