THE SEISMICITY FEATURES IN SOUTH NINGXIA

The variation features of synthetic focal mechanism solution of small earthquakes before mid strong earthquakes in south Ningxia region are studied.It is found that direction of the P axes drifted off average values and turned to E or SE,being accompanied by identified first motion mark of P n and P g waves of small earthquakes.The focal mechanism solutions of 6 mid strong earthquakes in south Ningxia region show that the earthquakes occurred under the affect of principal compressive stress of NE,with no somewhat change in comparison with direction of principal compressive stress of regional stress field.So the possibility of the destructive earthquakes to occur in the region is smaller .But patterns of seismicity in recent years show it is possible that seismicity is going to concentrate in Zhongwei,Zhongning and Tongxin region in the future.