Modelling magma storage and transport in Aira Caldera and Sakurajima Volcano, Japan.

Sakurajima volcano, located on the rim of the Aira caldera in Japan, represents a major hazard for the heavily populated area of Kagoshima Bay. In recent decades, ground deformation modelling and seismic imaging have inferred the presence of a large magma reservoir ~10-15 km below Aira caldera [1] and one or multiple shallower reservoirs below Sakurajima [2, 3]. Understanding the connectivity between these reservoirs is critical for hazard assessment, as deep-melt migration into the shallow system can trigger major eruptions [4]. To this end, accurate models of the magma plumbing system are needed, considering both realistic reservoir geometries and the possibility of magma storage in dynamic magma-mush systems rather than melt-filled cavities. Modelling reservoir stability and magma transport also requires realistic estimates of the state of stress underground. In this regard, the location of Aira caldera within the Kagoshima graben offers a unique case study, as the regional stress field is likely modulated by various factors beyond reservoir pressurisation. In this study, we employ Finite-Element numerical modelling [5] and recent GNSS and seismic tomography data to investigate the coupled plumbing systems of the Aira-Sakurajima complex, describing the deep reservoir as a poroelastic magma mush. First, we use ground deformation data to constrain the geometry and location of the reservoirs, as well as melt supply parameters. We introduce a complex geometry for the deep reservoir inferred from seismic tomography [1], assessing its influence on deformation modelling compared to previously employed simplified geometries. We also estimate the volume of the active magma source, providing an upper limit to the magnitude of current eruptions. Finally, we integrate the best-fit model of plumbing system architecture and pressurisation into stress models including gravitational loading and tectonic stress to identify the conditions for magma exchange between the deep and shallow reservoirs, which might escalate volcanic risk at Sakurajima.

References:

[1] Tameguri et al. (2022) Bulletine Volcanological Society Japan, https://doi.org/10.18940/kazan.67.1.69

[2] Araya et al. (2019). Scientific Reports, https://doi.org/10.1038/s41598-019-38494-x

[3] Hotta et al. (2016). Journal of Volcanology and Geothermal Research. http://dx.doi.org/10.1016/j.jvolgeores.2015.11.017

[4] Hickey et al. (2016). Scientific Reports, https://doi.org/10.1038/srep32691

[5] Mantiloni et al. (2026). Journal of Geophysical Research: Solid Earth, under review.