Earth Planets Space, Vol. 50 (Nos. 11, 12), pp. 939-952, 1998
Yasuyuki Nakamura and Takuo Shibutani
Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan
(Received March 31, 1998; Revised July 23, 1998; Accepted July 27, 1998)
The three-dimensional shear wave velocity structure in the upper mantle beneath the Philippine Sea was investigated with Rayleigh wave phase velocities in the periods 30-100 s. 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 Philippine Sea with 2-D tomographic technique without any a priori regionalization. The resolutions of the tomographic analysis were quite good in almost of the target region. The phase velocity maps were inverted for the 3-D shear wave velocity structure in the upper mantle down to 220 km.
In the shallow depths lateral heterogeneities with short wavelength were seen 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 explained by the evolution history of the Philippine Sea. The older western Philippine Sea had higher shear wave velocities than the younger eastern Philippine Sea. In the western Philippine Sea the central basin ridge, which is the youngest in this area, showed the low velocity anomaly. This is supported by the fact that the West Philippine Basin was formed in this area. In the depths 150-200 km the low velocity anomaly was dominant inside the Philippine Sea, which might suggest the existence of the mantle return flows. The thickness of the lithosphere in the south of the West Philippine Basin reached about 100 km, which is much thicker than the results of previous studies for this region.