Fig. 9From: Source constraints for the 2015 phreatic eruption of Hakone volcano, Japan, based on geological analysis and resistivity structureSchematic geological model of 2015 eruption center and adjacent area. Beneath the eruption center area, a vapor pocket is formed, and the steam migrates from the pocket to the surface through Layer IIa, which is a sub-unit of Layer II characterized by mineral formation due to boiling. Mineral assemblage from the borehole data indicates an underground temperature of approximately 200 °C, even at the bottom of SPW52. The open crack formed at 7:32 on June 29, 2015 (JST), triggered the eruption; however, the uppermost tip did not reach the surface. The steam emitted by the eruption originated from the vapor pocket, and craters were formed by the erosion of Layer A or Layer IIaBack to article page