Variation Characteristics of Regional Stress Field in Datong Basin and Its Relationship with Medium Strong Earthquakes

Based on many results of focal mechanism solutions,the authors concluded that the principal compressive stress orientation of present crustal stress field in Datong basin is in NEE with indefinite angle of altitude,meanwhile the principal strain stress orientation is in NNW-SSE with small angle of altitude.This conclusion is consistent with the result of the compression structure of regional stress field along NE-NEE direction.According to analysis,the P-axis appeared in Feb.1997 tended to coincide with that before Jan.10th,1998 Zhangbei earthquake of Ms 6.2 on and 1st Nov.,1999 Datong-Yanggao earthquake of Ms 5.6,similar to the variation characteristics of stress field before the 1989 Datong-Yanggao main earthquake.During the low value of ratio of inconsistent signs,if the variations of orientation of principal stress axis also tend to coincide,this kind of anomaly is usually related with medium earthquakes in basin.Percentage of accumulated times of Up-down of first motions of Shanzizao and Yangyuan stations change synchronously before the Mar.26th 1991 earthquake of Ms 5.8,indicating the increase in stress field before strong earthquakes.