Abstract
Ground deformation often precedes volcanic eruptions, and results from complex interactions between source processes and the thermomechanical behaviour of surrounding rocks. Previous models aiming to constrain source processes were unable to include realistic mechanical and thermal rock properties, and the role of thermomechanical heterogeneity in magma accumulation was unclear. Here we show how spatio-temporal deformation and magma reservoir evolution are fundamentally controlled by three-dimensional thermomechanical heterogeneity. Using the example of continued inflation at Aira caldera, Japan, we demonstrate that magma is accumulating faster than it can be erupted, and the current uplift is approaching the level inferred prior to the violent 1914 Plinian eruption. Magma storage conditions coincide with estimates for the caldera-forming reservoir ~29,000 years ago, and the inferred magma supply rate indicates a ~130-year timeframe to amass enough magma to feed a future 1914-sized eruption. These new inferences are important for eruption forecasting and risk mitigation, and have significant implications for the interpretations of volcanic deformation worldwide.
Highlights
Ground deformation often precedes volcanic eruptions, and results from complex interactions between source processes and the thermomechanical behaviour of surrounding rocks
This is pertinent when assessing if a volcano is escalating towards initial, renewed, or heightened eruptive activity. The latter may currently be the case at Aira caldera, where ongoing uplift signifies major concerns for volcanic hazard and risk assessment
The Finite Element (FE) models employed do not account for faulting related processes, which can act as strain barriers[38] and may reactivate due to magmatic accumulation[39], and could provide an explanation for the discrepancy between modelled and observed deformation vectors
Summary
The geodetic models used to date to explain the various periods of deformation at Aira caldera have all been based on the assumption of a simple, homogeneous, elastic, half-space[2]. To account for the known three-dimensional subsurface mechanical heterogeneity from the seismic velocity data (Fig. S2, Fig. S3, and Table S1), as well as surface topography, we use Finite Element (FE) analysis for our geodetic modelling (Fig. S4). We assess the influence of including (TOPO), or excluding (HALF), topography and bathymetry, as the caldera depression and steep stratovolcano could induce surface strain localisation not accounted for with a flat half-space approach. This procedure produces a total of six different model classes. Inversions were run using spherical, prolate, and oblate shaped deformation sources of varying size (Table S2) to obtain the optimal location and over-pressure to fit the surface displacement data (Methods)[34]
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