Fig. 1

Tectonic setting of Mount Ontake. a Bathymetry of the central Japan region. Red triangles denote active volcanoes on the Japanese islands. Red contours are isodepth contours of the upper surface of the subducting Philippine Sea plate at 10-km intervals (Hirose et al. 2008), and blue dashed contours denote the upper surface of the Pacific plate at 50-km intervals (Nakajima and Hasegawa 2006). The black square indicates the location of the study area (Fig. 1 b). b Topographic map of Mount Ontake. Black circles denote the epicenters of relocated events (Fig. 4). Blue and red squares represent short-period (1-s natural resonance) and broadband seismic stations whose waveforms were analyzed in the present study. Stations with prefix “NU,” “V,” and “N” have been operated by Nagoya University, JMA, and NIED, respectively. ROPW and GNDT have been operated by Nagano Prefecture and Gifu Prefecture. Seismometer of V.ONTN, N.ASSH and N.KADH are emplaced in a borehole at a depth of ~100 m. Green arrow shows downward tilt changes recorded during the final 6 min before the 2014 phreatic eruption