10:05 AM - 10:25 AM
[SEM15-10] Observation of fumarole area using electromagnetic survey and prediction of hydrothermal eruption
★Invited Papers
Keywords:hydrothermal eruption, CSAMT, fumarole, UAV
The 2014 eruption of Ontake volcano was Japan’s deadliest volcanic disaster since World War II, causing over 60 casualties and leading to major reforms in volcanic disaster prevention. Since then, several eruptions have drawn public attention, including the 2015 eruption of Hakone volcano, the 2018 eruption of Kusatsu-Shirane (Moto-Shirane) volcano, and the 2019 eruption of White Island. These are classified as hydrothermal eruptions, a type of phreatic eruption. Hydrothermal eruptions occur in fumarolic areas due to explosions caused by phase transitions of underground hydrothermal water. Since magma is not directly involved, precursor signals such as earthquakes and crustal deformation are subtle, making prediction difficult.
Fumarolic zones often serve as tourist attractions, increasing the risk of casualties in case of an eruption. This causes a mismatch between eruption scale and disaster severity and underscores the need for improved monitoring. Advanced electromagnetic surveys are expected to play a key role in the new monitoring system. This lecture discusses the current understanding of fumarolic areas and the potential of electromagnetic surveys for future monitoring.
Volcanoes with fumarolic activity are thought to have a hydrothermal system, a water circulation network between the surface and the deep magma chamber. Heat in fumarolic zones is transported by hydrothermal fluids, ultimately originating from deep magma. Gases from fumaroles also come from magmatic degassing, yet their chemical composition differs significantly from that of pure magmatic gas. Similarly, the composition of hot springs in fumarolic areas differs from hydrothermal fluids due to underground chemical reactions and fractionation, especially between liquid and vapor phases.
A vapor-dominated zone is a geological structure where interconnected pores are filled with steam rather than liquid water, playing a key role in fractionation. Hot springs in fumarolic zones typically characterized by (1) low silica content, (2) low chloride ion concentration, and (3) low discharge rates. These suggest that (1) the water has not experienced high temperatures, (2) it formed through vapor-liquid separation, and (3) its catchment area is limited to the fumarolic zone and its surroundings. This implies that hot springs in fumarolic areas mainly consist of recently fallen rainwater heated by steam from a near-surface vapor-dominated zone, with additional acidic components from oxidized vapor.
While the existence of vapor-dominated zones is widely accepted, few have been confirmed. Steam has significantly higher resistivity than liquid-phase water, particularly when the latter contains dissolved components. Thus, electromagnetic surveys can help identify these zones. Additionally, their steam fraction is thought to vary with temperature and pressure.
CSAMT surveys in Ōwakudani, the largest fumarolic zone of Hakone volcano, revealed a cap rock (low-resistivity zone) covering the hydrothermal system and an underlying vapor-dominated hydrothermal system (high-resistivity zone). Small vapor pockets were also detected within the cap rock. Notably, dominant hot springs and fumaroles were found directly above these vapor pockets. This suggests that vapor pockets act as conduits for surface emissions and as sites of chemical fractionation. After an eruption, these pockets grew larger and showed increased resistivity, indicating a higher steam fraction due to depressurization. Before an eruption, rising pressure likely reduces the steam fraction, causing the pockets to shrink and their resistivity to decrease.
As discussed, electromagnetic surveys are a promising tool for understanding fumarolic zones, assessing hydrothermal eruption potential, and detecting precursors. Future research should focus on (1) integrating electromagnetic surveys with geochemical analysis to enhance fumarolic zone modeling and (2) using automated UAV-mounted electromagnetic surveys for high-frequency, high-resolution resistivity monitoring.
Fumarolic zones often serve as tourist attractions, increasing the risk of casualties in case of an eruption. This causes a mismatch between eruption scale and disaster severity and underscores the need for improved monitoring. Advanced electromagnetic surveys are expected to play a key role in the new monitoring system. This lecture discusses the current understanding of fumarolic areas and the potential of electromagnetic surveys for future monitoring.
Volcanoes with fumarolic activity are thought to have a hydrothermal system, a water circulation network between the surface and the deep magma chamber. Heat in fumarolic zones is transported by hydrothermal fluids, ultimately originating from deep magma. Gases from fumaroles also come from magmatic degassing, yet their chemical composition differs significantly from that of pure magmatic gas. Similarly, the composition of hot springs in fumarolic areas differs from hydrothermal fluids due to underground chemical reactions and fractionation, especially between liquid and vapor phases.
A vapor-dominated zone is a geological structure where interconnected pores are filled with steam rather than liquid water, playing a key role in fractionation. Hot springs in fumarolic zones typically characterized by (1) low silica content, (2) low chloride ion concentration, and (3) low discharge rates. These suggest that (1) the water has not experienced high temperatures, (2) it formed through vapor-liquid separation, and (3) its catchment area is limited to the fumarolic zone and its surroundings. This implies that hot springs in fumarolic areas mainly consist of recently fallen rainwater heated by steam from a near-surface vapor-dominated zone, with additional acidic components from oxidized vapor.
While the existence of vapor-dominated zones is widely accepted, few have been confirmed. Steam has significantly higher resistivity than liquid-phase water, particularly when the latter contains dissolved components. Thus, electromagnetic surveys can help identify these zones. Additionally, their steam fraction is thought to vary with temperature and pressure.
CSAMT surveys in Ōwakudani, the largest fumarolic zone of Hakone volcano, revealed a cap rock (low-resistivity zone) covering the hydrothermal system and an underlying vapor-dominated hydrothermal system (high-resistivity zone). Small vapor pockets were also detected within the cap rock. Notably, dominant hot springs and fumaroles were found directly above these vapor pockets. This suggests that vapor pockets act as conduits for surface emissions and as sites of chemical fractionation. After an eruption, these pockets grew larger and showed increased resistivity, indicating a higher steam fraction due to depressurization. Before an eruption, rising pressure likely reduces the steam fraction, causing the pockets to shrink and their resistivity to decrease.
As discussed, electromagnetic surveys are a promising tool for understanding fumarolic zones, assessing hydrothermal eruption potential, and detecting precursors. Future research should focus on (1) integrating electromagnetic surveys with geochemical analysis to enhance fumarolic zone modeling and (2) using automated UAV-mounted electromagnetic surveys for high-frequency, high-resolution resistivity monitoring.