Abstract: Based on processing, analyzing and interpreting the gravimetry and magnetometry data, it is confirmed that the segment of the coastal fracture zone in the Pearl River mouth is existent, it is a great fracture zone with large scale, depth over 30 km, obvious characteristics, and development in NEE direction. And it is a geologic block zone with high gravity, bounded by the sea coast fracture zone and the littoral fracture zone, and image of the gradient zone and grey degree zone of the gravimetry and the aeromagnetometry also showed that clearly. In the shallow part, it was intersected by NW and NE faults, but the NW faults intersected the littoral fracture zone and the NE fracture zone more frequently and clearly. However,in the deep layer with the depth over 10 km, the NE and NW fractures disappeared on the whole. According to some reference materials it is considered that the Lianhuashan fault belt in the NE direction extended to SW and went down to the sea in the research area, then the trend changed to the NEE direction, and the gravimetry and magnetometry data indicated that these faults in the NEE direction were a part of the coastal fracture zone actually. But entering the researeh area, the Lianhuashan fault belt extended still in the direction about N 45W, then went through Shenzhen and Hongkong. However,it was intersected by the coastal fracture zone at the Wanshan Islands. The image showed that quite clearly, but could not find the change of the trend from NE to NEE.
The coastal fracture zone had high activity,intersected with the NW and NE faults in the Pearl River mouth segment,and there was seismotectonic condition of M ≥ 5 in the places where the fractures dissected mutually, especially in the places where the coastal fracture intersected with the NW faults which devel-oped later and of high activity (such as Zhenhai Bay,Guanghai Bay, Yamen, Macao, and Dangan Chain Island area). But in the research area,the NW fault is shallower than in the border lands of Guangdong-Fujian Provinces and Guangdong-Guangxi Provinces (where h≥ 30 km), therefore in this area the earthquake strength should be lower accordingly.