Viscoelastic crustal response to magma supply and discharge in the upper crust: Implications for the uplift of the Aira caldera before and after the 1914 eruption of the Sakurajima volcano

Crustal viscoelasticity is expected to influence volcano deformation as the crustal viscosity at volcanoes is lowered from regional values by higher crustal temperatures and the eventual presence of magma. In this study, we examine viscoelastic crustal response to continuous magma supply into, and sudden discharge from, a sill emplaced in the upper crust, using a three-dimensional (3-D) finite element model composed of an elastic layer underlain by a linear Maxwell viscoelastic layer with spatially uniform viscosity. We find that viscoelastic response to sill deflation due to magma discharge is an important driving force for post-eruption surface displacement, influenced both by how much viscoelastic relaxation occurs in response to sill inflation due to magma supply in the pre-eruption period and by the amount of coeruptive sill deflation. Following an eruption, the surface elevation is always recovered later than the sill volume, because the viscoelastic response to the sill inflation detracts from the uplift. We apply this viscoelastic model to geodetic observations in the Aira caldera, southern Kyushu, Japan, over a time span of more than 100 yr. It is found that a sill emplacement with constant inflation rate can well explain the observed first-order exponential-like surface recovery after the 1914 eruption until similar to 1975 if the effective crustal viscosity is similar to 5 x 10(18) Pa s, the sill emplacement with an equatorial radius of similar to 2 km occurs at a depth of similar to 11 km, and the sill is deflated by similar to 0.4 km(3) during the 1914 eruption, similar to 4 times less than the eruptive volume. The sill emplacement is also required to have begun similar to 50 yr before the 1914 eruption with a constant inflation rate of similar to 0.009 km(3)/yr. In the post-eruption period, the accumulation of magma in the sill during the first similar to 50 yr is lower than that estimated by an elastic model, but larger thereafter. It is necessary to introduce temporal variation of the inflation rate for higher-order improved fitting to the geodetic data after similar to 1975. Our results suggest that geodetic signals observed before and after magma discharge due to an eruption need to be revisited, considering the effects of crustal viscoelasticity. (C) 2019 Elsevier B.V. All rights reserved.