Seismogenic crustal structure affected by the Hainan mantle plume

Mantle plumes are one of the most influential first-order tectonic systems on Earth, playing a prominent role in controlling local magmatic activity and seismicity. Recent studies have shown that the Hainan mantle plume originates from the lower mantle and its volcanic activity is still intense during the Holocene. However, the effect of the Hainan plume on local seismicity is still unclear. In this study, we conducted the first wide-angle seismic survey in the source area of the 1969 earthquake doublet (M 5.1 and 5.2) offshore Hainan Island. A high-resolution 3-D P-wave velocity model of the crust is obtained using a large amount of Pg and PmP arrival times recorded by a portable array consisting of 17 ocean bottom seismographs (OBSs). Our results show that the 1969 earthquake doublet occurred in high-to-low velocity transitional zones which correspond to active faults. Prominent low-velocity anomalies are revealed in the crust of the study area, which may reflect magmatic intrusion channels. On the basis of multiple geophysical evidence including multi-channel seismic reflection profiles, geothermal data, and seismic tomography results, we infer that the low-velocity anomalies in the source area may reflect magmas and fluids from the Hainan plume, and high-angle normal faults in the crust serve as conduits for ascending of the magmas and fluids. The 1969 earthquake doublet offshore Hainan Island can be attributed to the magma-hydrothermal alteration processes in the fault zones, which led to the significant reduction of the fault strength and friction coefficient, facilitating the shear failure propagation. Furthermore, the seismicity in and around Hainan Island exhibits distinct swarm-like behaviors, which is also evidence for the Hainan plume affecting the crustal seismicity.