Magma-hydrothermal System of Hakone Volcano

Abstract

 Hakone volcano has been in an active phase since 2001, as implied by frequent volcanic unrest every 2-5 years, with each accompanied by deep inflation (6-10 km), increase of deep low-frequency events (DLFEs) at a depth of ∼20 km, increase of CO₂/H₂S ratio in fumarole gas, and surge of volcano tectonic earthquakes (VT; < 6 km deep). A series of episodes of volcanic unrest culminated in a small phreatic eruption (erupted volume; ∼100 m³) in 2015; however, lesser unrest in terms of seismic activity occurred in 2017 and 2019. Recent studies on crustal structures based on seismic tomography indicate a magma chamber 10-20 km beneath the volcano, which might be connected to a large magma chamber beneath Fuji volcano, approximately 30 km NW of Hakone. Interestingly, the DLFEs beneath Hakone volcano seem to take place in a high attenuation zone that connects the magma chambers. Deep inflation beneath Hakone volcano, however, is clearly located at a shallower location than the magma chamber of Hakone. The increases of CO₂ and He within the fumarole of Hakone during its unrest may indicate degassing of magma at depth. The maximum fumarole temperature after the eruption and constraints on subsurface temperature (∼200°C at 400 m deep indicated by the mineral assemblage and ∼370°C at 4 km below sea level where is the lower depth limit of VT) imply a vapor-dominated hydrothermal system in the volcano from the bottom of the cap structure (∼100 m deep) to a depth of possibly 2-4 km. Such a vapor-dominated system may allow rapid transfers of magmatic gases and their emission from the fumarole area in the very early phase of volcanic unrest, as was observed. Hakone lacks long period events (LF) and volcanic tremors, which are common at many active volcanoes. Because such events are considered to be related to fluid migration, the vapor-dominated system can be attributed to their absence in Hakone. An estimation of the water mass balance implies that the amount and rate of inflation in the hydrothermal system are comparable to those emitted from the fumarole area in pre-eruptive calm periods. Thus, continuous inflation at depth can be explained by crystal depositions from the hydrothermal fluid. The high temperature of steam emitted in the fumarole area after the eruption indicates destruction of the container of the hydrothermal system, which also caused the lower VT activity and CO₂/H₂S ratio during post-eruptive unrest.