P-wave anisotropic tomography of the central and southern Philippines

The first P-wave anisotropic tomography of the crust and upper mantle beneath the central and southern Philippines is obtained by inverting a large number of P-wave arrival-time data of local earthquakes and teleseismic events, which sheds new light on the geodynamic evolution of the study region. Our results show that fast-velocity directions of azimuthal anisotropy in the upper mantle below 250 km depth and the mantle transition zone change gradually from NE-SW to NW-SE northwards, possibly indicating a horizontal mantle flow in and around the mantle transition zone. The existence of the horizontal flow suggests that basalts of the Indian type mantle in the eastern part of the South China Sea possibly originated from the mantle beneath the Celebes and Molucca seas. Our results also reveal a positive radial anisotropy (V-pv < V-ph) reflecting a horizontal mantle flow between the Manila Trench and the northern part of the Philippine Trench, which may facilitate the subduction initiation along the northern Philippine Trench.