Abstract: By using the data of mechanism solutions of small earthquakes, we have analysed the temporal and spatial changes of stress field in different tectonic positions of the focal region and its adjacent areas before the Haichang earthquake of magnitude 7.3 in 1975,Tangshan earthquake of magnitude 7.8 in 1976, Heze earthquake of magnitude 5.9 in 1983, and Cangshan earthquake of M_S 5.2 in 1995. It is verified that there were some anomaly changes of stress field around the focal region before earthquakes. For instance, the axes of principal stresses had orientation coincidence in focal area as long as 4 years before the Tangshan earthquake. The P axis azimuths of composite mechanism solutions of small earthquakes concentrated first, and turned then, before the Heze earthquake. The azimuths of P axes deflected and its coincidence increased before the Cangshan earthquake. In the meanwhile, the stress field changes before the Tangshan earthquake may began earlier than 1972. The composite mechanism solutions of small earthquakes recorded by Douhe station, which is in the focal region, is different from that of Changli station, which is outside the focal area. The focal-mechanism solutions and the seismic activity in different stress areas near the focal fault have dynamic differences. To a certain extent, these phenomena indicate that the mechanical state in different tectonic positions of focal areas is not same and small earthquakes are developing in focal areas before strong earthquakes under different tectonic conditions and stress background. This has practical meanings for studying the earthquake preparation process and differences for different earthquakes.

Key words: Earthquake focal mechanism solution, Stress field, North China

摘要： 使用小震机制解资料,分析了1975年海城7.3级和1976年唐山7.8级强震及1983年菏泽5.9级和1995年苍山5.2级中强震前,震源区及其周围不同构造部位应力场的时、空变化,证实震前震源区附近应力场曾有某些异常改变,如唐山强震前震源区周围出现长达4a多的小震机制解主应力轴一致性取向的现象,菏泽地震前小震机制解P轴“集中—转向”,苍山地震前P轴偏转且一致性增强。同时还发现,唐山地震前应力场异常变化开始时间可能早于1972年;震源区内的陡河台与源外区的昌黎台小震综合机制解反映出震前的受力差异;震源断层附近不同应力区内震源机制解和地震活动有时空动态差异。这些现象一定程度上提供了不同构造条件和应力背景下,中、强震前震源区不同构造部位力学状态的改变或地震孕育过程的信息,对研究不同地震的孕震过程及差异有一定意义。

关键词: 地震震源机制解, 应力场, 华北