中文摘要：

用美国南加州地区的震源机制资料,研究了以1992年Landers Mw7.3和1999年Hector Mine Mw7.1地震矩心为圆心,分别以300、250、200、150、100和50km为半径的圆形区域内10年内地震震源机制相对主震震源机制平均空间旋转角的变化。结果表明,两个地震前均有小震震源机制趋于主震机制的现象。震前小震震源机制与主震震源机制的平均空间旋转角约在大震前半年开始减小,搜索平均空间旋转角降低的最优半径分别为250km和100km。震前所出现的平均空间旋转角最小值均低于其2倍标准差。该标准似乎可以作为大地震危险性增加的判据。

英文摘要：

This study investigates the average rotation angles between focal mechanisms of Landers and Hector Mine earthquakes in United State and that of their preshocks in ten years before the mainshocks and in circles with radius of 300km, 250km, 200km, 150km, 100km, and 50 km around their centroids. The results show the phenomenon of consistency of focal mechanisms of preshocks and that of mainshocks in both earthquakes. The average rotation angles between the focal mechanisms of mainshocks and that of the preshocks is decreased in 0.5 year before the mainshocks. The optimal radii of average rotation angle decrease are 250km and 100km, respectively. The lowest values of average rotation angle in both earthquakes are lower than twice of their standard errors. It seems that this criterion can be used to judge the probability increasing of a big earthquake.