Abstract: The Bohai earthquake (M=7.4,38°20′N,119°27′E) occurring on July 18,1969 is typical of a main-aftershock.Analyses of regional seismicity indicate some anomalous variations,such as b-value decrease and belt-shaped distribution and so forth.Based upon detailed study for revised aftershock data,we have found characterized spatial distribution of aftershocks from lateral cluster,vertical diffusion to lateral one,vertical cluster,b-value decreasing and p-value high,and the like.Also discussed in this paper are some details about Bohai earthquake and M=7.3 Haicheng earthquake of 1975.

摘要： 1969年7月18日渤海海域(北纬38°20′,东经119°27′)发生7.4级地震,属主震-余震型。 区域地震活动分析表明,主震前出现小震条带、b值低等异常现象。研究重新校订的余震目录资料,发现余震空间分布具有随时间由横向集中、垂向扩散到横向扩散、垂向集中的过程,及b值高、P值大等特点。另外,对于渤海地震与1975年海城7.3级地震的同异点等问题进行了讨论。