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根据我国近几年来的震例资料,不少强烈地震发生之前,往往有震群活动。震群发生的部位,有的在未来地震的震中区及其附近,有的在与主震发震构造呈共轭关系的活动构造带上,也有的在主震发震构造带的延长线上,并与主震震中相隔一定的距离。震群出现的时间多在震前五个月,这种震群我们称之为前兆震群。前兆震群中最大震级M_b、震群持续时间T、震群距主震震中的距离D都与未来主震震级M_主大小无关。结合前兆观测资料分析,似乎可以认为震前5个月是强震短期前兆出现的上限异常时间。在这个时间段,前兆观测曲线或呈现转折,或呈现明显的加速现象,即由中长期缓变阶段开始转为短期加速变化阶段。另外,在强震震中沿活动构造带或沿块体边缘迁移时,迁移时间也有相隔5个月的现象。
According to the earthquake data in recent years in our country, there are often earthquake swarm activities before many strong earthquakes happen. Some parts of the earthquake swarm occur on or near the epicenter of the future earthquakes, some on the active tectonic belt in conjugate relationship with the seismogenic structure of the main shock, and some on the extension line of the seismogenic structural belt of the main shock , And separated from the main shock by a certain distance. The swarm occurred more than five months before the earthquake, which we call a precursor swarm. The maximum magnitude M_b, the duration T of the swarm, and the distance D between the swarm and the main epicenter in the precursor swarm are all independent of the magnitude of the future mainshock M_main. Combined with the precursory observation data analysis, it seems that 5 months before the earthquake can be considered as short-term precursory strong earthquake occurrence of the upper limit of anomalous time. During this period, the precursor observation curve may show a turning point or show a marked acceleration phenomenon, that is, from the mid-long-term gradual change phase to the short-term accelerated change phase. In addition, during the epicenter of strong earthquakes along the active tectonic belt or migration along the edge of the block, the migration time is also separated by 5 months.