11:00 AM - 11:15 AM
[SVC29-08] Construction of a conceptual model for the evaluation of volcanic activity on Izu-Oshima
Keywords:Izu-Oshima volcano, Mt. Mihara, conceptual model
In the eruption scenario of Izu-Oshima, which is the background of the eruption alert level criteria, summit eruption is considered the most likely case. Activities of magma ascent, lava overflow, abd magma descent were repeated at the central cone Mt. Mihara, suggesting that a stable conduit is being formed. The eruption alert level criteria are based on observations at the time of past eruptions. In future, it will be necessary to have a unified understanding and modeling of the observed phenomena. Here, a conceptual model for the summit eruption is presented based on existing knowledge and observation to provide the background of the eruption alert level criteria and the viewpoint of the surveillance and evaluation.
The groundwater level in the island is very low so as almost at sea level along the coast, and 36 m ASL at the western part of the caldera, except for the northern part of the caldera, which is around 200 m ASL. Although springs from higher altitudes exist, the amount of discharge is 2~3 orders of magnitude smaller than precipitation on the entire island. Thus, it is expected that most of the recharged meteoric water will penetrate the thick unsaturated zone, reach the watertable, and then be discharged into the sea due to the hydraulic gradient. In addition, there is a concept of Ghyben-Herzberg lens as a way of groundwater on islands. At least in wells along the coast, freshwater overlying on salt water is observed. By numerical simulations of precipitation infiltration, the permeability of the whole volcanic edifice above sea level was estimated to be 3×10-12~3×10-11 m2 in the horizontal direction and 10-14~10-13 m2 in the vertical direction (Onizawa et al., 2009). Such the high permeabilities are probably due to lithology such as alternation of scoria and volcanic ash deposits and lavas with cracks.
During the magma accumulation period after the 1986 eruption, the fumaroles in Mt. Mihara and the caldera region were mainly composed of H2O and CO2, and the H2O/CO2 weight ratio was in the range of 14~65. Based on the isotope ratios, it is estimated that H2O is of meteoric origin and CO2 is of magmatic origin (e.g., Ohba, 2007). Further, the soil CO2 discharge rate from Mt. Mihara region is estimated to be 2~27 ton/day (e.g., Hernandez et al., 2013). For meteoric origin H2O, the emission rate is 28~1,755 ton/day according to the CO2 discharge rate and the H2O/CO2 weight ratio. This discharge rate corresponds to precipitation in an area of 3.4×103~2.1×105 m2 (radius of 33~260 m in a circle), and it is thought that meteoric water that has penetrated into the conduit to crater scale area is released.
Referring to the above-mentioned hydrological settings and the volatile discharge from Mt. Mihara, we can conceptually consider the phenomena that occur due to magma ascent through the conduit. It is expected that degassing will be enhanced, and volatile components and heat will be supplied to the surroundings. However, at the stage where the supply rate is low, the groundwater below watertable and the meteoric water downflow in the unsaturated zone greatly affect, and little signs will appear at Mt. Mihara except for discharge of poorly water-soluble volatile as CO2. Rather, it is expected that the groundwater temperature increases and the highly soluble components such as sulfur dissolve in the groundwater. These signs appear in the groundwater flowing down the coast. If the supply rate increases, the groundwater around the conduit is expected to be depleted, and various phenomena will occur from the watertable to Mt. Mihara. Volcanic tremor and increased thermal activity at Mt. Mihara, which are criteria for the eruption alert level, are thought to occur during such a process. Besides the criteria, changes in the geomagnetic field, an increase in the proportion of magma-origin H2O in the fumaroles, and changes in the fumarole composition such as the addition of SO2 and H2S are also considered.
The groundwater level in the island is very low so as almost at sea level along the coast, and 36 m ASL at the western part of the caldera, except for the northern part of the caldera, which is around 200 m ASL. Although springs from higher altitudes exist, the amount of discharge is 2~3 orders of magnitude smaller than precipitation on the entire island. Thus, it is expected that most of the recharged meteoric water will penetrate the thick unsaturated zone, reach the watertable, and then be discharged into the sea due to the hydraulic gradient. In addition, there is a concept of Ghyben-Herzberg lens as a way of groundwater on islands. At least in wells along the coast, freshwater overlying on salt water is observed. By numerical simulations of precipitation infiltration, the permeability of the whole volcanic edifice above sea level was estimated to be 3×10-12~3×10-11 m2 in the horizontal direction and 10-14~10-13 m2 in the vertical direction (Onizawa et al., 2009). Such the high permeabilities are probably due to lithology such as alternation of scoria and volcanic ash deposits and lavas with cracks.
During the magma accumulation period after the 1986 eruption, the fumaroles in Mt. Mihara and the caldera region were mainly composed of H2O and CO2, and the H2O/CO2 weight ratio was in the range of 14~65. Based on the isotope ratios, it is estimated that H2O is of meteoric origin and CO2 is of magmatic origin (e.g., Ohba, 2007). Further, the soil CO2 discharge rate from Mt. Mihara region is estimated to be 2~27 ton/day (e.g., Hernandez et al., 2013). For meteoric origin H2O, the emission rate is 28~1,755 ton/day according to the CO2 discharge rate and the H2O/CO2 weight ratio. This discharge rate corresponds to precipitation in an area of 3.4×103~2.1×105 m2 (radius of 33~260 m in a circle), and it is thought that meteoric water that has penetrated into the conduit to crater scale area is released.
Referring to the above-mentioned hydrological settings and the volatile discharge from Mt. Mihara, we can conceptually consider the phenomena that occur due to magma ascent through the conduit. It is expected that degassing will be enhanced, and volatile components and heat will be supplied to the surroundings. However, at the stage where the supply rate is low, the groundwater below watertable and the meteoric water downflow in the unsaturated zone greatly affect, and little signs will appear at Mt. Mihara except for discharge of poorly water-soluble volatile as CO2. Rather, it is expected that the groundwater temperature increases and the highly soluble components such as sulfur dissolve in the groundwater. These signs appear in the groundwater flowing down the coast. If the supply rate increases, the groundwater around the conduit is expected to be depleted, and various phenomena will occur from the watertable to Mt. Mihara. Volcanic tremor and increased thermal activity at Mt. Mihara, which are criteria for the eruption alert level, are thought to occur during such a process. Besides the criteria, changes in the geomagnetic field, an increase in the proportion of magma-origin H2O in the fumaroles, and changes in the fumarole composition such as the addition of SO2 and H2S are also considered.