1605年琼山M 7½级地震区现今地应力动态变化特征及对东寨港沉陷的指示意义

Dynamic variation characteristics of in-situ stress in the 1605 Qiongshan M 7½earthquake area and its implications to the Dongzhaigang subsidence,northeastern Hainan Island,China

文章利用1605年琼山7½级地震区不同区块布设的3个地应力监测台站应变观测数据,进行了地应力动态变化分析并对其记录的应力突变信息进行了提取,分析了2016年3月—2018年5月地应力变化与构造活动情况,并探讨了东寨港地貌演化趋势及其沉陷机理。结果显示:研究区总体受北西向挤压应力场作用,使得位于马袅-铺前断裂与铺前-清澜断裂上盘的演丰和大致坡地区以拉张应力场为主,位于断层下盘的锦山地区以挤压应力场为主;马袅-铺前断裂和铺前-清澜断裂在区域应力场作用下,不断进行非震活动来调整局部应力场,其中,马袅-铺前断裂在2016年3—7月、2017年10月和2018年4月有过多次活动,铺前-清澜断裂在2017年10月和2018年4月有过2次活动,马袅-铺前断裂较铺前-清澜断裂活动能量强;应力场变化趋势指示以铺前-清澜断裂(F13-1)为界,东部有逐步抬升趋势,西部的东寨港可能还会持续沉降;断层活动趋势指示以马袅-铺前断裂(F2-2)为界,东寨港北部演丰地区的沉降速率应该大于南部三江地区;此外,体应变监测数据也指示了位于海南岛南北地震带下部的岩浆活动痕迹。综合研究认为,东寨港沉陷主要受深部岩浆上涌导致的马袅-铺前断裂与铺前-清澜断裂的正断活动控制,并受到全新世海平面变化与软土沉积地层性质导致软土流滑、砂土液化以及海水侵蚀等多方面影响。文章创新性的将钻孔应变观测技术应用于探索海岸带地区典型震陷地貌演化规律与趋势研究,在地应力监测与构造地貌研究领域都有着重要的学术价值,同时,该成果对东寨港地区红树林保护、城镇规划建设等方面亦具有重要的应用价值。

Based on the strain data of three in-situ stress monitoring stations in different sections of the 1605 Qiongshan 7½earthquake 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.