The Quaternary activity of Qiongzhou Strait segment of Chang Liu-Xian Gou fault

- Context

As the eastern edge fault of Fu Shan sag, Chang Liu-Xian Gou fault is one of the most important active faults in northern Hainan Island. It has been continuously activity since Cenozoic and controlled the Quaternary volcanic activity. It is located in the area of Chang Chang coal mine in the south, and extends northward through the Xian Gou, Rong Shan, Chang Liu into the Qiongzhou Strait. However, limited by the marine geophysical exploration technology, it is in low degree on study of the spatial distribution and the Quaternary activity of the Qiongzhou Strait segment yet. So that we carried out a single seismic detection work to track the fault break point in Qiongzhou Strait.

- Methods

Since 2014, our team has carried out a single seismic detection work of about 2000km in the Qiongzhou strait, completing 10 NEE direction survey lines and 51 NNW direction survey lines at 2km intervals. The source is a 200-electrode seawater electric spark source, the negative electrode discharge, the resulting pulse energy range is 2000 J, and the hydrophone is a standard 8-unit hydrophone cable, which is suitable for the measurement of the 300 m water depth.

- Results

After data analysis and geological interpretation, the fault section of the Chang Liu-Xian Gou fault zone is found on the NEE seismic profile. It composed of three groups of graben-type fault systems, which are arranged between Ying Bin peninsula and Dong Shui Bay. In NEE direction line, each group of faults is composed of two faults, and the strata between faults are graben-type subsidence, dislocation 2 to 4 m, and Interrupting the seabed. The breakpoints along the NW direction extend over 10km and correspond to the land section of the Chang Liu Xian Gou fault.

- Interpretation

Volcanic activity is in fissure-type along the fault in the southern part of the Strait segment, forming NW basalt dyke, and developed graben-type fault on both sides. On the land, there are a large number of late Pleistocene - Holocene craters along the Chang Liu - Xian Gou fault. It indicates that the fault had been active in the late Pleistocene and Holocene, and it was used as a channel for the upwelling of magma.

5．Conclusion

To sum up, the active age of the southern part of the Qiongzhou Strait segment can reach late Pleistocene and Holocene, which is great significance to the assessment of earthquake risk of fracture.