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.

中文翻译：

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基于PALSAR-2时间序列分析的与2018年草津-白根潜水喷发相关的火山喷发和后喷发地壳变形

协同变形有助于解释与火山爆发相关的物理过程。由于潜水喷发会导致小的局部协同变形，我们有时无法识别合理的变形信号。卫星合成孔径雷达 (SAR) 数据使我们能够以高空间分辨率识别广泛的变形场。在此，我们报告了与 L 波段卫星 SAR (ALOS-2/PALSAR-2) 数据的时间序列分析检测到的 2018 年草津-白根潜水喷发相关的爆发性地壳变形。从 SAR 时间序列分析得出的累积变形图表明，下沉和向东位移主导着一个直径约 2 公里的空间范围的喷发火山口的西南侧。尽管我们无法在 2018 年喷发之前识别出任何显着的变形信号，但火山口西南侧的后喷变形一直持续到 2019 年底。这种长时间的变形意味着在封闭的热液系统内进行了后喷的物理过程，例如作为火山流体排放，类似于 2014 年 Ontake 喷发期间观察到的过程。尽管积雪和茂密的植被阻碍了使用传统 InSAR 数据检测草津-白根火山上的变形信号，但具有各种时间基线的 L 波段 SAR 使我们能够成功提取协同和后期变形信号。提取的累积变形可以通过正断层作用与沿西南倾斜断层面的左侧滑动分量和各向同性收缩的组合得到很好的解释。根据草津-白根火山上浅层热液系统发育的地质背景，推测的位错面可以认为是浅层热液系统因潜水喷发而到地表的脱气通道。我们再次确认 SAR 数据是检测协同喷发和后喷发变形的强大工具，有助于理解与活火山潜水喷发相关的浅层物理过程。推断的位错平面可以被认为是由于潜水喷发从浅层热液系统到地表的脱气通道。我们再次确认 SAR 数据是检测协同喷发和后喷发变形的强大工具，有助于理解与活火山潜水喷发相关的浅层物理过程。推断的位错平面可以被认为是由于潜水喷发从浅层热液系统到地表的脱气通道。我们再次确认 SAR 数据是检测协同喷发和后喷发变形的强大工具，有助于理解与活火山潜水喷发相关的浅层物理过程。