Fig. 1

a Map showing the locality of Hachijojima Island and b topographical map. In a, triangles indicate the locations of active volcanoes. The slab depths were sourced from Kita et al. (2010) and Liu and Zhao (2016). The solid and dashed contour lines denote the depths of the upper boundaries of the subducting Pacific and Philippine Sea slabs, respectively (Liu and Zhao 2016). The blue lines denote the plate boundaries at the surface. In b, the localities of submarine “Hachijo NW chain”, “Hachijo-kojima chain”, and “NE edifices”, consisting of many satellite cones (small triangles) and chains of vents (bars), are also shown (Ishizuka et al. 2008)