2:00 PM - 2:20 PM
[SVC34-08] Monitoring and evaluation of Hakone volcano
★Invited Papers
Keywords:Hakone volcano, unrest, hydrothermal system
Hakone volcano seems to have been active since the beginning of the 21st century and volcanic unrests have been observed every few years. These unrests start with increase of baseline length across the volcano and surge of deep low-frequency events then followed by an increase of volcano tectonic earthquake (VT) and change in the gas ratios in the fumaroles. Sometimes, the unrest accompanies intensive steaming from steam production well in Owakudani steaming area. Based on these experiences, Japan Meteorological Agency introduced Volcano Alert Level (VAL) scheme to Hakone volcano in 2009 and three criteria, increase of baseline length, surge of VT and emergence of significant steaming activity are set to raise the VAL from 1 to 2. The 2015 eruption was a very small phreatic eruption but it occurred after declaration of VAL 2. Since such clear pattern of unrest has been recognized in Hakone volcano, the evaluation of volcanic activity in hakone appears to be successfully established; however, recent findings indicate there are clearly many challenges to improve evaluation of the volcanic activity. In this talk, I will review magma-hydrothermal system of the volcano based on analysis of the unrests and the eruption and points out the task ahead.
The unrest in Hakone volcano can be explained by the migration of magmatic fluid from the depths and its accumulation just below the brittle-ductile boundary, which causes pore pressure rise in the hydrothermal system. In the fumarolic gases, ratios of SO2, HCl, and He, which are considered as magmatic origin, are observed to increase during the unrests, but the total amount of SO2 released does not increase significantly. This observation suggests that magma intrusion into the shallows hydrothermal system is highly unlikely. In the post-eruptive unrests, the fumarole temperature does not change and seems to be constrained by temperature of vapor in the vapor-liquid coexistence system beneath the surface of the volcano. The fast change of volcanic gases almost simultaneously with the start of crustal deformation suggests that the shallow part of the hydrothermal system of Hakone volcano could be a vapor dominant system. The reason why the 2015 eruption of Hakone volcano ended in a small-scale eruption can be explained by that the explosion did not propagate to the deep part of the volcano due to the existence of a vapor-dominated system.
The activity of Hakone Volcano can be roughly explained by the above modeling, but geologically, there is a completely different mode of eruption. Although the 2015 eruption of Hakone volcano was very small in magnitude with a localized crater opening in Owakudani, a series of fissure vent up to more than 1 km in length, which may have been caused by past phreatomagmatic eruptions, have been recognized at Hakone volcano. It is noteworthy that one of them runs directly above the crack formed on the day of the 2015 eruption as observed by InSAR and tiltmeters. Phreatic eruptions from long multiple fissures have also been observed in the 2018 eruption of Kusatsu-Shirane (Moto-Shirane) volcano, which was triggered by the rapid supply of fluid just below the brittle-composition boundary into the shallow part of the volcano (Tseng et al., 2020; Terada et al., 2021). Such a rapid evolution of volcanic activity has not been assumed for Hakone volcano. In addition, the assumed crater area was set only in Owakudani, and other fumarolic zones and ancient fissures have not been considered.
The probability of eruptions outside of Owakudani needs to be assessed by detecting potential hydrothermal system beneath the volcano using InSAR and magnetotelluric surveys, and by analyzing phase distribution using hydrothermal system simulations. On the other hand, since there is no record of disasters caused by eruptions, it is hard to convince residents to be prepare for eruption. It is thus necessary to develop evaluation criteria of eruption outside Owakudani based on experience of past eruptions in other volcanoes. Also, probabilistic evaluation of disaster potential based on simulations will be needed.
The unrest in Hakone volcano can be explained by the migration of magmatic fluid from the depths and its accumulation just below the brittle-ductile boundary, which causes pore pressure rise in the hydrothermal system. In the fumarolic gases, ratios of SO2, HCl, and He, which are considered as magmatic origin, are observed to increase during the unrests, but the total amount of SO2 released does not increase significantly. This observation suggests that magma intrusion into the shallows hydrothermal system is highly unlikely. In the post-eruptive unrests, the fumarole temperature does not change and seems to be constrained by temperature of vapor in the vapor-liquid coexistence system beneath the surface of the volcano. The fast change of volcanic gases almost simultaneously with the start of crustal deformation suggests that the shallow part of the hydrothermal system of Hakone volcano could be a vapor dominant system. The reason why the 2015 eruption of Hakone volcano ended in a small-scale eruption can be explained by that the explosion did not propagate to the deep part of the volcano due to the existence of a vapor-dominated system.
The activity of Hakone Volcano can be roughly explained by the above modeling, but geologically, there is a completely different mode of eruption. Although the 2015 eruption of Hakone volcano was very small in magnitude with a localized crater opening in Owakudani, a series of fissure vent up to more than 1 km in length, which may have been caused by past phreatomagmatic eruptions, have been recognized at Hakone volcano. It is noteworthy that one of them runs directly above the crack formed on the day of the 2015 eruption as observed by InSAR and tiltmeters. Phreatic eruptions from long multiple fissures have also been observed in the 2018 eruption of Kusatsu-Shirane (Moto-Shirane) volcano, which was triggered by the rapid supply of fluid just below the brittle-composition boundary into the shallow part of the volcano (Tseng et al., 2020; Terada et al., 2021). Such a rapid evolution of volcanic activity has not been assumed for Hakone volcano. In addition, the assumed crater area was set only in Owakudani, and other fumarolic zones and ancient fissures have not been considered.
The probability of eruptions outside of Owakudani needs to be assessed by detecting potential hydrothermal system beneath the volcano using InSAR and magnetotelluric surveys, and by analyzing phase distribution using hydrothermal system simulations. On the other hand, since there is no record of disasters caused by eruptions, it is hard to convince residents to be prepare for eruption. It is thus necessary to develop evaluation criteria of eruption outside Owakudani based on experience of past eruptions in other volcanoes. Also, probabilistic evaluation of disaster potential based on simulations will be needed.