Fig. 1

a Index map showing Mt. Ontake (Ontake Volcano) and river systems on the southern slope of the volcano. The distribution of ashfall and pyroclastic density currents of the 2014 eruption is after Oikawa et al. (2014) and Takarada et al. (2016). These distribution maps are based on survey points mostly to the east, north, and west of the volcano; data for the southern medial–distal areas are very limited. The contour lines are the 50-cm and 0.1-cm isopachs and the 1-g/m² isopleth. b Field photograph taken at the river monitoring site showing the gray, turbid Akagawa River on the 46th day after the eruption