Seismogenic Structure of the 1605 Qiongshan M7$ \mathrm{½} $ Earthquake and Its Holocene Activity History in Northern Hainan Island, China: Evidence from Cross-Section Drilling and Shallow Seismic Profile

The 1605 M7$ \mathrm{½} $ Earthquake is the only earthquake in the history of China that has caused large-scale land subsidence into the sea, with the total area of land subsidence exceeding 100 km². The disaster has led to the sinking of 72 villages. There is still no clear understanding of the source seismogenic fault of this earthquake. In this work, we conducted a detailed study of the middle segment of the Maniao-Puqian fault (MPF), which is the epicenter area, through geomorphological survey, data collection, shallow seismic exploration, cross-section drilling, and chronological dating. The results showed that the middle segment of the MPF zone is composed of three nearly parallel normal faults with a dextral strike-slip: "Macun-Luodou fault (F2-1), Haixiu-Dongyuan fault (F2-2), and Changliu-Zhuxihe fault (F2-3)". And F2-2 is composed of two secondary faults, namely F2-2′ and F2-2″, with a flower-shaped structure buried under the ground. It is distributed nearly east-west, dipping to the north and has experienced at least five stages of activities since the Miocene. The vertical activity rates of F2-2′ and F2-2″ are ~2.32 and ~2.5 mm/a, since the Holocene, respectively. There were eight cycles of transgression and regression since the Miocene. The fault activity resulted in the thickening of the Holocene strata with a slight dip to the south, on the hanging wall, showing V-shaped characteristics. The MPF is likely the source seismogenic fault of the M7$ \mathrm{½} $ earthquake that hit Qiongshan in 1605.