11:30 AM - 11:50 AM
[SVC33-10] Assessment of lateral eruption hazards at Kusatsu-Shirane volcano: Mass transport in the subsurface as suggested by soil gas chemistry
★Invited Papers
Keywords:gaseous elemental mercury, Helium, soil gas, lateral eruption, Kusatsu-Shirane volcano, phreatic eruption
Kusatsu-Shirane is one of the most active volcanoes in Japan in terms of repeated phreatic eruptions and persistent release of a large amount of heat as hot springs from the flank of the volcano. It has been noted that the phreatic eruptions at Kusatsu-Shirane occurred not only in the craters at the summit but outside of them. If such a lateral eruption occurs near tourist facilities including restaurants and walking trails, even small phreatic eruptions can be expected to cause many fatalities. In this study we have developed a method for assessing the risk of lateral eruptions using soil gas.
We measured gaseous elemental mercury (GEM) fluxes from ground surface around the Yugama crater of the Shirane pyroclastic cone at Kusatsu-Shirane volcano. Since GEM in the atmosphere is very rare, it is expected to be a good tracer of magmatic or hydrothermal fluids from depth. GEM fluxes were measured in the range of 0.8 to 67×10-3 ng/m2/s, which is consistent with previous studies conducted at other volcanoes, including the Campi Flegrei geothermal area in Italy. In addition, we found high GEM fluxes on the southern slope of the Shirane pyroclastic cone where lateral eruptions have repeatedly occurred. Although no thermal features have been observed over the past 60 years, small amounts of magmatic fluids are suggested to be ascending beneath the southern slope. It has been noted that GEM is generated from not only magmas or hydrothermal reservoirs but also from deposits containing FeS and industrial wastes buried underground. Since Helium cannot originate from FeS or industrial wastes, we calculate Helium flux from the ground surface from measurements of soil diffuse CO2 flux and He/CO2 of the soil gas at 1 m depth each measurement sites. The result shows that relatively high He fluxes (up to 0.6 ng/m2/s in maximum) were found in the area where relatively high GEM fluxes were observed.
To examine the origin of the soil gases on the southern flank of the Shirane pyroclastic cone, we collected 20 cc of the soil gasses from a depth of 1 m at each measurement sites and analyzed 3He/4He, 4He/20Ne, 3He/CO2, and δ13C of the soil gas using equipment operated by Sumino Lab at Research Center of Advanced Science and Technology, the University of Tokyo. 4He/20Ne of the soil gases collected at sites on the southern flank reveal that the soil gases consist of several percent of gas originated from depth. Combining with the values of 3He/4He, we conclude magmatic Helium of 0.3 ng/m2/s is emitting from the southern flank.
Our results suggest that the southern flank of the Shirane pyroclastic cone is corresponding to the fracture zone which acts as a fluid pathway from a magmatic hydrothermal reservoir to ground surface, although no thermal features are observed. It is inferred that the risk of phreatic eruptions is relatively higher in such fracture zone. Our method is applicable even in areas without fumaroles and hot springs, therefore, it can be used to assess lateral eruption hazards in areas almost covered by vegetations. Because soil temperatures are not very high, gas sampling from soil is much easier than from fumaroles. In addition, since soil gas does not contain corrosive gases, equipment for soil gas monitoring can be installed on site. Thus, soil gas can be a new volcano monitoring tool.
This work is supported by Japan Society for the Promotion of Science under the Grants-in-Aid for Scientific Research of 18H01290 and 22K03735.
We measured gaseous elemental mercury (GEM) fluxes from ground surface around the Yugama crater of the Shirane pyroclastic cone at Kusatsu-Shirane volcano. Since GEM in the atmosphere is very rare, it is expected to be a good tracer of magmatic or hydrothermal fluids from depth. GEM fluxes were measured in the range of 0.8 to 67×10-3 ng/m2/s, which is consistent with previous studies conducted at other volcanoes, including the Campi Flegrei geothermal area in Italy. In addition, we found high GEM fluxes on the southern slope of the Shirane pyroclastic cone where lateral eruptions have repeatedly occurred. Although no thermal features have been observed over the past 60 years, small amounts of magmatic fluids are suggested to be ascending beneath the southern slope. It has been noted that GEM is generated from not only magmas or hydrothermal reservoirs but also from deposits containing FeS and industrial wastes buried underground. Since Helium cannot originate from FeS or industrial wastes, we calculate Helium flux from the ground surface from measurements of soil diffuse CO2 flux and He/CO2 of the soil gas at 1 m depth each measurement sites. The result shows that relatively high He fluxes (up to 0.6 ng/m2/s in maximum) were found in the area where relatively high GEM fluxes were observed.
To examine the origin of the soil gases on the southern flank of the Shirane pyroclastic cone, we collected 20 cc of the soil gasses from a depth of 1 m at each measurement sites and analyzed 3He/4He, 4He/20Ne, 3He/CO2, and δ13C of the soil gas using equipment operated by Sumino Lab at Research Center of Advanced Science and Technology, the University of Tokyo. 4He/20Ne of the soil gases collected at sites on the southern flank reveal that the soil gases consist of several percent of gas originated from depth. Combining with the values of 3He/4He, we conclude magmatic Helium of 0.3 ng/m2/s is emitting from the southern flank.
Our results suggest that the southern flank of the Shirane pyroclastic cone is corresponding to the fracture zone which acts as a fluid pathway from a magmatic hydrothermal reservoir to ground surface, although no thermal features are observed. It is inferred that the risk of phreatic eruptions is relatively higher in such fracture zone. Our method is applicable even in areas without fumaroles and hot springs, therefore, it can be used to assess lateral eruption hazards in areas almost covered by vegetations. Because soil temperatures are not very high, gas sampling from soil is much easier than from fumaroles. In addition, since soil gas does not contain corrosive gases, equipment for soil gas monitoring can be installed on site. Thus, soil gas can be a new volcano monitoring tool.
This work is supported by Japan Society for the Promotion of Science under the Grants-in-Aid for Scientific Research of 18H01290 and 22K03735.