Three-dimensional shear wave velocity structure in the upper mantle beneath the Philippine Sea region

The three-dimensional shear wave velocity structure in the upper mantle ben eath the Philippine Sea was investigated with Rayleigh wave phase velocitie s in the periods 30-100s. More than 900 Rayleigh wave phase velocity curves were obtained for this region with good path coverage. The phase velocity data were inverted for the phase velocity distribution maps in the Philippi ne Sea with 2-D tomographic technique without any a priori regionalization. The resolutions of the tomographic analysis were quite good in almost of t he target region. The phase velocity maps were inverted for the 3-D shear w ave velocity structure in the upper mantle down to 220 km. In the shallow depths lateral heterogeneities with. short wavelength were s een in the shear wave velocity maps. This might be related with complicated surface structures. In the middle depths the shear wave velocity was well correlated to the main tectonic features seen at the surface and well expla ined by the evolution history of the Philippine Sea. The older western Phil ippine Sea had higher shear wave velocities than the younger eastern Philip pine Sea. In the western Philippine Sea the central basin ridge, which is t he youngest in this area, showed the low velocity anomaly. This is supporte d by the fact that the West Philippine Basin was formed in this area. In th e depths 150-200 km the low velocity anomaly was dominant inside the Philip pine Sea, which might suggest the existence of the mantle return flows. The thickness of the lithosphere in the south of the West Philippine Basin rea ched about 100 km, which is much thicker than the results of previous studi es for this region.