5:15 PM - 6:45 PM
[SVC26-P03] Flow path estimation of volcanic fluids from soil diffuse CO2 fluxes and ground temperature measurements at Mt. Usu
Keywords:Mt. Usu, Soil diffuse CO2 fluxes, Ground temperature measurements
Volcanic gases are released from fumaroles and are also diffused from the ground surface. Because the spatial distribution of diffusive emission rates of gases is influenced by subsurface fractures, it can provide information about shallow structures, including gas flow paths. Measurements at Mt. Asama suggested that CO2 and heat were released through fractures extending along the crater rim, based on comparisons of CO2 flux distributions, geothermal anomalies, and location of past crater edges (Morita et al., 2016). Because hot volcanic gases transport heat from depth, heat transport by conduction, advection, or both can be observed in fumarolic areas. The heat transport processes affect the vertical profiles of ground temperature, and we thus can infer the transport processes of volcanic fluids underground from temperature measurements at a depth of one meter (Ehara and Okamoto, 1980).
At Mt. Usu, the target area of this study, CO2 fluxes were measured before and after the 2000 eruption (Hernández et al., 2001), and the flux in the summit area increased 6 months before the eruption, suggesting that repetitive observation of CO2 fluxes may provide valuable information for eruption forecasting. However, there have been no studies combining the CO2 fluxes and ground temperature measurements. This study aims to estimate the flow patterns and pathways of volcanic fluids from the CO2 flux distribution in the summit area, and the heat transport process by measuring vertical temperature distributions at Gin-numa crater.
CO2 flux measurements were performed in May and October 2023. The chamber method was used, where soil diffuse CO2 was collected in a chamber and the concentration rate was measured by a spectrometer in real time. Spatial interpolation by kriging produced a distribution map of CO2 emission from the entire area. We measured the ground temperature at depths of 25 cm, 50 cm, 75 cm, and 1 m at Gin-numa crater in October 2023. The trace of the temperature vs. depth graph and provided the information on whether the heat transport was due to conduction or advection.
The CO2 flux was low at most sites except the rim around Gin-numa crater, I crater, and Mt. Kita-byoubu ridge. The spatial distribution didn’t change significantly between May and October measurements, but the flux tended to be slightly higher in October than in May. The logarithm of the CO2 fluxes at the same place after 2020 fluctuated within a range of 1, and repeatedly increased and decreased at some points. However, compared to the values before the 2000 eruption, the maximum flux is substantially less than that time, so we consider that the Mt. Usu is still inactive. We expect that the CO2 flux may increase by several orders of magnitude if the volcano becomes more active in the future.
Temperature measurements at 1 m depth at the rim of Gin-numa crater showed high temperature near the boiling point which suggested the heat transport by advection; meanwhile low temperature suggesting heat conduction were observed in the central part of Gin-numa crater. The CO2 flux showed a weak correlation with the heat flux by conduction, but a slightly stronger correlation with the temperature at 25 cm depth. This suggests that heat and CO2 emission by fumaroles (advection) are dominant there. We estimate the depth of the boiling point at the center of Gin-numa crater to be 11.2 m, based on the linear temperature increase by conduction. This is close to the thickness of the sediments, which was estimated to be about 15 m from a comparison between topographic maps just after the 1977 eruption and present. The bottom surface of the sediments may be altered by the high temperature volcanic fluids and prevent the fluid transportation to the ground surface in the central part of the crater. The volcanic fluids are forced to rise along the funnel-shaped altered layer and appear at the rim of the Gin-numa crater floor.
At Mt. Usu, the target area of this study, CO2 fluxes were measured before and after the 2000 eruption (Hernández et al., 2001), and the flux in the summit area increased 6 months before the eruption, suggesting that repetitive observation of CO2 fluxes may provide valuable information for eruption forecasting. However, there have been no studies combining the CO2 fluxes and ground temperature measurements. This study aims to estimate the flow patterns and pathways of volcanic fluids from the CO2 flux distribution in the summit area, and the heat transport process by measuring vertical temperature distributions at Gin-numa crater.
CO2 flux measurements were performed in May and October 2023. The chamber method was used, where soil diffuse CO2 was collected in a chamber and the concentration rate was measured by a spectrometer in real time. Spatial interpolation by kriging produced a distribution map of CO2 emission from the entire area. We measured the ground temperature at depths of 25 cm, 50 cm, 75 cm, and 1 m at Gin-numa crater in October 2023. The trace of the temperature vs. depth graph and provided the information on whether the heat transport was due to conduction or advection.
The CO2 flux was low at most sites except the rim around Gin-numa crater, I crater, and Mt. Kita-byoubu ridge. The spatial distribution didn’t change significantly between May and October measurements, but the flux tended to be slightly higher in October than in May. The logarithm of the CO2 fluxes at the same place after 2020 fluctuated within a range of 1, and repeatedly increased and decreased at some points. However, compared to the values before the 2000 eruption, the maximum flux is substantially less than that time, so we consider that the Mt. Usu is still inactive. We expect that the CO2 flux may increase by several orders of magnitude if the volcano becomes more active in the future.
Temperature measurements at 1 m depth at the rim of Gin-numa crater showed high temperature near the boiling point which suggested the heat transport by advection; meanwhile low temperature suggesting heat conduction were observed in the central part of Gin-numa crater. The CO2 flux showed a weak correlation with the heat flux by conduction, but a slightly stronger correlation with the temperature at 25 cm depth. This suggests that heat and CO2 emission by fumaroles (advection) are dominant there. We estimate the depth of the boiling point at the center of Gin-numa crater to be 11.2 m, based on the linear temperature increase by conduction. This is close to the thickness of the sediments, which was estimated to be about 15 m from a comparison between topographic maps just after the 1977 eruption and present. The bottom surface of the sediments may be altered by the high temperature volcanic fluids and prevent the fluid transportation to the ground surface in the central part of the crater. The volcanic fluids are forced to rise along the funnel-shaped altered layer and appear at the rim of the Gin-numa crater floor.