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.