New evidence for Holocene activity of the northern segment of the Puqian-Qinglan fault in northern Hainan Province

The Puqian-Qinglan fault, located in northern Hainan Province, is closely related to the judgment of the seismogenic structure of the Qiongshan M7½ earthquake in 1605. To accurately determine the latest activity characteristics of this fault, this study, adopting artificial shallow seismic exploration with large-tonnage seismic sources combined with small-spacing cross-well stratigraphic correlation, investigated the sedimentary geological conditions of the Quaternary basalt cover and significantly thick marine sand layers in northern Hainan Province through exploration at Qishantou and Dongzhai villages in the Sanjiang Town, Jiangdong New Area, Haikou. Key results are as follows: (1) the Puqian-Qinglan fault dislocates multiple marker strata, with the shallowest dislocation occurring in the silt layer of the Holocene Yandun Formation and upper fault contacts’ burial depths of approximately 10 m. As revealed by drilling, the fault surface exhibits significant strike-slip and thrusting properties, suggesting a large-scale fault zone with a certain width, multiple branches, and multi-stage activity; (2) Within the depth range revealed by the cross-well stratigraphic correlation, the displacement of the dislocated strata increases with depth. The Puqian-Qinglan fault has been active since 8 346-7 153 a cal BP. It exhibited a cumulative vertical displacement of 4-5 m during the Holocene, with vertical displacement rates of 0.53‒0.63 mm/a. Accordingly, it can be inferred that the Puqian-Qinglan fault serves as the seismogenic fault of the Qiongshan M7½ earthquake in 1605. The new evidence for the Holocene activity of the Puqian-Qinglan Fault obtained in this study provides a scientific basis for spatial land planning and regional seismic hazard assessment.