Transient inflation event at Kussharo caldera, eastern Hokkaido, Japan in 2021–2023 captured by multiple GNSS observation networks

Abstract Three Global Navigation Satellite System observation networks realize the detection of an evident transient crustal deformation induced by volcanic inflation activity at the Kussharo caldera, eastern Hokkaido, Japan, from 2021 to 2023. To explain this deformation field, we model three types of volcanic deformation sources, the Mogi, spheroid, and sill, using the Markov Chain Monte Carlo method. Although the spheroid and sill models also show no large differences and can generally explain the observations, the Mogi model best represents the observations, with the source of deformation estimated to be in the center of the caldera at a depth of approximately 5 km. The location of the deformation source is roughly the same regardless of which model was assumed. The inflation source is located west of the Atosanupuri fumaroles, which is the most active area in this caldera, and consistent with the center of the volcanic inflation source in the 1990s detected by the Interferometric Synthetic Aperture Rater analysis. The estimated depth of the deformation source is roughly consistent with the upper limit of the low electrical resistivity region, suggesting the presence of some fluid-like media. The volume change is distributed around 1.1 × 10 7 m 3 , about half that of the 1990s event. This study suggests a possibility that the center of the Kussharo caldera usually contracts slowly and quietly but sometimes induces inflations repeatedly. It is still unclear what triggers the inflation event and how it relates to the long-term activity of the volcano, but future monitoring will provide further insight. Graphical Abstract