关键词: 地震震源机制解, 应力场, 华北

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