Regional variation in surface wave group velocities in the Philippine Sea

Abstract

Group velocities of Rayleigh and Love waves have been measured within the period range 30–80 s to study the regional variation of seismic wave velocity structures of the crust and upper mantle beneath the Philippine Sea. These group velocity measurements were made by applying the moving window method to long-period seismograms recorded at six WWSSN stations, which are located around the Philippine Sea plate. We divided the Philippine Sea region into seven small areas on the basis of tectonic features, age and topography of ocean floor, and determined the group velocities of the Rayleigh and Love waves in each region as a function of period. The surface wave group velocities in the Philippine Sea plate are higher than those in tectonic regions such as the Mariana arc, the Ryukyu-Japan arc and the Philippine trench. The low group velocities in tectonically active regions are attributable to the presence of an extremely low velocity layer in the upper mantle. In the Philippine Sea plate, the group velocities in the western region (the west Philippine basin) are higher than those in the eastern region (the Shikoku-Parece Vela basin). This result is interpreted as being due to the difference in the lithospheric thicknesses of the two regions. The lithospheric thickness estimated from the group velocity data of the Rayleigh wave is about 55 km in the western region and 40 km in the eastern region. The northern part of the west Philippine basin, covering the Amami plateau, the Daito ridge and the Oki-Daito ridge, has a thick crust, which is needed to explain the fact that the short-period group velocities are lower than those in the western and eastern regions of the Philippine Sea plate. These features are commonly found in the seismic velocity structures inferred from the group velocity data of Rayleigh and Love waves.