Seismic activity is the direct reflection of tectonic stress fields.Under a regional stress field,the cumulative strain reaches a certain level before the crustal rock fractures and is disloca-ted,resulting in an earthquake.The accumulation of tectonic stress follows the temporal variation and spatial migration of the stress field.The study of stress field trends in wide regions may deter-mine the epicenters of future large earthquakes to provide a scientific foundation for the physical meaning of earthquake prediction.Current research methods of tectonic stress fields mainly in-clude geological structure analysis,seismic analysis,in-situ stress measurement,numerical simula-tion,and experimental simulation.The most direct method for reflecting the stress state of the crust is to examine the focal mechanism of earthquakes in the deep crust.Compared with strong earthquakes of lesser quantity and concentrated distribution,small and moderate earthquakes are numerous and widespread;these conditions provide the possibility of detecting the stress state of the crust in the study area.Some of the most intense seismic activities in China have occurred in Sichuan-Yunnan,located in the southeastern region of the Tibetan Plateau.Several large earth-quakes have occurred in this area during the past 30 years,and its unique tectonic environment has a specific meaning in the dynamics of the continent.In this paper,we calculate the focal mecha-nism of 1893 M ≥3.0 small to moderate earthquakes that occurred from 2003 to 2012 by using the Hash method on the basis of P-wave first motion and S-wave amplitude ratios,and we analyze their characteristics.Moreover,we investigate the focal mechanisms of moderate and strong shocks occurring from 1 976 to 2012.The regional stress field is inverted by using the spatial and temporal stress inversion (SATSI)method for damping regional stress with 1 65 1 reliable focal mechanism solutions,and its space characteristics are analyzed.Our research shows that the focal mechanism solutions of small and moderate M ≥3.0 earthquakes were dominated by strike slip from 2003 to 2012 in the Sichuan-Yunnan region,with some reverse and normal faulting M ≥4. 0 earthquakes were dominated by strike slip,which is consistent with the regional background. The fault type is indicative of 3.0 ≤M ≤3.9 random earthquakes.The Sichuan-Yunnan area is divided into five zones of A,B,C,D,and E according to geological and seismic activity and other characteristics to examine the characteristics of the focal mechanism in detail.The results show that characteristics differ among zones.The stress action is mainly horizontal,and strike-slip faul-ting is dominant in the study area.The maximum horizontal principal stress orientation of the tow side of the Hong-he fault tail is NW-SE.In the Sichuan-Yunnan area,the rhombic block stress is divided into two different states by the Lijiang-Xiaojinhe fauthe stress orientation of the south region is NW-SE,and that of the north region is N-S.The stress orientation has a rotating clock-wise tendency from inner the Tibetan Plateau to the east border of the Sichuan-Yunnan rhombic block.In the eastern boundary area,there is a gradual transition from NE to SW in the north to NW-SE in south.The maximum horizontal principal stress orientation is parallel to the fault ori-entation in western and southwestern Yunnan.We compared our results with the Harvard Cen-troid Moment Tensor (CMT)database and determined that our results are comparatively accu-rate.%利用目前国际上较为流行和普遍接受的基于 P 波初动符号和振幅比 HASH 方法，研究给出川滇地区2003年1月1日-2012年12月31日1893次 M 3.0以上地震震源机制解。选择1651个可靠的中小地震震源机制解，采用基于中小地震震源机制解的 Hardebeck 和 Michael 的阻尼区域应力场反演方法，研究区域的水平最大主压应力方向。结果表明：川滇菱形块体南段红河断裂带尾部左右两侧应力方向相同，水平最大主压应力 NW-SE 向；川滇菱形块体内部以丽江-小金河为界呈现出不同的应力状态，位于分界线以南的滇中块体水平最大主压应力 NW-SE 向，而在分界线以北取向基本上沿 NS 方向；从青藏高原内部到川滇菱形块体东边界应力方向整体有一定的顺时针旋转趋势；块体东边界从北部的 NNE-SWW 向逐渐过度到南部的 NW-SE 向。滇西及滇西南水平最大主压应力方向与构造方向基本平行，为近 SWW-NEE 向。
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