Abstract
Long-term deformation of Kusatsu-Shirane and Asama volcanoes in central Japan were investigated using Global Navigation Satellite System (GNSS) measurements. Large postseismic deformation caused by the 2011 Tohoku earthquake—which obscures the long-term volcanic deformation—was effectively removed by approximating the postseismic and other recent tectonic deformation in terms of quadrature of the geographical eastings/northings. Subsequently, deformation source parameters were estimated by the Markov Chain Monte Carlo (MCMC) method and linear inversion, employing an analytical model that calculates the deformation from an arbitrary oriented prolate/oblate spheroid. The deformation source of Kusatsu-Shirane volcano was found to be a sill-like oblate spheroid located a few kilometers northwest of the Yugama crater at a depth of approximately 4 text {km}, while that of Asama was also estimated to be a sill-like oblate spheroid beneath the western flank of the edifice at a depth of approximately 12 text {km}, along with the previously reported shallow east–west striking dike at a depth of approximately 1 text {km}. It was revealed that (1) volume changes of the Kusatsu-Shirane deformation source and the shallow deformation source of Asama were correlated with the volcanic activities of the corresponding volcanoes, and (2) the Asama deep source has been steadily losing volume, which may indicate that the volcano will experience fewer eruptions in the near future.
Highlights
Monitoring of volcanic deformation is a powerful tool to assess a potential volcanic hazard
I observed the corresponding enhancement of volcanic activities around the Yugama crater in 2014 and 2018 onward as summarized in the introduction
I suggest this correlation implies that the KusatsuShirane source acts as a heat-source for the shallow source that triggers volcanic activity
Summary
Monitoring of volcanic deformation is a powerful tool to assess a potential volcanic hazard. With the processed coordinate time series, I determine the possible source locations of Kusatsu-Shirane and Asama volcanoes using the Markov Chain Monte Carlo (MCMC) method (e.g., Munekane et al 2016), employing an analytical model that calculates the deformation from an arbitrary oriented prolate/ oblate spheroid (Cervelli 2013). The Kusatsu‐Shirane source To represent the deformation source around KusatsuShirane volcano, I used an analytical model for an arbitrarily oriented spheroid with a point-source approximation where the ratio of the horizontal axis to the vertical axis (axis ratio) is estimated, instead of considering the absolute semi-axis dimensions (Cervelli 2013; Xue et al 2020) This source is intended to represent a deep source that is supposed to be located in the north or north-western part of the Yugama crater (Japan Meteorological Agency 2018b). This procedure is repeated for variables β02, β12, β22 to conduct a grid search for minimizing ABIC and the estimated parameter values for which ABIC is minimum are adopted
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