ANALYSIS OF RELOCATED EARTHQUAKES IN NORTH CHINA REGION
华北地区地震重新定位结果分析

We relocuted epicenters of totally 6 771 earthquakes which occurred in North China region (35°～42°N, 111°～120°E) from 1993 to 2004. The relocation shows those formerly randomly-distributed earthquakes are clustered and are aligned tending to the North-East and North-West directions. It follows that there is a close relationship between the seismicity and known major active faults. From the cross sectional profile beneath the Xinhe fault, the relocated seismic sources reveal two seismic belts of different depths. One of them is around 60km long and in the direction of north-east, its depth varies from the surface downwards to 20 km underground. The other is about 40 km in length and is from 25km to 30 km in depth.At the same time, we found that the 83 percent relocated hypocenters vary from 0 km to 15 km in depth. This is certainly more shallow than that from the previous studies which are based on the roubine earthquake location, whose average focal depth is within a range of 7～21km. This result indicates that the contemporary seismicity in North China region is mainly concentrated in the upper and middle crusts. Furthermore, about 92 percent focal depth is ranged from 1 km to 24 km, which is in good agreement with the consensus that the lower bound of the focal depth in North China region is 24 km.