The stress field and seismogenic structure in northern Hainan island based on current small earthquakes

Based on the seismic waveforms recorded by Hainan seismic network in the northern Hainan island since 2000,103 earthquakes were relocated by double-difference method and some focal mechanisms were determined by amplitude ratio. We inverted the regional stress field from the focal mechanism and two focal fault planes from the precise hypocenters, and inferred the slip type of the focal fault under the regional stress field. Contrasting with the isoseismal line of the historical earthquake,the WNW focal fault located in the eastern central part of the meizoseismal area, which has the same direction as the major axis of the meizoseismal area.This indicates the focal fault may be the seismogenic structure of Qiongshan earthquake in 1605. The northeast wall of the high angle focal fault slipped down to southeast, which makes the NNW quadrant extensional and leads to the land subsidence into the sea. Another NS focal fault just located on the boundary between the Quaternaryvolcanic rocks and Quaternary basin where the lifting movement is the most intense.The eastern wall of the normal focal fault dropped, which was consistent with the sedimentary basin. The results show that this two focal faults are related to the historical earthquake and tectonic movement,but do not coincide with the surface fault, indicating an uncoordinated phenomenon between the shallow and deep tectonics.