New evidence for Holocene activity of the northern segment of the Puqian-Qinglan fault in northern Hainan Province
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摘要: 琼北地区铺前−清澜断裂是一条关系到1605年琼山7½级大地震发震构造判断的重要断裂。为准确厘定该断裂的最新活动特征，针对琼北地区第四纪玄武岩盖层和巨厚海相砂沉积等地质条件，采用大吨位震源的浅层人工地震勘探和小间距钻孔联合地质剖面探测相结合的方法，分别在海口江东三江镇岐山头村和东寨村展开探测。结果表明：(1)铺前−清澜断裂断错了多个标志地层，最浅断错至全新统烟墩组淤泥层，上断点埋深10 m左右，钻探所揭示的断层面明显具有走滑兼具逆冲性质，是一条具有一定宽度、多分支、多期次活动的大规模断裂带；(2)钻孔联合地质剖面所揭示的地层深度范围内，被断错地层的位移量随着深度增加而加大，铺前−清澜断裂自8 346~7 153 a cal BP以来有过活动，全新世累计垂直位移量4~5 m，垂直位移速率为0.53~0.63 mm/a，推断其为1605年琼山大地震的发震断裂。本研究取得的铺前−清澜断裂全新世活动的新证据，为国土空间规划和区域地震危险性评价提供了科学依据。Abstract: The Puqian-Qinglan fault, located in northern Hainan Province, is closely related to the judgment of the seismogenic structure of the Qiongshan M7.5 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 PuqianQinglan 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 M7.5 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.
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