2011 年 63 巻 3 号 p. 139-151
Recently, several researchers have elucidated crustal structures over a large area of the Japanese Islands from travel time inversion analyses. However, very few studies have paid attention to velocity discontinuities due to the limitations of spatial resolution. In this study, we apply a receiver function analysis to estimate seismic velocity structure and seismic velocity discontinuities of the crust in the Japanese Islands. We search for the best-correlated velocity structure model between an observed receiver function at each station and synthetic ones using a grid search method. Synthetic receiver functions are calculated from many assumed simple velocity structures that consist of a sediment layer to compensate for the effects of the low-velocity sediment layer and two velocity discontinuities. As a result, we clarified the spatial distributions of the crustal S-wave velocities and the tops of mantle depths. Several plain and basin areas are covered with thick low-velocity sediment layers. There are low-velocity layers corresponding to volcanoes in the upper crust (5-15 km deep). In the lower crust (15-25 km deep), our results show low-velocity structures in the eastern part of the Niigata-Kobe Tectonic Zone (NKTZ) and the Median Tectonic Line. Around the crust-mantle boundary, we see clear low-velocity zones beneath volcanoes, the western part of the NKTZ, and in the occurrence regions of the non-volcanic low-frequency tremor. High velocities near the southern coastline of the Japanese Islands correspond to the crust-mantle discontinuity of the subducting Philippine Sea plate. The crustal structure beneath the Itoigawa-Shizuoka Tectonic Line (ISTL) shows relatively low velocities in the shallower part and high velocities in the deeper part compared to neighborhood areas. The ISTL is also the boundary of the velocity structure of the upper crust in the Japanese Islands. The northeastern Japan region has heterogeneities of velocity perturbations, whereas the southwestern Japan region has the relatively stable high-velocity zones. The tops of mantle depths tend to increase in mountain regions with some undulations. The discontinuity of the subducting Philippine Sea plate at a depth of more than 40 km is extracted in several areas from the Kanto district to the Kyushu district. This suggests that the velocity discontinuity of the subducting Philippine Sea plate is larger than that of the overriding plate.