Current state of tectonic stress in northwestern Yunnan and its relationship with earthquakes

Based on collected the focal mechanism solutions of M_S≥3.0 earthquakes from 2000 to 2018 in northwestern Yunnan, we analyze the current state of tectonic stress in northwestern Yunnan by Michael’s linear superposition stress inversion method, and further investigate the spatio-temporal distribution of stress tensor variance and its relationship with seismic activity. The results show that the focal mechanism solutions of earthquakes in northwestern Yunnan are complex in types, mainly strike-slip type （46%）, followed by normal-faulting type （27%）. Tectonic stress field is overall consistency and partial inhomogeneity. The tectonic stress field shows strike-slip faulting stress regime with NNSwards principal compressive stress and ENEwards principal tensile stress in the studied area. It suggests that the study area is subjected to horizontal compression from NNW direction, which plays a dominant role in the upper crustal movement and fault activities in this area. Regional grid stress inversion shows that the variance is mostly below 0.2 in northwestern Yunnan, which indicates that the stress field is in a uniform state except for some northern regions. According to the variation of variance with time and subsequent earthquakes, moderate to strong earthquakes in the studied area mostly occurred when the stress tensor variance was lower than 0.2. Most earthquakes mainly occurred during the process that the stress tensor variance decreased, that is, when the focal mechanism solutions tend to be consistent. And spatially these earthquakes basically occur in the low value zones of the variance or near the edge of the zones, which provides reference for judging the earthquake location and comprehending the regional stress concentration enhancement process in the future.