Joint Drilling Reveals No Detectable Quaternary Displacement at Selected Sites Along the Western Maniao–Puqian Fault, Northern Hainan

The 1605 Qiongshan M7½ earthquake resulted in the submergence of more than 100 km2 of coastal land. While the eastern segment of the roughly EW-trending Maniao–Puqian Fault (MPF) has been proposed as one of the seismotectonics that triggered this earthquake, the activity of its western segment, which may extend into the Yangpu free trade port, remains highly disputed due to a lack of evidence. On the basis of comprehensive interpretation from shallow seismic exploration and previous trench excavation, two joint drilling geologic sections were bored along each of the two branches of the western segment with a probing depth of approximately 100–120 m to determine whether it was active. Combined with chronological dating, these sections revealed Quaternary sedments with a thickness of approximately 15–50 m, underlain by the Pliocene Haikou Formation (N2h). Even the lowest part of the Haikou Formation was not drilled to the faults explained by the geophysical data. The suspected “fault” exposed by the trenches did not extend downward and was not formed by faulting. Consequently, the western MPF poses a relatively low surface-rupture hazard to the Lingao–Chengmai area and its surroundings.