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[O11-P15] Proposal of subsurface models based on fumarolic observations at Ibusuki volcanoes
Keywords:phreatomagmatic eruption, volcanic gases, earthquake, SO2/H2S fluctuation
1. Introduction
To predict future phreatomagmatic eruptions, we started regular volcanic gas observations from 2022 at Ibusuki volcanoes, located at the center of the active volcanoes “Ikeda-Yamagawa” where earthquakes have been increasing since 2000 as shown in Figure 1. We selected the Gongen, Minami-Sakoda, and Unagi-Ike alteration zones of Fig.2 in Ibusuki volcanoes as the study area, and evaluated the volcanic earthquakes and composition ratio variation of volcanic gases by periodic observations using a simplified method.
2. Method
Since 2021,regular monthly observations have been conducted.The components measured are CO2 and SO2 from magma and H2S from hydrothermal water. CO2 and H2S were measured with gas sensors and gas detector tubes
by a simple dilution method using a plastic bag with a zipper as shown in Fig.3. SO2 was measured by diluting it by about 3 times. We also collected condensate and surface water and measured the dissolved volcanic gases using the turbidimetric method.
3. Results and Discussion
(1) Volcanic gas composition variation
Fig.5 and 6 were prepared to compare SO2/H2S and CO2/H2S with those of the sensed earthquakes. The SO2/H2S fluctuations in the graphs show that the three sites show a large decrease to an increase after the November 15, 2023 shallow earthquake (crustal movement) directly under the gensen, suggesting that the stress acting on the crust was released by the earthquake and magma-induced SO2 rose from the subsurface. On the other hand, in Fig. 6, the effect of the November 15, 2023 earthquake is seen only at Unagi-Ike, with little variation at the other two points.
(2) Hydrothermal reservoir model of Ibusuki volcanoes
According to the NEDO reports2)3) , volcanic gases rise from magma in the subsurface along tectonic lines and faults and emanate from fumaroles. On the other hand, hydrothermal water is considered to be advected through permeable layers to the Minami-Sakoda and Gongen alteration zones, where groundwater formed by seawater and lake water seepage is heated and rises under Unagi-Ike alteration zone to form large hydrothermal pools. In particular, considering that Unagi-Ike alteration zone is located on the shore of Unagi-Ike Lake and has a high concentration of H2S of hydrothermal origin, a large hydrothermal pool is located shallow underground, and SO2 dissolves due to this abundant amount of hydrothermal water, suggesting that the SO2 concentration in the normal fumarole is the lowest among the three sites. Based on these facts, we propose the model shown in Fig. 7.
On the other hand, the outcrop at Minami-Sakoda alteration zone is the driest among the three sites, and there is less hot spring water and mud on the surface than at the other two sites. The hydrothermal reservoir in the subsurface of the Gongen alteration zone is also supplied by the same aquifer immediately below Unagi-Ike as at Minami-Sakoda. Furthermore, SO2 levels at Unagi-Ike as well as Gongen were elevated immediately before the Gongen earthquake. Based on these facts, the increase in SO2 immediately before the earthquake may be due to the release of stresses applied to the crust and a model like the one shown in Figure 8 is proposed.
(3) Proposal of a model showing advection of volcanic gases due to earthquakes directly under Gongen zone.
In order to explain the rise of SO2 in the Ibusuki volcano group due to earthquakes directly under the gonads as described in (2), we proposed a subsurface model as shown in Fig.9. This model assumes that magma-derived SO2 are prevented from rising by crustal stresses, and that they rise and are released into fumaroles due to crustal rupture caused by earthquakes. This suggests that magma-derived volcanic gases normally tend to decrease and increase during earthquakes.
4 Future Plans
(1) Continue regular observations of volcanic gases and hot spring water, aiming for continuous observation using instruments.
(2) We will continue to investigate the relationship between the concentrations of volcanic gases and volcanic earthquakes, and to verify the model.
Through these efforts, we will continue to accumulate long-term data that will lead to the prediction of volcanic activities at the active volcanoes "Ikeda and Yamagawa".
References
1) NEDO, Geothermal Development Promotion Study Report “Tsujinodake Area”,2001.
2) NEDO, Geothermal Development Promotion Study Interim Report, “Eastern Lake Ikeda Area (Phase I)”,2008.
3) JMA seismic intensity database search
4) https://gbank.gsj.jp/volcano/Act_Vol/
5) Shirota,N. et al., Journal of Earth Sciences, 2021,783-796
6) J.Hirabayashi,Volcanoes,Vol.2, No.30 (1986), 327-338
To predict future phreatomagmatic eruptions, we started regular volcanic gas observations from 2022 at Ibusuki volcanoes, located at the center of the active volcanoes “Ikeda-Yamagawa” where earthquakes have been increasing since 2000 as shown in Figure 1. We selected the Gongen, Minami-Sakoda, and Unagi-Ike alteration zones of Fig.2 in Ibusuki volcanoes as the study area, and evaluated the volcanic earthquakes and composition ratio variation of volcanic gases by periodic observations using a simplified method.
2. Method
Since 2021,regular monthly observations have been conducted.The components measured are CO2 and SO2 from magma and H2S from hydrothermal water. CO2 and H2S were measured with gas sensors and gas detector tubes
by a simple dilution method using a plastic bag with a zipper as shown in Fig.3. SO2 was measured by diluting it by about 3 times. We also collected condensate and surface water and measured the dissolved volcanic gases using the turbidimetric method.
3. Results and Discussion
(1) Volcanic gas composition variation
Fig.5 and 6 were prepared to compare SO2/H2S and CO2/H2S with those of the sensed earthquakes. The SO2/H2S fluctuations in the graphs show that the three sites show a large decrease to an increase after the November 15, 2023 shallow earthquake (crustal movement) directly under the gensen, suggesting that the stress acting on the crust was released by the earthquake and magma-induced SO2 rose from the subsurface. On the other hand, in Fig. 6, the effect of the November 15, 2023 earthquake is seen only at Unagi-Ike, with little variation at the other two points.
(2) Hydrothermal reservoir model of Ibusuki volcanoes
According to the NEDO reports2)3) , volcanic gases rise from magma in the subsurface along tectonic lines and faults and emanate from fumaroles. On the other hand, hydrothermal water is considered to be advected through permeable layers to the Minami-Sakoda and Gongen alteration zones, where groundwater formed by seawater and lake water seepage is heated and rises under Unagi-Ike alteration zone to form large hydrothermal pools. In particular, considering that Unagi-Ike alteration zone is located on the shore of Unagi-Ike Lake and has a high concentration of H2S of hydrothermal origin, a large hydrothermal pool is located shallow underground, and SO2 dissolves due to this abundant amount of hydrothermal water, suggesting that the SO2 concentration in the normal fumarole is the lowest among the three sites. Based on these facts, we propose the model shown in Fig. 7.
On the other hand, the outcrop at Minami-Sakoda alteration zone is the driest among the three sites, and there is less hot spring water and mud on the surface than at the other two sites. The hydrothermal reservoir in the subsurface of the Gongen alteration zone is also supplied by the same aquifer immediately below Unagi-Ike as at Minami-Sakoda. Furthermore, SO2 levels at Unagi-Ike as well as Gongen were elevated immediately before the Gongen earthquake. Based on these facts, the increase in SO2 immediately before the earthquake may be due to the release of stresses applied to the crust and a model like the one shown in Figure 8 is proposed.
(3) Proposal of a model showing advection of volcanic gases due to earthquakes directly under Gongen zone.
In order to explain the rise of SO2 in the Ibusuki volcano group due to earthquakes directly under the gonads as described in (2), we proposed a subsurface model as shown in Fig.9. This model assumes that magma-derived SO2 are prevented from rising by crustal stresses, and that they rise and are released into fumaroles due to crustal rupture caused by earthquakes. This suggests that magma-derived volcanic gases normally tend to decrease and increase during earthquakes.
4 Future Plans
(1) Continue regular observations of volcanic gases and hot spring water, aiming for continuous observation using instruments.
(2) We will continue to investigate the relationship between the concentrations of volcanic gases and volcanic earthquakes, and to verify the model.
Through these efforts, we will continue to accumulate long-term data that will lead to the prediction of volcanic activities at the active volcanoes "Ikeda and Yamagawa".
References
1) NEDO, Geothermal Development Promotion Study Report “Tsujinodake Area”,2001.
2) NEDO, Geothermal Development Promotion Study Interim Report, “Eastern Lake Ikeda Area (Phase I)”,2008.
3) JMA seismic intensity database search
4) https://gbank.gsj.jp/volcano/Act_Vol/
5) Shirota,N. et al., Journal of Earth Sciences, 2021,783-796
6) J.Hirabayashi,Volcanoes,Vol.2, No.30 (1986), 327-338