Possible crustal stress change in middle section of Xiaojiang fault and its adjacent area

The study on temporal and spatial crustal stress change is important for exploring earthquake origin and seismogenic condition. Based on 740 focal mechanism solutions of MLge;2.5 earthquakes occurred during Jan. 1998 to Jan. 2009 in middle Xiaojiang fault and its adjacent area, the studied region was divided into sub-regions, and temporally further divided into phases with the larger earthquakes as boundary of the sub-regions. After removal of the larger earthquakes this paper inverted the focal mechanism data with the method of Gephart for the stress in main phase of each sub-region. The result indicates that the azimuth of maximum principal stress sigma;1 in 4 temporal phases of the sub-region A is approximately NNW-SSE, and in northern part of sub-region B are approximately NNW-SSE，NNW-SSE and NNW-SSE. The azimuth of sigma;1 in southern part of sub-region B is approximately NNW-SSE. The azimuth of sigma;1 in northern part of sub-region B changed significantly, while the azimuth of sigma;1 in sub-region A is relatively stable. This result may be related to the complicated structure and possible stress adjusting in northern sub-region B.The sigma;1 directions in sub-region A may indicate seismically relative stability of the central Yunnan crustal block.