Abstract
Coeruptive deformation helps to interpret physical processes associated with volcanic eruptions. Because phreatic eruptions cause small, localized coeruptive deformation, we sometimes fail to identify plausible deformation signals. Satellite synthetic aperture radar (SAR) data allow us to identify extensive deformation fields with high spatial resolutions. Herein, we report coeruptive crustal deformation associated with the 2018 Kusatsu-Shirane phreatic eruption detected by time series analyses of L-band satellite SAR (ALOS-2/PALSAR-2) data. Cumulative deformation maps derived from SAR time series analyses show that subsidence and eastward displacement dominate the southwestern side of an eruptive crater with a spatial extent of approximately 2 km in diameter. Although we were unable to identify any significant deformation signals before the 2018 eruption, posteruptive deformation on the southwestern side of the crater has been ongoing until the end of 2019. This prolonged deformation implies the progression of posteruptive physical processes within a confined hydrothermal system, such as volcanic fluid discharge, similar to the processes observed during the 2014 Ontake eruption. Although accumulated snow and dense vegetation hinder the detection of deformation signals on Kusatsu-Shirane volcano using conventional InSAR data, L-band SAR with various temporal baselines allowed us to successfully extract both coeruptive and posteruptive deformation signals. The extracted cumulative deformation is well explained by a combination of normal faulting with a left-lateral slip component along a southwest-dipping fault plane and an isotropic deflation. Based on the geological background in which the shallow hydrothermal system develops across Kusatsu-Shirane volcano, the inferred dislocation plane can be considered as a degassing pathway from the shallow hydrothermal system to the surface due to the phreatic eruption. We reconfirmed that SAR data are a robust tool for detecting coeruptive and posteruptive deformations, which are helpful for understanding shallow physical processes associated with phreatic eruptions at active volcanoes.
Highlights
A phreatic eruption is a hazardous volcanic activity event that includes sporadic ejections of volcanic ash, steam and volcanic gases induced by transient pressureHimematsu et al Earth, Planets and Space (2020) 72:116 less damage than magmatic eruptions, but can occasionally produce destructive hazards such as lahars that wash out infrastructure (e.g., Naranjo et al 1986)
In the path 19 averaged interferometric SAR (InSAR) data, we identified a displacement discontinuity with a WNW– ESE strike at the 2018 crater where the path 19 multi-temporal InSAR (MTI) data show missing data (Figure S5 in Additional file 1)
We successfully extracted the coeruptive and posteruptive deformation signals associated with the 2018 Kusatsu-Shirane phreatic eruption based on Advanced Land Observing Satellite 2 (ALOS-2)/ PALSAR-2 MTI data
Summary
A phreatic eruption is a hazardous volcanic activity event that includes sporadic ejections of volcanic ash, steam and volcanic gases induced by transient pressureHimematsu et al Earth, Planets and Space (2020) 72:116 less damage than magmatic eruptions, but can occasionally produce destructive hazards such as lahars that wash out infrastructure (e.g., Naranjo et al 1986). Ground deformation of active volcanoes is usually helpful for interpreting the physical processes of volcanic activity; most of ground deformation associated with phreatic eruptions is characterized as small and localized ones with a sudden onset. These characteristics hinder the detection of plausible deformation signals prior to phreatic eruptions due to sparse observation networks or limited measurement accuracy. Several studies have successfully detected coeruptive deformation associated with phreatic eruptions using satellite SAR data (e.g., Hamling 2017; Doke et al 2018; Narita and Murakami 2018). If precursors of phreatic eruptions, such as overpressure in shallow hydrothermal systems, persist for a long time, satellite SAR data can identify the precursory deformation signals of a phreatic eruption (Kobayashi et al 2018)
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