11:45 AM - 12:00 PM
[SVC32-20] Assessment of subsurface structure based on measurements of soil gas fluxes from the surface
Keywords:gaseous elementary mercury, Helium, soil gas, lateral eruption, Kusatsu-Shirane volcano, phreatic eruption
1. Introduction
Kusatsu-Shirane volcano is one of the most active volcanoes in terms of frequent phreatic eruptions. Recent phreatic eruptions have repeatedly occurred outside the main crater, Yugama crater, indicating that future eruptions likely occur at outside the main crater where tourist facilities are established. Since many tourists can easily visit outside the crater, it is important to assess the possibility of such lateral eruptions in advance. To overcome this problem, we focus on volcanic gases in soil outside the main crater. For example, spatial distribution of CO2 emitting from the ground surface on volcanoes may reflect subsurface structure such as permeability. When magmatic gases containing CO2 rise from a hydrothermal reservoir to shallow part of volcanoes, they selectively pass through areas with high permeability, such as fracture zone. Because fracture zones have a high potential for future lateral phreatic eruptions, measurements of the spatial distribution of magmatic soil gas emitted from the ground surface on volcanoes may provide risk information. However, especially in vegetated areas, including outside the main crater of Kusatsu-Shirane volcano, there are non-negligible emissions of organic-derived CO2, which may interfere with magma signals.
In this study, gaseous elementary mercury (GEM) is applied to assess subsurface structure. In active volcanoes, GEM is mainly derived from magmatic hydrothermal sources and can be measured with high accuracy in the field. In addition, the influence of organic matter on flux measurements is smaller than that of CO2. In this presentation, we discuss a validity of GEM measurement on volcanoes based on the field observation of GEM and CO2 fluxes and He isotope ratios at Kusatsu-Shirane volcano. Additionally, a preliminary report on the subsurface structure south of the main crater of Kusatsu-Shirane Volcano is discussed.
2. Method
A closed chamber system was used for the GEM flux measurements, as was the CO2 flux measurement. A mercury analyzer, Lumex RA-915M, was used to measure GEM concentration in the air inside the chamber. We modify the closed chamber system by use of a GEM removal filter to reduce interference form GEMs in the ambient and shortened the measurement time. Applying this system, we measured GEM fluxes at 73 sites in 17 hours. Since the GEM flux from the surface varies with ambient temperature, the Arrhenius equation was used to correct for temperature.
3. Results and discussion
Most of the GEM concentrations measured more than 1 km from the main crater were 0-4 ng/m3, corresponding to 0-3.6 pg/m2/s of GEM flux, which is consistent with the typical value for non-volcanic areas. High GEM fluxes were detected only on the southern outside of the main crater and an area 3 km southeast of the main crater (fumaroles and steaming grounds, called Sesshogawara). The maximum GEM concentration of 174 ng/m3 recorded near the rim of the main crater is several tens of times higher than the atmospheric GEM concentration of 2 ng/m3. The calculated GEM flux was 179 pg/m2/s, exceeding several times higher than the GEM flux of about 2 pg/m2/s in non-volcanic areas.
Magmatic He gases whose 3He/4He were higher than the representative atmospheric value were observed only at sites where the High GEM fluxes were measured, suggesting that the high GEM flux outside the main crater originates from magmatic or hydrothermal fluid. On the other hand, no correlation was found between CO2 fluxes and 3He/4He ratios. This may be due to the predominance of organic-derived CO2 in the area, which prevent the detection of magmatic CO2.
Spatial distribution of GEM flux on the outside the main crater indicates a high GEM flux zone which extends approximately 500 m southwest from the main crater, corresponding to the region where phreatic eruptions repeatedly occurred between 1902 and 1942. Our GEM flux measurements indicate that the subsurface in this zone remains highly permeable and fractured, suggesting future phreatic eruptions.
This work is supported by Japan Society for the Promotion of Science under the Grants-in-Aid for
Scientific Research of 22K03735.
Kusatsu-Shirane volcano is one of the most active volcanoes in terms of frequent phreatic eruptions. Recent phreatic eruptions have repeatedly occurred outside the main crater, Yugama crater, indicating that future eruptions likely occur at outside the main crater where tourist facilities are established. Since many tourists can easily visit outside the crater, it is important to assess the possibility of such lateral eruptions in advance. To overcome this problem, we focus on volcanic gases in soil outside the main crater. For example, spatial distribution of CO2 emitting from the ground surface on volcanoes may reflect subsurface structure such as permeability. When magmatic gases containing CO2 rise from a hydrothermal reservoir to shallow part of volcanoes, they selectively pass through areas with high permeability, such as fracture zone. Because fracture zones have a high potential for future lateral phreatic eruptions, measurements of the spatial distribution of magmatic soil gas emitted from the ground surface on volcanoes may provide risk information. However, especially in vegetated areas, including outside the main crater of Kusatsu-Shirane volcano, there are non-negligible emissions of organic-derived CO2, which may interfere with magma signals.
In this study, gaseous elementary mercury (GEM) is applied to assess subsurface structure. In active volcanoes, GEM is mainly derived from magmatic hydrothermal sources and can be measured with high accuracy in the field. In addition, the influence of organic matter on flux measurements is smaller than that of CO2. In this presentation, we discuss a validity of GEM measurement on volcanoes based on the field observation of GEM and CO2 fluxes and He isotope ratios at Kusatsu-Shirane volcano. Additionally, a preliminary report on the subsurface structure south of the main crater of Kusatsu-Shirane Volcano is discussed.
2. Method
A closed chamber system was used for the GEM flux measurements, as was the CO2 flux measurement. A mercury analyzer, Lumex RA-915M, was used to measure GEM concentration in the air inside the chamber. We modify the closed chamber system by use of a GEM removal filter to reduce interference form GEMs in the ambient and shortened the measurement time. Applying this system, we measured GEM fluxes at 73 sites in 17 hours. Since the GEM flux from the surface varies with ambient temperature, the Arrhenius equation was used to correct for temperature.
3. Results and discussion
Most of the GEM concentrations measured more than 1 km from the main crater were 0-4 ng/m3, corresponding to 0-3.6 pg/m2/s of GEM flux, which is consistent with the typical value for non-volcanic areas. High GEM fluxes were detected only on the southern outside of the main crater and an area 3 km southeast of the main crater (fumaroles and steaming grounds, called Sesshogawara). The maximum GEM concentration of 174 ng/m3 recorded near the rim of the main crater is several tens of times higher than the atmospheric GEM concentration of 2 ng/m3. The calculated GEM flux was 179 pg/m2/s, exceeding several times higher than the GEM flux of about 2 pg/m2/s in non-volcanic areas.
Magmatic He gases whose 3He/4He were higher than the representative atmospheric value were observed only at sites where the High GEM fluxes were measured, suggesting that the high GEM flux outside the main crater originates from magmatic or hydrothermal fluid. On the other hand, no correlation was found between CO2 fluxes and 3He/4He ratios. This may be due to the predominance of organic-derived CO2 in the area, which prevent the detection of magmatic CO2.
Spatial distribution of GEM flux on the outside the main crater indicates a high GEM flux zone which extends approximately 500 m southwest from the main crater, corresponding to the region where phreatic eruptions repeatedly occurred between 1902 and 1942. Our GEM flux measurements indicate that the subsurface in this zone remains highly permeable and fractured, suggesting future phreatic eruptions.
This work is supported by Japan Society for the Promotion of Science under the Grants-in-Aid for
Scientific Research of 22K03735.