5:15 PM - 7:15 PM
[SVC32-P02] Volcanic gas measurement by using drone and Multi-GAS at Mt. Tokachi, Hokkaido in 2023 and 2024
Keywords:Mt. Tokachi, Multi-GAS
Mt. Tokachi, located in central Hokkaido, is an active volcano that has experienced three magmatic eruptions in the 20th century. Among the volcanoes in Japan during non-eruptive periods, some exhibit vigorous fumarolic activity, maintaining SO2 fluxes ranging from 100 to 2000 t/day (Yamaguchi et al., 2023). Therefore, understanding the gas supply mechanism at Mt. Tokachi is crucial for elucidating its volcanic activity dynamics. In this study, volcanic gas composition observations were conducted at Mt. Tokachi using drones and Multi-GAS in 2023 and 2024. The composition ratios of gases, including H2O, SO2, CO2, and H2S, were determined. Furthermore, temporal variations in the volcanic gas composition were analyzed to investigate the shallow gas supply system of Mt. Tokachi.
The volcanic gas composition varied among the primary gas emission sources of Mt. Tokachi: Crater 62-2, Taisho Crater, and the Furikozawa fumaroles. This suggests that the volcanic gas ascent processes differ among the gas emission sources. The higher CO2/SO2 ratio and lower SO2/H2S ratio observed at the Taisho Crater, compared to the 62-2 Crater, are considered to reflect the reduction of SO2 due to disproportionation reactions within the hydrothermal system (Kusakabe et al., 2000). This is consistent with the results of AMT observations conducted around the summit of Mt. Tokachi, which revealed a low-resistivity structure extending from beneath the 62-2 Crater to beneath the Taisho Crater, interpreted as reflecting the presence of a hydrothermal system (Tanaka et al., 2021). On the other hand, the Furikozawa fumaroles are considered to be the least influenced by the hydrothermal system and to emit magmatic gas, as indicated by its relatively high SO2/H2S ratio and low CO2/H2O ratio compared to other fumarolic sources. Based on these results, it is inferred that magmatic gas ascends toward the Furikozawa fumaroles and beneath the 62-2 Crater. At the Furikozawa fumaroles, the gas is released with minimal influence from the hydrothermal system, while at the 62-2 Crater and Taisho Crater, the gas is emitted after passing through the hydrothermal system.
Furthermore, a comparison of data observed at Crater 62-2 and Taisho Crater in 2024 with that from 2023 indicates an increase in the proportion of H2O in the volcanic gas composition at both craters. This suggests a decrease in magmatic gas supply and a shift toward a composition more strongly influenced by the hydrothermal system.
The volcanic gas composition varied among the primary gas emission sources of Mt. Tokachi: Crater 62-2, Taisho Crater, and the Furikozawa fumaroles. This suggests that the volcanic gas ascent processes differ among the gas emission sources. The higher CO2/SO2 ratio and lower SO2/H2S ratio observed at the Taisho Crater, compared to the 62-2 Crater, are considered to reflect the reduction of SO2 due to disproportionation reactions within the hydrothermal system (Kusakabe et al., 2000). This is consistent with the results of AMT observations conducted around the summit of Mt. Tokachi, which revealed a low-resistivity structure extending from beneath the 62-2 Crater to beneath the Taisho Crater, interpreted as reflecting the presence of a hydrothermal system (Tanaka et al., 2021). On the other hand, the Furikozawa fumaroles are considered to be the least influenced by the hydrothermal system and to emit magmatic gas, as indicated by its relatively high SO2/H2S ratio and low CO2/H2O ratio compared to other fumarolic sources. Based on these results, it is inferred that magmatic gas ascends toward the Furikozawa fumaroles and beneath the 62-2 Crater. At the Furikozawa fumaroles, the gas is released with minimal influence from the hydrothermal system, while at the 62-2 Crater and Taisho Crater, the gas is emitted after passing through the hydrothermal system.
Furthermore, a comparison of data observed at Crater 62-2 and Taisho Crater in 2024 with that from 2023 indicates an increase in the proportion of H2O in the volcanic gas composition at both craters. This suggests a decrease in magmatic gas supply and a shift toward a composition more strongly influenced by the hydrothermal system.