Pn anisotropic tomography underneath the South China Sea and surrounding areas and its geodynamic implications

Abstract In the present study, we present a new Pn anisotropic tomographic model of the uppermost mantle underneath the South China Sea and surrounding areas through the inversion of 23,042 high-quality hand-picked Pn arrival-time data. Our model shows obvious lateral heterogeneities in velocity and anisotropy that are closely correlated to surface geological structure in the study region. High Pn velocity anomalies are generally observed in the structurally stable areas, such as the South China block and Pearl-River Mouth basin. Obvious low Pn velocity anomalies are imaged underneath the Tengchong volcano, the Hainan volcano and other volcanoes along the Ryukyu, Luzon, Philippine and Java arcs, whereas the low-to-high-velocity transition zone is visible underneath the volcanoes in the Sumatra arc. Most large crustal earthquakes occurred in the low Pn velocity areas in the Ryukyu, Luzon, and Philippine arcs and the low-to-high Pn velocity transition zones in the Sumatra and Java arcs. These results suggest that these volcanism and earthquake occurrences may be closely related to the deep dynamics of the subduction of the Philippine Sea and Australian plates. High Pn velocity areas with trench-perpendicular fast directions of Pn propagation are revealed in the Ryukyu, Philippine, Sumatra, and Java trenches, presenting the subducting Philippine Sea and Australian plates, whereas low Pn velocity areas with trench-parallel fast directions are visible in the arc areas, suggesting the compressed mantle flows there. Our results provide new seismological evidence for understanding the mantle geodynamics beneath the South China Sea and surrounding areas.