Dizhi lixue xuebao (Jun 2023)
Dynamic variation characteristics of in-situ stress in the 1605 Qiongshan M 7½ earthquake area and its implications to the Dongzhaigang subsidence, northeastern Hainan Island, China
Abstract
Based on the strain data of three in-situ stress monitoring stations in different sections of the 1605 Qiongshan 7½ earthquake area, we studied the dynamic variations of in-situ stress and extracted the sudden stress changes recorded by them. We analyzed the in-situ stress variations and tectonic activity between March 2016 and May 2018 to discuss the geomorphological evolution trends and subsidence mechanisms at the Dongzhai Port. The results show that the study area was generally subjected to the NW-compressive stress field, which makes the tensile stress field dominate in the Yanfeng and Dazhipo areas in the hanging wall of the Maniao-Puqian fault（MPF） and the Puqian-qinglan fault（PQF), while the Jinshan area in the footwall of the faults by compressive stress. The MPF fault and the PQF fault have been constantly engaged in non-seismic activities to adjust the local stress field under the regional stress field, among which the MPF Fault had several activities in March–July 2016, October 2017, and April 2018, and the PQF fault had two activities in October 2017. The energy of the MPF activities is more intensive than that of the PQF. The variation trend of the stress field indicates a gradual upward trend in the east bounded by the MPF fault (F13-1) and possible continued subsidence in Dongzhaigang in the west. The fault activity trend implies that the subsidence rate in Yanfeng, the northern part of Dongzhaigang, bounded by the MPF fault (F2-2), should be greater than that in the southern Sanjiang area. In addition, the volume strain monitoring data also reveals traces of magmatic activity in the lower part of the N–S seismic zone of Hainan Island. The comprehensive study concluded that the Dongzhaigang subsidence is mainly controlled by the positive fault activity of the MPF and PQF faults due to the upwelling of deep magma and influenced by the Holocene sea level change and the properties of soft soil depositional strata leading to soft soil flow slip, sand liquefaction, and seawater erosion. We innovatively apply the borehole strain observation technology to explore the evolution law and trend of typical earthquake subsidence landforms in coastal zones, which has essential academic value in the fields of in-situ stress monitoring and tectonic geomorphology research, and the results also have significant application value for mangrove protection and urban planning and construction in Dongzhaigang area.
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