Plume‐Fed Magmatic Fluids Weakened Crust and Promoted a Large Intraplate Earthquake in Northern Hainan (South China)

Abstract Large intraplate earthquakes are paradoxical because they occur far from active plate boundaries. Northern Hainan Island (South China) hosts the 1605 Qiongbei M7.5 earthquake within a volcanic province linked to the Hainan mantle plume. New shear‐wave velocity images reveal two shallow low‐velocity zones extending to ∼10 km depth: a stronger anomaly beneath young volcanic cones and a weaker anomaly beneath the seismogenic fault system. Both anomalies connect near ∼6 km depth and merge into a deeper anomaly at 12–30 km depth that spatially coincides with low‐resistivity structures imaged by magnetotellurics, indicating a vertically connected crustal magmatic‐fluid system. Locally thinned crust and low uppermost‐mantle velocities further suggest ongoing plume‐fed mantle upwelling. We infer that volatile‐rich fluids ascended into shallow fault zones, reducing rock strength and promoting rupture during the 1605 earthquake. These results link mantle upwelling to brittle fault failure and suggest that plume‐derived magmatic fluids may govern large intraplate earthquakes.