Investigating the 2023 Beibu Gulf ML5.4 earthquake sequence: seismicity, faulting, and the influence of the Hainan mantle plume fluids?

The Beibu Gulf and its surroundings are a significant intraplate seismic zone in the northwest of the South China Sea. At least 15 M>5 earthquakes and 4 M>6 events have occurred historically. In 1605, a destructive earthquake with M7.5 occurred in the southern Beibu Gulf, causing villages to submerge into the sea with an area exceeding 100 km² of land subsidence into the sea. This seismic zone has serious earthquake risk, but few studies focused on the moderate to strong earthquake sequence in this area, leading to unclear seismic structures, mechanisms, and triggers of earthquakes in the region. In this work, waveform data of the 2023 Beibu Gulf earthquake sequence (M_L5.4) are well analyzed based on 4-month data recorded at 51 broadband stations. One foreshock and 73 aftershocks were detected by template matching techniques. The entire earthquake sequence is relocated using the HypoDD relocation method and the focal mechanism solutions of 12 M>1.5 earthquakes are inverted. The results show that the 2023 Beibu Gulf earthquake sequence forms a belt in the NWW-SEE direction, with a lateral extent of ~5.0 km and a width of ~1.5 km. Vertical slices show that the earthquake sequence is distributed continuously and densely with a dip angle of about 50° toward the NNE direction. Most foreshocks and aftershocks have similar source mechanisms to the main shock including the NWW-trending fault plane having a strike of 288°, a dip angle of 51°, and a slip angle of 177°. The location and attitude of the seismic fault match well with the NWW-trending Wei-2 Fault (F1) identified by petroleum exploration. The stereographic projection of the P-axes indicates that the principal compressive stress direction is NW-SE in the source area, which intersects obliquely with the F1, resulting in the right-lateral strike-slip faulting of the 2023 Beibu Gulf earthquake sequence. The seismic images in the research area show that the location of the 2023 Beibu Gulf earthquake sequence corresponds well to the deep columnar low-velocity zone, indicating that the triggering of the event may be related to the magmatic-hydrothermal fluids produced by the Hainan mantle plume. By comparing the seismic activity characteristics of the surrounding areas, and combining geological with geophysical survey information, we believe that deep hydrothermal fluids and fault structures or weak zones are the main driving factors of seismicity in this area.