宁河6.9级强震前小震主应力轴取向的变化/

本文分析了1976年11月15日天津市宁河县6.9级强震前、后小震震源机制解,表明在强震前3个月内主应力轴方位角曾发生紊乱——定向——偏转的变化.P 轴偏转约40度.根据宁河地区共轭性断裂发育的地质构造特点,对 P 轴的方向变化作了初步解释.另外,还利用微小地震初动符号叠加的方法,求得平均机制解,显示了紊乱——定向变化,但未观察到重新取向变化.以上的差异可能与资料和方法有关. 

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.