15:30 〜 17:00
[SVC35-P14] 伊豆大島1986年溶岩流出噴火におけるマグマ溜り・火道システムへの制約
キーワード：マグマ溜り、火道、溶岩流出噴火、伊豆大島
A variation in volcanic activity has been detected in volcanic eruptions based on multi-observation data. Understanding the physical processes of subsurface magma movement is essential to relate volcanic activity with the observation data. The subsurface magma movement is characterized by a magma ascent from a magma chamber through a volcanic conduit to the surface. During the 1986 eruptions of Izu-Oshima volcano, Japan, multi-observations based on geological, petrological, and geophysical methods enabled us to investigate the magma movement dynamics accompanied by various eruption styles. In this study, we investigated the dynamics of the magma chamber-volcanic conduit system during the 1986 lava flow eruption of Izu-Oshima volcano based on a physics-based model.
During the 1986 lava flow eruption from the summit crater at Izu-Oshima volcano, an exponential decrease in the discharge rate was observed by the geological method. When magma erupts from the elastically deformable magma chamber through the conduit to the surface during an eruption, the discharge rate decreases exponentially, and the time scale for the exponential decrease depends on two parameters C and Ω. The parameter C controls a variability of the chamber pressure with respect to magma outflow, and it depends on the chamber volume, the magma compressibility in the magma chamber, and the effective compressibility of the magma chamber. The parameter Ω is the conduit conductivity, depending on the magma viscosity and the conduit geometry. In this study, we estimated the magma and effective chamber compressibilities from the ratio of the erupted lava volume to geodetic volume change and rigidity of surrounding host rocks inferred from seismic velocity structure. We also estimated Ω from a numerical analysis of a 1-dimensional steady conduit flow model in which magma viscosity constrained from petrological data and various conduit geometry are considered. By integrating these estimations, we obtained the relationship between the chamber volume and the conduit geometry needed for reproducing the observed time scale for the exponential decrease in the discharge rate. We found that a nearly cylindrical conduit is necessary for a chamber volume of approximately less than 10 km3. This result is consistent with the fact that a dyke intrusion at a shallow level was not detected by the geodetic observations during the summit eruption.
During the 1986 lava flow eruption from the summit crater at Izu-Oshima volcano, an exponential decrease in the discharge rate was observed by the geological method. When magma erupts from the elastically deformable magma chamber through the conduit to the surface during an eruption, the discharge rate decreases exponentially, and the time scale for the exponential decrease depends on two parameters C and Ω. The parameter C controls a variability of the chamber pressure with respect to magma outflow, and it depends on the chamber volume, the magma compressibility in the magma chamber, and the effective compressibility of the magma chamber. The parameter Ω is the conduit conductivity, depending on the magma viscosity and the conduit geometry. In this study, we estimated the magma and effective chamber compressibilities from the ratio of the erupted lava volume to geodetic volume change and rigidity of surrounding host rocks inferred from seismic velocity structure. We also estimated Ω from a numerical analysis of a 1-dimensional steady conduit flow model in which magma viscosity constrained from petrological data and various conduit geometry are considered. By integrating these estimations, we obtained the relationship between the chamber volume and the conduit geometry needed for reproducing the observed time scale for the exponential decrease in the discharge rate. We found that a nearly cylindrical conduit is necessary for a chamber volume of approximately less than 10 km3. This result is consistent with the fact that a dyke intrusion at a shallow level was not detected by the geodetic observations during the summit eruption.