Fig. 1

a Tectonic map showing the study area. Gray areas delimit regions deeper than 3000 m, highlighting the contrasting structure of the Philippine Sea plate between the Izu-Bonin volcanic arc and the Shikoku basin, consisting of typical oceanic crust. Black rectangle indicates location of plot (b). b Seismic experiments in and around the Izu collision zone. In this study, we used active-source data whose shot points are shown by red stars. Previous studies in the collision zone include seismic profiles whose shot points are indicated by yellow stars (Sato et al. 2005; Arai et al. 2009), green stars (Arai et al. 2013), and blue stars (Arai and Iwasaki 2014). Record sections of SP-W1, SP-W5, and SP-E4 are shown in Figs. 2 and 3. The Itoigawa-Shizuoka Tectonic Line (ISTL), Median Tectonic Line, and geological boundaries between the Izu-Bonin and Honshu arcs are shown by dashed blue lines. Geologically, the accreted Izu-Bonin arc blocks (hatched area) are delimited by the southernmost part of the ISTL. Black arrow indicates relative plate motion between the Philippine Sea plate and Eurasian plate (Seno et al. 1993). The GMT software (Wessel and Smith 1998) was used to draw the maps