【摘要】 本文利用邢台地震台网的地震目录资料,研究了邢台地区强震前小震的“密集—平静”特点及其空间分布的立体图象.研究结果表明: (1)1966年3月邢台地区发生的几个6级以上强震前小震均有“密集—平静”现象,但明显程度随时间减小.部份4.5级地震前小震也有“密集—平静”现象,但因出现在6级以上地震后的余震活动背景上,情况比较复杂.相对平静期的长度,密集期中最大地震的震级均与相应主震震级之间有一定关系.一般来讲,主震震级越大,平静期越长,密集期中最大地震震级也越大. (2)3月22日7.2级主震前,小震的“密集—平静”过程在空间上有清晰的物理图象.3月8日6.8级地震发生后,3月20日在其余震区的两个端部中介质强度较弱的南西端首先达到破裂强度,发生一系列中小地震,对应于主震前的密集期.3月21日至22日主震前小震明显减少,是主震前的相对平静期,而且小震有由南西端向北东方向迁移的趋势.显示震区应力场在迅速地改变,应力由南西端向北东端进一步集中,并促使22日在北东端尖端部位失稳而发生6.7级及7.2级主震.3月26日6.2级强震前小震的“密集—平静”过程在空间上也有类似清晰的物理图象. 此外,本文对大震前小震的“密集—平静”特点作初步的解释.更多还原

【Abstract】 The "dense-quiet" characteristics of small shocks prior to major shocks of the 1966 Xingtai earthquakes is investigated in this paper using an earthquake catalogue given by the Xingtai seismic network and so a 3-D picture of hypocentral distribution is established. Results show (1) small shocks before all major earthquakes of M≥6.0 that occurred in the Xingtai region in March 1966 are characterized by the "dense-quiet" phenomena,but is less distinguishable as time increases. Similar features are noted in small shocks before some of those Ms=4.5 strong shocks. In such cases,however,it is rather complicated as the small shocks emerge on the background of aftershocks of much stronger earthquakes of M≥6.0. The duration of a relative "quiet" period as well as the magnitude of the largest shock in a "dense" period is related to the magnitude of the succeeding major event. In general the higher the magnitude of the major event is,the longer the "quiet" period will last and the higher the magnitude of the largest shock in the "dense" period will be; (2) The "dense-quiet" process of small shocks before the Ms=7.2 mainshock of March 22 was clearly a physical process which was revealed by spatial distribution maps of the hypocenters. After the Ms=6.8 earthquake of March 8,the breaking strength of the crust was reached first at the southwest end of the aftershock zone,where the medium was weaker,causing a series of minor and moderate shocks on March 20. This corresponded to the "dense" period prior to the main shock of March 22. By contrast,the time period from March 21 to 22 before the main shock was the relatively "quiet" period during which seismicity was evidently lower and had a tendency to migrate from the SW end to the NE. This indicated a rapid change in the stress field of the earthquake area and a further transfer of stress concentration from the SW end to the NE end. It was responsible for the instability at the tip of the NE end that resulted in the Ms=6.7 and 7.2 major shocks of March 22. The same was true for the Ms=6.2 strong shock of March 26.In addition,the present paper intends to give a tentative explanation for the "dense-quiet" characteristics of small shocks before a strong earthquake.更多还原