Rayleigh-wave Tomography and Azimuthal Anisotropy of the Philippine Sea Basin

The Philippine Sea plate (PSP) is surrounded by convergent boundaries. On the eastern side, the Pacific plate subducts beneath the PSP along the Izu-Bonin trench and Mariana trench. On the western side, the PSP subducts to the Eurasian continental margin along the Ryukyu trench, driving the arc-continent collision at Taiwan. Further south, the Caroline plate subducts under the southern part of the PSP along Yap trench. There are three major basins in the Philippine Sea: the West Philippine basin developed in 35-45 Ma at west, and the Shikoku and Parece Vela basins opened in 15-30 Ma at east. The back-arc spreading Mariana trough is active since 7 Ma, next to the Mariana trench. Previous studies show a significant discrepancy in seismic anisotropy structures for the PSP region, which perform different scenarios of tectonic evolution for this area. In this study, we conduct both isotropic and anisotropic Rayleigh-wave phase velocity structure of the PSP. The earthquakes larger than Mw 5.0 are screened out from the global seismic databases. Totally, 7,914 events are screened out in the period of 1998-2014 which can form the measurements of Rayleigh-wave dispersion curves along 397 station-pairs over the PSP. The measured dispersion curves are then inverted into the isotropic and azimuthally anisotropic (2Ψ) velocity maps at different periods with the proper damping and lateral smoothing in the LSQR inversion. The inversion is framed by the knots’ spacing in 400 km. The consequent velocity anomalies are referenced to the averaged values corresponding to the periods between 60 and 160 seconds. The derived velocity anomalies are consistent with the basins’ ages. The positive velocity anomalies are seen in the West Philippine basin, whereas the negative anomalies are found in the Shikoku and Parece Vela basins. A positive anomaly is also seen in the eastern PSP along the Izu-Bonin trench and north part of Mariana trench. It may correspond to the subducting Pacific slab beneath the PSP. On the other hand, the fast direction of the Rayleigh-wave anisotropy is found in NE-SW in the northern PSP at 60-100 s, which is parallel to the direction of extension. The anisotropy in the southern PSP is relatively minor. It is found to follow the direction of absolute plate motion (APM) of the PSP and the neighboring Caroline plate at 80-140 s. Such the APM-parallel pattern may reflect the mantle materials flowing as the direction of APM in the south. The plate collision around Taiwan forms a natural barrier to mantle flow resulting an ENE-WSW flow dominated by the NW subducting Ryukyu slab. Comparing to the other studies for the PSP, our result provides large scope of the mantle flows at the level of lithosphere and asthenosphere, reflecting the plate motions joint around the PSP. The variation in kinematic behaviors of the mantle flow at different subduction zones is also revealed.