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

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