Abstract:

Located at the southernmost margin of the South China Block, Hainan Island is influenced by both the spreading of the South China Sea and the upwelling of the Hainan mantle plume, resulting in regional relatively higher seismicity. To better investigate the characteristics of microseismic activity and its relationship with deep crustal structures in this region, we applied the LOC-FLOW earthquake-location workflow to continuous waveform data recorded from March 2023 to August 2024. The dataset comprises observations from 40 HAVESArray broadband portable stations and 28 provincial permanent stations. These portable and permanent stations complement each other and provide good azimuthal coverage, thereby enabling the construction of a high-resolution earthquake catalogue. A total of 2,671 high-quality earthquake events were obtained, with local magnitudes M_L ranging from -1.2 to 5.1. Most events with magnitudes greater than 3.0 occurred in the Beibu Gulf, north of the Leiqiong Strait. After relocation, the mean travel-time residual is 0.11s, and the average location uncertainties in the north-south, east-west, and depth directions are 0.50km, 0.48km, and 0.66km, respectively. The number of events identified in this catalogue is 7.5 times that reported by the China Earthquake Networks Center catalogue for the same period, and the magnitude of completeness Mc is reduced from 1.3 to 0.4. Our results reveal a distinct circum-island coastal distribution of microseismicity around Hainan Island. Four major seismic swarms are identified in northeastern Hainan, Wanning-Wenchang, Ledong, and Dongfang. The northeastern Hainan swarm forms a nearly north-south-trending seismic belt, which spatial distribution is inconsistent with mapped faults in the region, suggesting a complex local geological structure. The newly identified Wanning-Wenchang swarm exhibits a segmented NNE-SSW distribution and can be divided into northern, central, and southern segments, forming a bowl-shaped hypocentral geometry. The central segment reaches the greatest depth, extending to approximately 18km. The Ledong swarm generally follows a NW-SE trend, which is consistent with the principal compressive stress axis and indicates the structural control of the regional stress field on swarm development. The Dongfang swarm is characterized by clustered seismicity, steep faulting, and clear segmentation. By integrating the relocated catalogue with existing velocity models, we further discuss the roles of crustal heterogeneity and deep structures in controlling these seismic swarms. Under the influence of deep fluids associated with the Hainan mantle plume, crustal heterogeneities such as buried and deep-seated faults appear to exert a primary control on microseismicity. In addition, deep-fluid migration may contribute to earthquake generation, while the potential influence of seawater infiltration cannot be excluded. These results provide new constraints on the seismogenic mechanisms of Hainan Island and offer scientific support for regional seismic hazard assessment in and around the island.

Key words: HAVESArray, LOC-FLOW, Hainan mantle plume, Hainan Island

摘要：

海南岛位于华南块体最南部, 受南海扩张伸展与海南地幔柱上涌等动力作用的共同影响, 区域内地震活动较为活跃。为深入研究该区域微震活动与深部构造关系, 文中基于LOC-FLOW算法, 从HAVESArray台阵的40个宽频带流动台站及省级28个固定台站2023年3月—2024年8月的连续波形资料中拾取震相到时。研究结果显示, 共获得2 671次高质量地震事件, 震级为M_L-1.2~5.1。 其中, 3级以上地震多发生在雷琼海峡以北的北部湾海域。文中目录中的地震数量为同期中国地震台网目录(CENC)的7.5倍, 完备震级M_c从1.3降低至0.4。定位结果显示, 微震活动呈现出明显的环岛沿海分布特征, 分为琼东北、 万宁—文昌、 乐东和东方等4个主要震群。文中根据地震分布新发现了万宁-文昌断裂, 并讨论了其活动特性。结合已有的速度结构模型, 探讨了这些震群地壳介质和深部构造对地震活动的控制作用。在海南地幔柱的深部流体作用背景下, 地壳介质的非均质性, 如隐伏断裂和深大断裂起到了关键的控制作用, 深部流体作用可能对地震的发生发挥着重要作用, 但也不排除海水下渗的影响作用。文中结果为理解海南岛及邻区的地震孕育机制及区域强震风险评估提供了科技支撑。

关键词: HAVESArray台阵, LOC-FLOW, 海南地幔柱, 海南岛