Seismicity, faulting, and the magmatic fluid influence: Insights from the 2023 ML5.4 earthquake sequence in the Beibu Gulf

The Beibu Gulf and its surroundings represent a significant intraplate seismic zone in the northwest of the South China Sea. Historically, this region has experienced at least 15 M > 5 earthquakes and 4 M > 6 events, including a devastating M7.5 earthquake in 1605. Despite the serious earthquake risk in this seismic zone, few studies have focused on the moderate to strong earthquake sequences, resulting in unclear seismic structures, mechanisms, and triggers. This study investigates the 2023 Beibu Gulf (BBG) earthquake sequence (M_L5.4) using four months of waveform data from 51 broadband stations. We detected one foreshock and 73 aftershocks using template matching techniques, relocated the earthquake sequence with the HypoDD method, and inverted focal mechanisms for 12 M > 1.5 earthquakes. The results indicate that the 2023 BBG earthquake sequence forms a belt in the NWW-SEE direction, extending laterally ∼5.0 km and vertically ∼1.5 km, with a dip angle of ∼52° toward the NNE. Most foreshocks and aftershocks exhibit source mechanisms similar to the mainshock, corresponding with the NWW-trending Wei-2 Fault (F1). The principal stress direction in the source region is NW-SE, intersecting obliquely with the F1, resulting in the right-lateral strike-slip faulting of the 2023 BBG earthquake sequence. Ambient noise tomography reveals a deep columnar low-velocity zone beneath the sequence, suggesting a possible relationship with magmatic-hydrothermal fluids from the Hainan mantle plume. By integrating seismic activity characteristics and geological data, we conclude that deep hydrothermal fluids and fault structures or weak zones are the primary driving factors of seismicity in this region.