张北6.2级地震的孕育过程和前兆特征研究

张北地震发生在通常认为稳定的、浅部构造不明显，历史上地震活动微弱的地区，但是根据小震活动条带推测可能在一条ＮＷ走向的深部断裂。张北地震的孕育可能与区域断层的长期活动，中小地震持续活跃以及外围强震的增震作用有关。震前存在地震活动空段、４级地震条带、区域小震群活跃以及平静－收缩－信号震－外围活动增强等特征。有比较丰富的中长趋势异常，持续时间长达３年以上，而短临异常并不突出。对张北地震异常进行信度综合分

A STUDY OF DEVELOPMENT PROCESS OF THE ZHANGBEI M6. 2 EARTHQUAKE AND ITS PRECURSORY CHARACTERISTICS

The Zhangbei earthquake occurred in an area which was considered to be stable and where shallow tectonic structures were not obvious and seismic activity were week in history; but according to small earthquake activity belts, it can be inferred that there might be a NW strike deep fault. The development of the Zhangbei earthquake may be related to a long term activity of the regional fault,the continued activity of moderately strong and small earthquakes and the earthquake increasing action of strong earthquakes occurred in the outside area. Before the earthquake, there were seismic activity gap stage, magnitude 4 earthquakes belt, regional active small earthquake group and the characteristics of quiet-contraction-signal earthquake-activity increase in the outside area. There were relatively abundant mid-term tendency anomalies which lasted for more than 3 years,but the short-term and imminent anomalies were not outstanding.After the confidence comprehensive analysis for the anomalies of the Zhangbei earthquake,it is found that on matter how complicated and nonuniform the spacial distribution of the precursory anomalies are,the comprehensive confidence field always showed an extremely big value point,and showed a regular elliptical distribution comered around the extreme value,the earthquake occurred near the extreme value area.