Mathematical simulation of magma-hydrothermal activity associated with the 1977 eruption of Usu volcano

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During the 1977 eruption of Usu volcano, magma was emplaced at shallow crust. This intrusion induced fumarole activity immediately after the eruption. Based on the repeated thermal observations, the amount of heat discharged by this thermal activity is estimated to be 2 × 1017J. The corresponding volume of the intrusion is 6 × 107m3. The inferred intrusion volume is comparable to the volume of the resistive block beneath the major faults formed by this eruption, which was interpreted as a cooled intrusion on the basis of recently conducted MT surveys. The heat discharge rate is a surface boundary condition for an underlying magma hydrothermal system. A mathematical simulation, which accounts for multiphase mass and heat transport within a porous media, is conducted to reproduce the thermal activity of Usu volcano. The simulation incorporates the supply of latent heat by solidifying magma and heat transfer by degassing. Permeability conditions are important factors to fit the simulated heat discharge rate with the observation. Increased permeability of surrounding formations causes early appearance and high amplitude of surface heat discharge rate and reduced permeability causes opposite effect. The intrusion permeability has a strong influence on the surface thermal activity. High permeability is needed for the early appearance of the surface heat discharge rate, although it results in the maximum intrusion temperature that is much lower than the observed fumarolic temperature. To avoid the inconsistency, temperature dependent permeability is used in the simulation. The inside of the intrusion with the temperature dependent permeability that has low initial values and the high permeable margin are supposed to satisfy the conditions of the observed surface heat discharge rate and fumarolic temperature.

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