Tectonic map in and around the Izu-Honshu collision zone. The black broken lines show the depth contours of the upper surface of the Philippine Sea (PHS) plate beneath Kanto region, as estimated by Ishida (1992). The gray broken line indicates the location of the volcanic front as reported by Kaizuka (1975). The PHS plate is subducting northwestward beneath the Eurasian plate at a rate of 4 to 5 cm/yr, as indicated by the black arrow (Seno, 1993). The squares and solid and open triangles indicate the locations of the stations used in the present study (squares: NIED Hi-net, solid triangles: HSRI, open triangles: JMA). The rectangle indicates the region shown in Fig. 2.