Numerical simulation of magma migration prior to the eruption at the Showa crater of Sakurajima volcano: Implications from the ground deformations in 2009

Understanding the migration of magma before volcanic eruption is of importance to evaluate the eruptive mechanism and to promote the prediction of volcanic eruption. We use a model with two magma reservoirs located beneath the Showa crater of Sakurajima volcano at depths of 4.0 km and 0.1 km interconnected with each other through a conduit. The locations of reservoirs were inferred using the Mogi's model from the past geodetic observation data. The change of tilt and strain data observed 36 hours before an eruption on April 9, 2009 is analysed in terms of the magma migration prior to the eruption. The data indicates that the fluctuation in the volumes of two reservoirs takes place in the course to the eruption with a slight phase lag of a few hours between the two, i.e., the volumetric increase of the shallower is initiated when the volume of the deeper starts to decrease, and vice versa. We conduct numerical simulations for the magma plumbing system with the focus on magma flow between the two reservoirs for explaining the two observed features, i.e., the phase lag and the volumetric changes. In the simulations, a constant magma supply and a pressure threshold to initiate the ascent of magma from the deeper to the shallower are assumed. During the ascent of magma, we allow the escape of gas, the effects of bubble deformation, the vesiculation of volatiles in magma, and the existence of overpressurized bubbles. The wide range in the physical properties of magma from basaltic to rhyolitic melts is covered in the equations applied in our simulation. Possible flow regimes from mono-phase gas flow through the porous network to two-phase flow of gas and melt mixture are considered to explain both the relative rate of the volumetric changes in the two reservoirs and the time lag between the two. Finally, we found that the observed phase lag and the relative rate of volumetric changes would be reproduced in the simulations. We would like to propose that the numerical simulation of magma migration leading to an eruption could offer quantitative and powerful insights with respect to eruption forecasting.