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.