THE SYNTHETICAL STRESS FIELD ANALYSIS OF SMALL EARTHQUAKES BEFORE AND AFTER MODERALTELY STRONG EARTHQUAKES IN NINGXIA AND ITS ADJACENT AREAS

The studies in this paper have.showed that before and after several moderately strong shocks in Ningxia, the strikes of P (main pressure stress) axes have changed distinctly, deviating from the strikes of P axes in normal structural stress field or drawing no conclusions. The longest anomaly before earthquakes can last as long as 4 years. So it is obvious that the variation before shocks can be a significant clew for the moderately strong earthquakes prediction.