Surface wave tomography and azimuthal anisotropy of the Philippine Sea Plate

Long period and broadband waveform data from 1984 to 2007 extracted mainly from the IRIS Data Center with additional data from 1997 to 2007 extracted from the Ocean Hemisphere Project is used to perform a detailed mapping of Rayleigh wave group velocity and anisotropic structures of the Philippine Sea Plate (PSP). Our results show prominent high Rayleigh wave group velocity anomalies and heterogeneity in three marginal basins of the PSP at period up to 30 s. The convergent boundaries around the PSP are associated with low-velocity anomalies. Heterogeneity of velocity distribution at deep depths along the Ryukyu trench is also observed. Our results indicate that the azimuthal anisotropy in the PSP is depth dependent. The fast direction in the West Philippine Basin and the Shikoku Basin is in NE–SW at shorter periods, which is correlated with fossilized fabric produced by final phase of deformation, whereas the fast direction in the Parece Vela Basin is complex at shorter periods and becomes uniformly NE–SW-oriented at longer periods. At the depths near the lithosphere–asthenosphere boundary, the fast directions become parallel to the absolute plate motion for various parts of the PSP. The fast directions along subduction zones around the PSP are predominately trench-normal. The relatively simple patterns of azimuthal anisotropy may imply that the observed anisotropy originates from the A-type LPO of olivine fabric at shallow depths in the back-arc region and with slab-entrained mantle flow at deep depths.

► Prominent velocity heterogeneity is observed at lithospheric depths. ► The signature of azimuthal anisotropy is clearly depth dependent. ► The anisotropy at shorter periods is related to lattice-preferred orientation. ► Influence on LPO is replaced by absolute plate motion with increasing depth. ► Trench-normal fast directions are observed along convergent boundaries.