Numerical simulation of the hydrothermal system of Hakone volcano

The eruption of fumarolic gases and hot springs from active volcanoes indicates the existence of magmatic hydrothermal systems. Numerical simulations were performed to reproduce the surface manifestations at Hakone volcano. At this site, high-chloride hot springs are concentrated at the northeastern foot of the central cone. The simulation indicates that this distribution of hot springs can be explained by topographic effects. When supercritical water containing NaCl rises from depth beneath the central cone, vapor–liquid separation occurs near sea level, and the NaCl-rich liquid phase flows laterally because of the influence of the topography and discharges from hot springs. Meanwhile, the vapor phase continues to rise, forming a vapor-dominated hydrothermal system beneath the central cone, which leads to fumarolic activity at the summit area. This relatively simple model can reproduce the hydrothermal system of Hakone volcano, but only if we consider topography. By comparison between observation and numerical simulations, we estimated the rate and location of supercritical water upflow. This upflow location corresponds with the source area of crustal deformation observed immediately before the 2015 eruption. We suggest that a disturbance in this flow system may have triggered the eruption.