LAUSR

Since phreatic eruptions often occur near the surface, it is important to understand the shallow subsurface structure of a volcano to clarify the eruption mechanism. The 2015 phreatic eruption at Owakudani, Hakone Volcano, Japan, was preceded by localized uplift and intensified steam activity. However, the cause of these events is unknown due to a poor understanding of the shallow subsurface structure. We report on a three‐dimensional resistivity structure inferred from dense audio‐frequency magnetotelluric surveys conducted in Owakudani that revealed a ∼100‐m thick conductive layer at the surface underlain by a resistive section, at the edge of which eruptions craters were located. This conductive layer is estimated to be a low‐permeability cap layer, because the inflation source prior to the eruption is located beneath it and the ejecta from the eruption was reported to have originated from a depth of 100 m or less.