Presentation Information
[O12-P15]Volcanic Gas Composition Monitoring Using the Simple Dilution Method at Ibusuki Volcanoes
*Riku Yokoyama1, *Reo Sonoda1 (1. Ikeda Senior Highschool)
Keywords:
volcanic gas composition,Low cost dilution approach using detector tubes and gas sensors,CO2/H2S ratio,SO2/H2S ratio
1 Introduction
Prediction of phreatic eruptions like Ontake is difficult because preeruption magma volume changes are small compared with magmatic eruptions such as Sakurajima, so crustal deformation precursors are hard to detect. Changes in magma-derived components in volcanic gas can reflect magma activity or hydrothermal system changes and may serve as precursors. We began monthly observations in December 2021 at the active Ikeda-Yamakawa area, where seismicity has increased since 2007 (Fig.1). Because conventional monitoring uses expensive instruments, we developed a safe, low-cost method usable by high school students. We selected three fumarole fields in Ibusuki volcanoes (Gongen, Minamisakoda, Unagi-ike; Fig.2) for monthly sampling.
2 Methods
From 2021 we conducted monthly monitoring at three sites. For trace volcanic gas components CO2, SO2, and H2S, we developed a method to accurately dilute samples into the effective measurement ranges of detector tubes and low-cost commercial gas sensors (Fig.3).
Using these instruments we measured concentrations and calculated CO2/H2S and SO2/H2S ratios for each monthly survey. Tracking ratio changes may reveal variations in magma activity and hydrothermal conditions. Based on four years of ratio data, we proposed a subsurface volcanic gas behavior model and compared results with other Kyushu fumarole fields.
3 Results and Discussion
(1) Subsurface model: We propose a subsurface structural and hydrothermal model for Ibusuki Volcanoes (Fig.4). Magma-derived gases are normally impeded by crustal stress (Fig.5); earthquakes release stress and allow trapped gases to rise to the hydrothermal system and vent, so gas signals may increase after seismic stress release.
(2) Composition ratio variations: Shallow earthquakes beneath Ibusuki since 2007 include events with hypocenters <10 km that may be volcanic. We used SO2/H2S and CO2/H2S as indicators. All three sites showed a clear SO2/H2S increase after the Gongen earthquake on 15 Nov 2023 (Fig.6, Fig.7). Earthquakes after 2025 showed no CO2/H2S increase and only slight SO2/H2S rises.
(3) Comparison with Kyushu volcanoes: Gongen and Minamisakoda have CO2/H2S averages similar to high-ratio volcanoes such as Aso and Unzen; Unagi-ike is lower (Fig.8, Fig.9).
4 Conclusion and Future Work We constructed a subsurface structural model for Ibusuki volcanoes from volcanic gas ratio variations. Ibusuki volcanoes belongs to the high CO2/H2S ratio group among Kyushu volcanoes. We will continue monthly monitoring, aim for continuous low-cost observations, and further study magma and hydrothermal dynamics via gas composition changes.
5 References
1)Oba, T. 2020. Phreatic Eruptions and Chemistry. Chemistry and Education, 68(5), 204-207
2)Japan Meteorological Agency. Seismic Intensity Database Search
3)NEDO. 2001. Geothermal Development Promotion Survey Report: Tsujinodake Area
4)NEDO. 2008. Interim Report of the Geothermal Development Promotion Survey: Eastern Ikeda Lake Area (First Survey)
5)Kawanabe, Y., and Sakaguchi, K. 2005. Geology of the Kaimondake District. Geological Survey of Japan, AIST, Regional Geology Report
6)Geological Survey of Japan, AIST. Active Volcano Database. https://gbank.gsj.jp/volcano/Act_Vol/
7)Iwasaki, S., et al. 1962. Volcanic Gases in Japan. Bulletin of the Tokyo Institute of Technology, 47, 1-54.
8)Fournier, R. O. 1999. [Title needed]. Economic Geology, 94, 1193
9)Shirota, Y., et al. 2021. Forecasting Volcanic Activity Based on the Composition of Volcanic Gases at Hakone Volcano.
Journal of Geography (Chigaku Zasshi), 783-796
10)Sugawara, M., et al. 2023. Measurement Methods and Usefulness of Volcanic Gas Component Observations Using Gas Detector Tubes. Bulletin of the Volcanological Society of Japan, 86(5), 1-23
Prediction of phreatic eruptions like Ontake is difficult because preeruption magma volume changes are small compared with magmatic eruptions such as Sakurajima, so crustal deformation precursors are hard to detect. Changes in magma-derived components in volcanic gas can reflect magma activity or hydrothermal system changes and may serve as precursors. We began monthly observations in December 2021 at the active Ikeda-Yamakawa area, where seismicity has increased since 2007 (Fig.1). Because conventional monitoring uses expensive instruments, we developed a safe, low-cost method usable by high school students. We selected three fumarole fields in Ibusuki volcanoes (Gongen, Minamisakoda, Unagi-ike; Fig.2) for monthly sampling.
2 Methods
From 2021 we conducted monthly monitoring at three sites. For trace volcanic gas components CO2, SO2, and H2S, we developed a method to accurately dilute samples into the effective measurement ranges of detector tubes and low-cost commercial gas sensors (Fig.3).
Using these instruments we measured concentrations and calculated CO2/H2S and SO2/H2S ratios for each monthly survey. Tracking ratio changes may reveal variations in magma activity and hydrothermal conditions. Based on four years of ratio data, we proposed a subsurface volcanic gas behavior model and compared results with other Kyushu fumarole fields.
3 Results and Discussion
(1) Subsurface model: We propose a subsurface structural and hydrothermal model for Ibusuki Volcanoes (Fig.4). Magma-derived gases are normally impeded by crustal stress (Fig.5); earthquakes release stress and allow trapped gases to rise to the hydrothermal system and vent, so gas signals may increase after seismic stress release.
(2) Composition ratio variations: Shallow earthquakes beneath Ibusuki since 2007 include events with hypocenters <10 km that may be volcanic. We used SO2/H2S and CO2/H2S as indicators. All three sites showed a clear SO2/H2S increase after the Gongen earthquake on 15 Nov 2023 (Fig.6, Fig.7). Earthquakes after 2025 showed no CO2/H2S increase and only slight SO2/H2S rises.
(3) Comparison with Kyushu volcanoes: Gongen and Minamisakoda have CO2/H2S averages similar to high-ratio volcanoes such as Aso and Unzen; Unagi-ike is lower (Fig.8, Fig.9).
4 Conclusion and Future Work We constructed a subsurface structural model for Ibusuki volcanoes from volcanic gas ratio variations. Ibusuki volcanoes belongs to the high CO2/H2S ratio group among Kyushu volcanoes. We will continue monthly monitoring, aim for continuous low-cost observations, and further study magma and hydrothermal dynamics via gas composition changes.
5 References
1)Oba, T. 2020. Phreatic Eruptions and Chemistry. Chemistry and Education, 68(5), 204-207
2)Japan Meteorological Agency. Seismic Intensity Database Search
3)NEDO. 2001. Geothermal Development Promotion Survey Report: Tsujinodake Area
4)NEDO. 2008. Interim Report of the Geothermal Development Promotion Survey: Eastern Ikeda Lake Area (First Survey)
5)Kawanabe, Y., and Sakaguchi, K. 2005. Geology of the Kaimondake District. Geological Survey of Japan, AIST, Regional Geology Report
6)Geological Survey of Japan, AIST. Active Volcano Database. https://gbank.gsj.jp/volcano/Act_Vol/
7)Iwasaki, S., et al. 1962. Volcanic Gases in Japan. Bulletin of the Tokyo Institute of Technology, 47, 1-54.
8)Fournier, R. O. 1999. [Title needed]. Economic Geology, 94, 1193
9)Shirota, Y., et al. 2021. Forecasting Volcanic Activity Based on the Composition of Volcanic Gases at Hakone Volcano.
Journal of Geography (Chigaku Zasshi), 783-796
10)Sugawara, M., et al. 2023. Measurement Methods and Usefulness of Volcanic Gas Component Observations Using Gas Detector Tubes. Bulletin of the Volcanological Society of Japan, 86(5), 1-23