Configuration of the Philippine Sea plate in the Kanto district, Japan, estimated from SP and PS converted waves

In the Kanto district, the Philippine Sea plate (PHS) is subducting northwestward beneath the inland plate from the Sagami trough and the Pacific plate (PAC) is subducting westward beneath the PHS from the Japan trench. Because of the complexity of the geological structure, various models for the configuration of the PHS have been proposed (e.g. Ishida, 1992; Kimura et al. 2006; Hirose et al., 2007). In this study, we used SP and PS waves converted at the upper boundary of the PHS to estimate its depth variation. The advantage of the depth estimation utilizing converted waves is that the boundary can be estimated without seismicity near the boundary. On the other hand, small repeating earthquakes are also useful to identify plate boundaries since they are thought to be distributed on the boundaries (e.g. Kimura et al., 2006). We identified small repeating earthquakes in the Kanto district and selected earthquakes that occurred between the PHS and PAC (i.e., bottom of the PHS) based on the waveform similarity and their focal mechanisms. In the seismograms of the earthquakes, we found prominent phases between P and S waves, that are identified as SP and PS waves converted at the upper boundary of the PHS. We measured the SP-P (N=794) and S-PS (N=212) times for the earthquakes and estimated the upper boundary. We expressed the depth of the upper boundary of the PHS as a function of latitude and longitude in the form of power series and the factors of power series were determined by the inversion of the SP-P and S-PS time data, following Horiuchi et al. (1982). The estimated plate boundary was in good agreement with the location of small repeating earthquakes on the upper boundary of the PHS, and thus we also included the depths to the repeaters (N=48) as constraints in the inversion. The depth to the upper boundary of the PHS shows a very small inclination in the depth range between 30-40km beneath northern Chiba prefecture, where no small repeating earthquakes on the top surface of the PHS have been found. We infer the shape is due to the existence of the PAC under the PHS. From the comparison with tomographic study (Hirose et al., 2007), we found the estimated boundary was, in most cases, located near the upper boundary of low Vs, high Vp/Vs layer of the PHS which is interpreted as the oceanic crust of the PHS. Acknowledgements: The waveform data were provided by National Research Institute for Earth Science and Disaster Prevention (Hi-Net), University of Tokyo, Japan Meteorological Agency and Tohoku University.