Toward joint muography and ground deformation monitoring for volcanic unrest assessment

Muography is a geophysical monitoring technique that allows passive, remote, and high-definition density imaging of volcanoes by measuring the yield of penetrated cosmic-ray-induced muon particles. This review focuses on joint muon and ground surface deformation monitoring of active volcanoes and its application for exploring the structures of upper plumbing systems, characterizing the eruptive activities, and contributing to intermediate-term (multi-month) hazard assessment. Our work is based on the data acquired at the active Sakurajima volcano in Japan by the Sakurajima Muography Observatory and synthetic aperture radar. An inverse correlation was found between the muographically measured densities through the conduits of the adjacent craters, suggesting a branched linkage between these conduits. The muographically measured densities were related to the vertical ground surface displacements and monthly number of eruptions and gas emission rates. Periods of low eruption frequency were associated with the formation of a dense plug in the conduit, which caused the inflation of the edifice by trapping pressurized magmatic gas and increased mass density. Periods of high eruption frequency were associated with the release of volcanic gases that caused the deflation of the volcanic edifice and decrease in the mass density. Muography can be utilized to explain the linkage between ground surface deformation and eruptions by revealing the causal physical mechanism. The volcanic unrest index was determined using mass density, vertical displacement, and gas flux data monitored from September 2018 to July 2023. Minor unrest was quantified from September 2019 to December 2020.