Dynamic variation of ambient shear stress field before and after three strong earthquakes in Yunnan

A method calculating relative shear stress values in seismic source areas using data of seismic wave is used in this paper which is introduced by Pei-Shan CHEN, based on a rupture model studying seismic rupture process from theory of fracture mechanics. The shear stress values τ ₀ in mid-small earthquakes occurred in 1986–June of 1997 in Yunnan and its adjacent areas are used to trace the variation process of ambient shear stress field before and after three strong earthquakes in Yunnan and China-Myanmar border areas. The results show that there exists a background of high values in ambient shear stress field. In the development process of a strong earthquake, the ambient shear stress field in and nearby its potential source area increases obviously, for a long time, experiences a process of multiple down — up — down — up. The pattern of dynamic variation of ambient shear stress field has reflected the multi-source fields of seismogenesis in and nearby Yunnan. There exist multi-increased areas of ambient shear stress fields synchronously in southwest, northwest, west and east of Yunnan, and three strong earthquakes of magnitude about 7 occurred in the most obviously increased areas of ambient shear stress fields. This paper is supported by the Project 95-04-04-02-03 from China Seismological Bureau.