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Abstract
Ground deformation in and around Jigokudani valley during the period from September 2021 through September 2022 detected from leveling surveys and GNSS observations was modeled to interpret what occurred beneath the Jigokudani valley accompanied by volcanic activities. Most leveling benchmarks showed an uplift of up to 0.9 cm during the 1-year period. Some benchmarks located in the west part of Jigokudani valley showed subsidence. Two GNSS stations in the valley also showed uplift up to 1.1 cm. These two GNSS stations (MDGB and MDGF) showed horizontal displacements up to 1.6 cm. The GNSS station locates out of fumarolic areas of the valley (J470) showed displacements of less than 0.1 cm. A dislocation model was applied to the deformation. Assuming a Poisson’s ratio of 0.25, the optimal parameters for the source were searched using Adam, one of a gradient method. A vertical rectangle crack with a length of 549 m, a width of 3.94 m and a dip of 83.2° is located at a depth of 2.94 m from Hyakusho jigoku through the new fumarolic area, which has recently become highly activated. The strike of the crack is N59.9°E. The opening of the crack of 15.4 cm yields a volume increase of 333 m3. The obtained crack is considered to correspond to the shallow aquifer, and the aquifer pushed out by hot volcanic gas and generated water vapor. Further investigation is needed to confirm the result and interpretation of the present study. Although overall deformation can be broadly explained by opening of the vertical crack, there are some differences between observation and calculation probably because the actual shape of the crack is more complex. In addition, it is necessary to construct a unified model, combining not only geodetic data, but also other data.Graphical
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