【摘要】 <正> 凡余震频次超过余震衰减曲线误差包线,同时满足以下条件:在不超过半个月的时间内,至少有三个震级相近的主要地震M₁>M₂>M₃,△M_1,2≤1、△M_2,3≤1(△M_1,2=M₁—M₂,△M_2,3=M₂—M₃)时,我们称其为余震震群。 作者曾研究过邢台、海城、唐山等强震的余震活动与华北地区地震活动的关系,本文拟对发生在海域内的1969年7月18日渤海7.4级地震进行类似的研究。由于没有地更多还原

【Abstract】 For studying the window effects of aftershock swarms for a series of strong earthquakes in the North. China region, the detailed studies of the aftershocks for the Bohai earthquake of Ms=7.4 which occurred on July 18,1969 are not made possible since there were not local stations for controlling them. In this paper, the statistical studies are made on the Bohai aftershocks with Ms ≥1.8 recorded at Changdao seismic station about 120km away from the mainshock since 1973, and the catalogue of Bohai aftershocks compiled by Seismological Bureau of Shandong Province is also used for research of the aftershocks before 1973. The results show that the Bohai aftershock swarm is featured with the "window" effects of the regional stress fields in the North China region, similar to that of the aftershock swarms for the Xingtai, Haicheng, and Tangshan earthquakes. It appeared that the frequency was increased, the 6 value dropped, and the average magnitude of the monthly maximum earthquakes risen, for the Bohai aftershock series before the mid-strong earthquakes in the North China region. These are the facts indicating the increases of the stress fields of the aftershock areas, and the responces to the variations of the regional stress field in the North China region before the mid-strong earthquakes.The predictability of the "windows" of the Bohai aftershocks is also estimated in accordance with the probabilistic gain concept proposed by Aki and the method of statistical assessment of R values suggested by the Geophysical Institute of State Seismological Bureau.Finally, the preliminary analyses are made on the relation between the two characteristic stages of the Bohiai aftershock series and the occurrences of mid-strong earthquakes.更多还原