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1956—1976二十年来,华北地区(106°E 以东,33°N 以北,42°N 以南,黄海以西)共发生震群37次,其中1966年邢台地震后到1976年唐山地震前,震群次数比1956—1966增加三倍多,近十年来华北四次七级以上地震就是在震群活动水平不断增高的背景上出现的.从空间分布上来看,1967—1972年震群分布范围相对缩小,仅靠近大震震中;1974年后,震群活动区域明显扩大,唐山地震前不到半年的时间内,震群活动沿唐山地震震中的 NE-SW 和 NW-SE 方向上分别延伸达一千多公里.这种地震活动随时间由未来大震震中逐步向外扩散的现象,在几次历史大地震前也类似出现过,据此粗略估计平均扩散速度约为100公里/年.除上述特点外,还观测到大震发生前一、二年至数月,在距未来震中不太远处(百公里)常发生前兆震群.分析了前兆震群与一般震群在震源机制、震源动力学参数、b 值以及 P 和 S 波振幅比等方面可能存在的差异.初步结果表明:与一般震群相比,前兆震群具有较高的应力降.最后,简要地分析了易于发生前兆震群可能的物理和构造条件.
In the past 20 years, a total of 37 swarms occurred in North China (east of 106◦E, north of 33◦N, south of 42◦N, and west of the Yellow Sea). After the Xingtai earthquake in 1966 and before the 1976 Tangshan earthquake , And the number of swarms more than tripled from 1956-1966. In the recent ten years, the earthquakes with magnitude more than seven occurred in North China appeared on the background of increasing earthquake activity. From the point of spatial distribution, the distribution of swarm in 1967-1972 The scope of the earthquake swarms narrowed relatively and only approached the epicenter of the epicenter. After 1974, the swarm activity area expanded obviously. Within less than half a year before the Tangshan earthquake, the swarm activity extended along NE-SW and NW-SE respectively in the epicenter of Tangshan earthquake This phenomenon of seismic activity spreading outwards over time from the epicenter of a large earthquake has also appeared before several major earthquakes, and a rough estimate of the average rate of diffusion is about 100 km / year. In addition to the above-mentioned features, it is also observed that one or two years to several months before the occurrence of a major earthquake and a precursor earthquake swarm often occurs not too far away from the future epicenter (100 km) , Source kinetic parameters, b values and P and S wave amplitude ratio The face possible differences preliminary results indicate that: Compared with the general swarms, Precursor Swarm last drop with higher stress, briefly analyzed Precursor Swarm possible physical configurations and conditions apt to occur.