2018年台湾海峡6.2级地震前地震活动及应力状态研究

通过对2018年台湾海峡6.2级地震震中及邻区地震活动、小震震源机制解、地震视应力等资料进行综合分析, 研究了6.2级地震前区域地震活动及应力状态特征, 结合大陆和台湾岛GPS测量资料的分析, 探讨岛陆两地相对运动对区域应力场等的影响.结果表明: (1)地震前5年起上述区域地震活动明显由弱转强, 中小地震及震群活动由海域逐步向粤闽赣湘交界广泛扩散, 表明地震活动增强可能是较大范围应力场增强所致; (2)震源机制和应力场的分析表明, 区域应力场呈现拉张(震前15年)-挤压(震前5年)-拉张(震前半年)的演化特征; 地震视应力计算结果同样显示震前5年开始区域应力水平快速大幅上升并持续至临震前半年转为下降; (3) GPS基线长期观测表明台湾岛相对大陆存在年度"分离-相聚"交替运动, 6.2级地震前两岸经历了"分离-相聚-分离"2次明显的倒转运动, 运动方式倒转时间与应力场及地震活动变化存在准同步性.分析认为台湾海峡及粤闽赣湘交界等区域应力状态及地震活动可能主要受制于两岸相对运动方式, 台湾岛震前持续近5年朝大陆挤压造成区域应力持续增强可能是导致6.2级地震以剪切破裂方式发生的直接因素.菲律宾海板块位于台湾岛东部的动力触角作用可能是造成台湾岛相对大陆往返运动最主要的驱动力.

Research on the seismic activity and stress state before the Taiwan strait 6.2 earthquake in 2018

Based on the comprehensive analysis of the seismic activity data, the focal mechanism of small earthquakes and the seismic apparent stress data in the epicenter of the 2018 M6.2 Taiwan Strait earthquake and its adjacent areas, the characteristics of regional seismic activity and stress state before the M6.2 earthquake were studied. Combined with the analysis of the GPS measurement data of Chinese continent and Taiwan Island, the influence of the relative motion between Chinese continent and Taiwan Island on the regional stress field was discussed. The results show that: (1) in the 5-year period prior to the 6.2 earthquake, the seismicity in the study area had obviously changed from weak to strong, with small-and-medium earthquakes and swarm activities gradually spreading from the sea area to the border region of Guangdong-Fujian-Jiangxi-Hunan provinces, indicating that the enhancement of seismic activity may be caused by the strengthening of the stress field in a larger area; (2) the analyses of focal mechanism and the stress field show that the evolution of the regional stress field in the study area is characterized by tension (15 years before the 6.2 earthquake)-compression (5 years before the 6.2 earthquake)-tension (6 months before the 6.2 earthquake). Calculated seismic apparent stress also showed that the regional stress level in the study area increased rapidly, and followed by substantially declining for a period from 5 years before the 6.2 earthquake to six months before the impending earthquake; (3) the GPS baseline long-time observation showed that there is annual "convergence-separation" alternating movement in the Taiwan Island relative to the continent. Before the 6.2 earthquake, the cross-strait experienced two obvious reverse movements of "separation-convergence-separation" and the reverse time in the convergence-separation mode had quasi-synchronous with changes of the stress field and seismic activity in the study area. The analyses shows that the stress state and seismic activity in the Taiwan Strait and bordering region in Guangdong-Fujian-Jiangxi-Hunan may be mainly controlled by the relative motion modes of the cross strait, and that the occurrence of the 6.2 earthquake in shear rupture mode could be led by the direct factor as in the continuous strengthening of the regional stress field caused by the Taiwan Island extrusion towards the continent for nearly 5 years prior the 6.2 earthquake. The dynamic tentacle effect of the Philippine Sea plate to the east of Taiwan may be the main driving force of the round-trip movement of the Taiwan Island relative to the continent.