Large earthquakes that occur away from plate boundaries (i.e., large intraplate earthquakes), though rare, could cause heavy damage. Understanding their causes could help with seismic hazard assessment. In this study, we image the seismogenic structure under the 1969 Ms6.4 Yangjiang intraplate earthquake within the stable South China Block with a high-resolution three-dimensional shear-wave velocity model constructed from ambient noise tomography based on a high-density seismic array. The model images relatively low velocities at a fault intersection region from the surface to at least 13 km depth in the Yangjiang seismogenic zone. Given spatial links among the faults, surface seawater, and low velocities, we interpret the low velocities to reflect the presence of seawater-filled highly-fractured rocks in fault zones. We infer that the seawater infiltration could have elevated pore pressure in the long term and lowered effective stress, as evidenced by low earthquake b values at depths of 8-13 km. This process could explain the generation of the Ms6.4 Yangjiang earthquake. We further hypothesize that the large intraplate earthquakes within stable plates are prone to occur in regional weakening regions that tend to build up stress. Given sufficient tectonic stress accumulation, the long-term hydrologically-driven crustal stress variations via increasing pore pressure could play an essential role in triggering large intraplate earthquakes. This study could help seismic hazard assessment in the Guangdong-Hong Kong-Macao-Greater-Bay-Area, China.