Seismological Constraints on the Causes of Intraplate Earthquakes in Yangjiang Region, South China.

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 dense nodal 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 intersection zone of faults, surface seawater, and low velocities, we interpret the low velocities to reflect the presence of seawater‐filled, highly fractured rocks created by the intersection of faults. We infer that the seawater infiltration through densely‐fractures could have elevated pore pressure in the long term, as evidenced by low earthquake b values at 8–13 km depths. We hypothesize that this long‐term seawater infiltration could have lowered the stress threshold of earthquake occurrence and contributed to the generation of the 1969 Ms6.4 Yangjiang earthquake. We propose that the large intraplate earthquakes within stable plates tend to occur in regional weakening regions (e.g., fault intersections) favorable to stress buildup. Given sufficient tectonic stress accumulation, the long‐term hydrologically driven crustal stress variations could play an essential role in triggering large intraplate earthquakes. This study could potentially contribute to improving seismic hazard assessment in the Guangdong‐Hong Kong‐Macao‐Greater‐Bay‐Area, China. Plain Language Summary: Large earthquakes usually occur at boundaries between two tectonic plates (i.e., interplate earthquakes). Away from a plate boundary, some large earthquakes, though rare relative to the interplate earthquakes, could occur within the interior of plates (i.e., intraplate earthquakes), such as the most notable Mw > 7.0 New Madrid earthquakes in 1811–1812. Why and how these large intraplate earthquakes occurred remain enigmatic. The Yangjiang region (South China) that hosted the 1969 Ms6.4 Yangjiang intraplate earthquake is one of the best places to study the causes of intraplate earthquakes. This study uses ambient noise tomography to image a low‐seismic‐velocity "chimney" (in the upper crust to 13 km depth) along the seismogenic fault zone. We interpret this low‐velocity "chimney" as a path for seawater infiltration through dense fractures created by the intersection of faults. The fault intersection zone at depths of 8–13 km is a favorable place for stress buildup, as indicated by low earthquake b‐values. We further hypothesize that seawater infiltration in the long term could have lowered effective stress through elevating pore pressure, probably explaining the cause of the Ms6.4 Yangjiang earthquake. We propose that long‐term hydrologically driven crustal stress variations could be essential in triggering large intraplate earthquakes given sufficient tectonic stress accumulation. Key Points: An upper crustal seismic shear‐wave velocity model was constructed under the 1969 Ms6.4 Yangjiang earthquake in South ChinaAmbient noise tomography images low velocities under the 1969 Yangjiang earthquake seismogenic regionWater‐filled dense fractures in fault zones can probably explain the presence of low velocities [ABSTRACT FROM AUTHOR]