Numerical study on the damped oscillation of effusive eruption: a case study of the 1991-1995 Mt. Unzen eruption (Japan)

Geophysical observations have detected damped oscillations in chamber pressure and discharge rate during effusive eruptions. This behavior is caused by the interaction between the chamber, which acts as an elastic capacitance, and the variable flow resistance of the conduit. Previous numerical approaches have shown that conduit deformation during eruptions causes damped oscillatory changes in the discharge rate and pressure. However, the change in the conduit radius due to the influence of irrational modeling, which does not consider changes in the physical properties of the magma, is overestimated. Therefore, to overcome this limitation, in this study, we constructed a new mathematical model that incorporates changes in the physical properties of ascending magma into a mathematical model of magma-supplying systems that consider conduit deformation. The proposed model revealed that changes in the physical properties of magma during ascent affect the flow resistance by influencing the efficient viscosity and bulk density, which in turn affect the amplitude of damping oscillation and scale of the conduit radius. The model was applied to the 1991-1995 eruption of Mt. Unzen, which showed two peaks of pulsating lava extrusion within a consistent size of conduit radius with the petrological studies. As an application, the model estimates the chamber volume to be [[EQUATION]], which is slightly smaller than that of the potential magma chamber zone detected by geophysical observations beneath Mt. Unzen, indicating that the proposed numerical model might constrain the mechanical size of the elastic chamber. Moreover, the estimated crustal viscosity is approximately 4 - 5 orders of magnitude smaller than typical values reported by geodetic observations. The underestimation may be caused by the effects of partial melting of the host rock and/or the dike-shaped conduit. Our results suggest that the effects of conduit deformation and changes in magma properties must be taken into account when considering the temporal changes in eruptions.