Abstract
In 2014, a phreatic eruption occurred at the Ontake Volcano in Central Japan causing multiple deaths and missing persons. Interferometric Synthetic Aperture Radar data showed local-scale subsidence around the newly created eruptive vents after the eruption. Source modeling resulted in a nearly spherical deflation source emplaced at a depth of 500 m below the vents reflecting post-eruptive depressurization in a shallow hydrothermal reservoir. The cumulative deflation volume reached 7 × 105 m3 3 years after the eruption. Comparison between our source model and GNSS data indicates that this shallow reservoir could have formed by 2007 and remained stable until the 2014 eruption. The absence of significant syn-eruptive subsidence indicates that the shallow reservoir was not the main water source driving the phreatic eruption. Under simple assumptions, mass balance between the shallow reservoir and the discharge plume from the vents indicates most of the water contained in the plume comes from greater depth than the shallow reservoir. To constrain the post-eruptive process, it is necessary to track not only deformation but also plume discharge for 3 years after the eruption.
Highlights
Phreatic eruptions sometimes cause human fatalities when they occur in close proximity to populated areas
It is difficult to clarify the structure of the subsurface hydrothermal system, which is a prerequisite for developing prediction strategies
Modeling To unveil the geometrical structure of the deflation source, we modeled the deformation using both analytical and numerical methods
Summary
Phreatic eruptions sometimes cause human fatalities when they occur in close proximity to populated areas Such disasters might be mitigated by advanced predictions of eruption. In contrast to magmatic eruptions, the precursors to phreatic eruptions are minute and only appear in small proximal areas (Rouwet et al 2014). Some very active hydrothermal systems occasionally reveal their activity through detectable geophysical signals. Such manifestations present a valuable opportunity to gain insights into the hydrothermal systems at the root of
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