One-dimensional shear-wave velocity structures beneath the Okinawa trough inferred from surface wave phase velocities

Structures beneath back-arc basins or marginal seas are as important as those beneath island arcs and forearcs to understand geodynamics in subduction zones. However a small number of studies focus on structures beneath backarc basins and marginal seas because of few seismic stations there. The present study focused on the structure beneath Okinawa trough. The Philippine Sea plate is subducting along the Ryukyu trench, forming the Ryukyu arc. The Okinawa trough is a back-arc basin behind the Ryukyu arc and is considered to be a rift zone. We inferred one-dimensional shearwave velocity structure of crust and uppermost mantle beneath the Okinawa trough. We also inferred one beneath the East China Sea continental shelf where is not a rift zone. We measured phase velocities of Rayleigh and Love waves. We then inverted the phase velocity dispersion curves for the one-dimensional shear-wave average structures along the paths by genetic algorithm. The shear wave velocities in the lower crust beneath the Okinawa trough is lower than that beneath the continental shelf. It may suggest magma intrusion in the lower crust. The shear wave velocities just below the Moho beneath the Okinawa trough is significantly lower than that beneath the continental shelf. It may suggest partial melting due to upwelling beneath the Okinawa trough. The SV velocities beneath the Okinawa trough are lower than SH velocities, while SV and SH velocities beneath continental shelf are similar to each other. These radial anisotropies beneath the Okinawa trough may be caused by the shape of the cracks or preferred orientations of mantle minerals