Fig. 1

Index map of Hakone volcano. a Location of Hakone volcano. b Shaded relief map of Hakone volcano. C Sampling locations. Blue lines indicate the Owakuzawa River and its tributary. Red star, soil gas location; yellow circles, fumarole; green squares, steam production well (SPW). The two digits before the hyphen indicate year of emergence (e.g., fumarole 15–2 was formed in 2015). Since the eruption crater and fumaroles are formed in an area within 100 m from the SPW52, the area is called an eruption center area