15:45 〜 16:00
[SVC31-06] プラグ形成を伴う火道流のダイナミクスが噴火前の地殻変動過程に与える効果
キーワード：火道流、地殻変動、数値モデル、噴火準備過程
Pre-eruptive crustal deformation is one of the key observations for eruption precursors, and it is caused by complex magma dynamics in volcanic conduit and magma chamber. In particular, the magma flow in the conduit (i.e., the conduit flow) shows wide variations in pressure, velocity, and magma viscosity during magma ascent, which may lead to a drastic crustal deformation. Most previous studies for conduit flow modeling have mainly focused on eruption phases such as effusive and explosive eruptions and a transition process between them. In this study, for the sake of modeling a pre-eruptive process in the conduit, we developed a 1-dimensional conduit flow model in which a formation of a solid plug beneath the vent is considered. By combining this model with a model for an elastic deformation of crust surrounding the conduit-chamber system, we investigated how the conduit flow dynamics affect the pre-eruptive crustal deformation which can be geodetically measured such as by tiltmeters and strainmeters.
The conduit flow model is based on Kozono and Koyaguchi (2012) and Wong et al. (2017), in which the magma ascent velocity is described by the sum of the velocity of viscous flow and that of the solid plug. In the viscous flow, essential processes controlling magma dynamics such as equilibrium vesiculation, vertical and lateral gas escapes, crystallization kinetics, and magma viscosity change are considered. The solid plug frictionally slips on the conduit wall, following rate-dependent friction law. The transition from the viscous flow to the solid plug is assumed to occur when the frictional plug velocity is greater than the viscous flow velocity. In the crustal deformation model, the elastic modulus surrounding the conduit-chamber system is set in the 2-dimensional axisymmetric domain, and we calculate the surface displacement caused by the normal and shear stresses acting on the conduit wall and the normal stress on the chamber wall which are obtained from the conduit flow model.
In the analysis of the conduit flow, we found that the solid plug is formed beneath the vent for a low magma flow rate. When the magma influx to the magma chamber reaches a critical value, the conduit-chamber system becomes unstable, leading to abrupt transition of the conduit flow accompanied by drastic increases in the magma flow rate, the magma porosity, and the overpressure inside the conduit. During the transition, the thickness of the plug decreases, and finally, the plug diminishes. This transition can be treated as a pre-eruptive process in the conduit. The analysis of the crustal deformation revealed that the increases in the overpressure and the magma ascent velocity beneath the plug cause near-field displacements just before the eruption, leading to observable tilt and strain changes. We also found that our results qualitatively agree with tilt and strain changes observed before the Vulcanian eruptions at Sakurajima volcano, suggesting that the conduit flow-plug dynamics play a key role in excitation of the pre-eruptive crustal deformation.
The conduit flow model is based on Kozono and Koyaguchi (2012) and Wong et al. (2017), in which the magma ascent velocity is described by the sum of the velocity of viscous flow and that of the solid plug. In the viscous flow, essential processes controlling magma dynamics such as equilibrium vesiculation, vertical and lateral gas escapes, crystallization kinetics, and magma viscosity change are considered. The solid plug frictionally slips on the conduit wall, following rate-dependent friction law. The transition from the viscous flow to the solid plug is assumed to occur when the frictional plug velocity is greater than the viscous flow velocity. In the crustal deformation model, the elastic modulus surrounding the conduit-chamber system is set in the 2-dimensional axisymmetric domain, and we calculate the surface displacement caused by the normal and shear stresses acting on the conduit wall and the normal stress on the chamber wall which are obtained from the conduit flow model.
In the analysis of the conduit flow, we found that the solid plug is formed beneath the vent for a low magma flow rate. When the magma influx to the magma chamber reaches a critical value, the conduit-chamber system becomes unstable, leading to abrupt transition of the conduit flow accompanied by drastic increases in the magma flow rate, the magma porosity, and the overpressure inside the conduit. During the transition, the thickness of the plug decreases, and finally, the plug diminishes. This transition can be treated as a pre-eruptive process in the conduit. The analysis of the crustal deformation revealed that the increases in the overpressure and the magma ascent velocity beneath the plug cause near-field displacements just before the eruption, leading to observable tilt and strain changes. We also found that our results qualitatively agree with tilt and strain changes observed before the Vulcanian eruptions at Sakurajima volcano, suggesting that the conduit flow-plug dynamics play a key role in excitation of the pre-eruptive crustal deformation.