Characterizing magma storage at Aira caldera and Sakurajima volcano (Japan) from geodetic inversions

Abstract:

Aira caldera is located within Kagoshima Bay at the southern end of Kyushu, in Japan. Sakurajima, a post-caldera andesitic stratovolcano, sits on the calderas southern rim and is Japan’s most active volcano. Historical records of activity at Sakurajima are punctuated with larger events: for example, in 1914 a major flank eruption ejected 0.52 km3 of pumice and ash, and 1.34 km3 of lava. The co-eruptive subsidence accompanying this eruption was around one metre in the vertical component, as determined from leveling surveys. Repeat surveys, and the addition of modern GPS data indicate that the present day level of ground uplift is approaching the level inferred before the 1914 eruption. This study is focused on interpreting the current deformation with integration of additional geophysical signals to constrain the subsurface magma storage conditions. Analytical geodetic models have been previously used to infer a deep spherical reservoir beneath Aira caldera and a smaller source beneath the summit craters of Sakurajima. However, subsequent geophysical explorations have shown that the assumptions inherent in these simple models may not apply. We use the Finite Element method to model the recorded deformation with an inverse least-squares approach. Our models incorporate the surrounding topography/bathymetry and three-dimensional medium heterogeneities inferred from seismic tomography. These are used to assess their respective significance when solving for the optimum source location. Subsequent forward Finite Element models quantify the importance of rheology and thermo-mechanical coupling owing to the observed high seismic attenuation and aseismic zones. The results are evaluated alongside the current volcanic activity to yield new insights into the behaviour of the Aira-Sakurajima sub-volcanic system.