Fig. 1

a A map of Japan showing the location of Mt. Ontake (solid triangle) in the central part of Japan. b A map of Mt. Ontake showing the locations of the summit peaks, eruption vents area (orange area). Kengamine (solid triangle) is the highest peak of Mt. Ontake (3067 m), and Otaki (triangle) is a peak about 500 m southeast of Kengamine. The 2014 eruption of Mt. Ontake volcano occurred from multiple vents (Eruption vents area) in Jigokudani area, southwest of Kengamine peak. Solid circles A, B and C are taking off and landing points, respectively, for the campaigns on November 20 and 21, 2014, and June 2, 2015. Contours are drawn every 100 m. The map was drawn with Kashmir 3D software (http://www.kashmir3d.com) using 10 m mesh DEM data of Geospatial Information Authority of Japan