The fine structure and seismogenic mechanism of the Yangjiang intraplate seismic zone in Guangdong Province, South China

Intraplate earthquakes occur within continental or oceanic plates, characterized by shallow focal depths, potential proximity to densely populated areas, and substantial destructive potential. To date, a scientific consensus of their controlling factors and seismogenic mechanisms remains elusive. Yangjiang, Guangdong, located along the southeastern coast of China, is a typical intraplate shallow-focus seismic zone. This study employed dense seismic array observations to identify and precisely locate seismicity in the Yangjiang region. We performed 3-D P-wave velocity (Vp) tomography using body-wave travel times and conducted joint inversions for S-wave velocity (Vs) structures by integrating body-wave travel times with surface-wave group-velocity dispersions extracted from ambient noise. Additionally, the Poisson's ratio structure was calculated. The results indicate that, except for the Xinhu Reservoir (Zone 1), seismic activity in Yangjiang primarily clusters in the Yangbian Bay (YBB), distributed along the NEE-trending Pinggang Fault. It can be further subdivided into two clusters: the western (Zone 2) and eastern (Zone 3) sectors. Zone 2, located at the intersection of the Pinggang and NW-trending Yangbianhai Faults, exhibits the highest seismicity, notably including a historic Ms6.4 earthquake. Tomographic results show that there are pronounced low Vs and high Poisson's ratio anomalies south of the Pinggang Fault beneath YBB (Zone3), suggesting seawater infiltration through the fault zone. A distinct low Vs and high Poisson's ratio anomaly at 5–13 km depth likely indicates enhanced seawater penetration through the fault intersection, potentially contributing to localized heightened seismicity. Compared to Zone 3, Zone 2 displays higher velocity and lower Poisson's ratio, indicating more rigid lithology that facilitates elastic strain energy accumulation and higher rupture thresholds. It could explain the occurrence of the Ms6.4 earthquake. This study advances the understanding of intraplate earthquake generation mechanisms and provides geophysical constraints for seismic hazard assessment and disaster prevention planning.