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

Abstract

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 × 10 17 J. The corresponding volume of the intrusion is 6 × 10 7 m 3 . 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.

Posteruptive Thermoelastic Deflation of Intruded Magma in Usu Volcano, Japan, 1992–2017

“…First, geophysical observations have revealed shallow intrusions that can serve as heat sources. Magnetotelluric surveys (Matsushima, ) have revealed a relatively high resistive block at depths between 200 and 800 m below sea level beneath the summit crater. The resistive block has been interpreted as an intruded magma body during the 1977–1982 eruption.…”

“…Studies have previously investigated the volumes of intruded magma during the recent eruptions at Usu volcano based on the coeruptive ground displacements, gravity changes or surface faulting (e.g., Goto & Johmori, ; Jousset et al, ; Miura & Niida, ). Intrusive volumes during the 2000, 1977–1982, and 1943–1945 unrests were estimated to be (150–220) × 10 6 (Jousset et al, ; Miura & Niida, ), (70–180) × 10 6 (Matsushima, ; Matsushima et al, ), and 33 × 10 6 m 3 (Goto & Johmori, ), respectively. Table S2 indicates the search bounds of the inverted parameters.…”

Ice Box Calorimetry: A useful method for estimating heat discharge rates through steaming ground

“…For the next-most recent eruption at Usu volcano in 1977, when the volume of intruded magma was similar to that in 2000, Matsushima (2003) reported a gradual increase in the heat discharge rate through steaming ground after the eruption, and a gradual decrease after 1980. The differences in geothermal activity between the 2000 and 1977 eruptions may reflect the contrasting development of the magma-hydrothermal system: magma intruded at the summit area in 1977 and upon a flank area in 2000, probably reflecting differences in underground conditions such as permeability and groundwater level.…”

“…Precise estimates of the heat discharge in such areas constrain the mechanism of growth of the hydrothermal system, the cooling process of intruded magma (Matsushima, 2003), and degassing in the magma reservoir and/or conduit (Kazahaya et al, 1994). Energy fluxes from fumaroles are quantitatively estimated using methods such as image analysis (Kagiyama, 1981) and UV spectrometry (Oppenheimer and McGonigle, 2004); however, large uncertainties are involved in estimating energy fluxes from steaming grounds.…”

Inter-eruptive volcanism at Usu volcano: Micro-earthquakes and dome subsidence

“…The estimated extent of the intrusion is 600 m high and 300 m wide, which becomes 1.8 × 10 8 m 3 assuming that the intrusion is a 1000-m-long slab. The volume of intrusive magma was also estimated as 8 × 10 7 m 3 by integration of the heat discharge rate (Matsushima, 1993). Matsushima and Oshima (2000) investigated whether the intrusive magma can explain the thermal activities observed at the surface by a numerical simulation of the hydrothermal system.…”