THE CHANGE OF ORIENTATION OF STRESS OF THE MINOR EARTHQUAKES BEFORE THE STRONG NINGHE EARTHQUAKE OF 1976

The focal mechanisms of minor earthquakes before the strong Ninghe earthquake of November 15, 1976 (Ms=6.9) are analysed. Within three months before this earthquake, the orientations of the principal stress axes started to be disordered and again oriented and finally deviated. The deviation of the strike of the P-axis was about 40. Based on the conjugate nature of the faults developed in this region, such deviation in stress direction has been priliminary explained geologically. In addition, by stacking up of all the signs of the first motions of minor earthquakes, the average focal mechanism solutions can be obtained. They show only disordering and reorientation but not deviation, which may be caused by the nature of the data and the method of analysis used.