Fig. 1

Location map. a Tectonic environment around Ioto Island. Lines show plate boundaries. Red triangles are active volcanoes in Japan. Ioto Island is a volcanic island belonging to the Izu-Bonin-Mariana volcanic arc. Square shows location of a reference GNSS station of GSI on Hahajima Island. b Map of Ioto Island. The broken circle is the estimated caldera rim of Ioto volcano using bathymetry. c Topography of Ioto Island and locations of continuous observation stations and eruption points. Motoyama is a resurgent dome formed in the central part of the caldera (Newhall and Dzurisin 1988). Suribachiyama is a monticule in the southwest of Ioto. Circles with numbers are eruption points after 1922. The numbers correspond to the numbers in Table 1. Crosses and squares are observation stations. Orange lines are active faults (Kaizuka et al. 1983, 1985). Contour intervals of topography are 10 m