oa Numerical Simulation of Magma Plumbing System Associated with the Eruption at the Showa Crater of Sakurajima Inferred from Ground Deformation

ground deformation associated with the eruption at the Showa crater of Sakurajima, which has been active since 2006. Using the Mogi’s spherical pressure model, a volume change of magma chambers can be estimated from the displacement, tilt, or strain observations near the ground surface. The tilt and strain data observed 36 hours before an eruption in April 9, 2009, are analyzed using the Mogi’s spherical model to reveal the behavior of magma leading to eruption. From these data, there seems to be a time lag in the inflation between the two magma chambers at a depth of 4km and 0.1km, respectively. Moreover, the volume change of the shallow source is about one order less than that of the deep one. A system which consists of shallow and deep magma chambers and several vertical conduits connecting them is numerically modeled to investigate the mechanism of the time lag and why the difference in the magnitude of the volumetric changes in the two chambers appears as described above. The initial values of magma properties in the deep magma chamber are assumed from the volcanic ejecta of Sakurajima volcano. We assumed that magma is supplied with a constant rate to the deep magma chamber. The pressure limit that the magma starts to ascend to the shallow chamber is assigned to the deep one. In a one-dimensional steady flow model of a magma conduit, we consider the vesiculation of volatile-bearing magma, gas escape and overpressure in the bubble due to the viscous resistance, which largely influences the physical properties of magma. We confirmed that our hypothetical model could explain the time lag of the inflation and the difference in the volume change although our simulation results cannot exactly describe the data. We would like to propose that the numerical simulation could be a powerful tool for understanding the behavior of magma before eruption once ground deformations are well observed.