Deformation source revealed from leveling survey in Jigokudani valley, Tateyama volcano, Japan

Abstract

We modeled the vertical deformation detected from a leveling survey in Jigokudani valley, Tateyama volcano, central Japan. In Jigokudani valley, uplift of 4 cm/year was previously detected during the period from 2007 to 2010 by interferometric synthetic aperture radar (InSAR). To confirm whether this inflation has continued to present day, we have conducted leveling surveys in Jigokudani valley since 2015. Most bench marks showed a subsidence of up to 5.6 cm during the 4-year period from October 2016 to September 2020, while a bench mark located at the center of the leveling route uniquely showed an uplift of 1.6 cm. We applied a dislocation source model to the deformation using a grid search method. A crack with a length of 650 m, a width of 425 m, a strike of N18° E and a dip of 67° is located at a depth of 50 m near the center of Jigokudani valley (Koya jigoku and the new fumarolic area) where higher activity has been observed recently. Closing of the crack of 59 cm yields a volume decrease of 163,000 m3. The closing direction of the crack is parallel to the line of old explosion craters (Mikurigaike and Midorigaike ponds) and corresponds to the current maximum compressive stress field in the region of the Hida Mountains, including Tateyama volcano. The deformation source of the previous period from 2007 to 2010 detected from InSAR was estimated to be at a depth of 50 m and a gas chamber was correspondingly found in an audio-frequency magnetotelluric (AMT) survey. The AMT survey also revealed that thermal fluid is accumulating from a magma chamber and the location of our crack is similar to uppermost part of the thermal fluid path. During the period from 2015 to 2016, the crack opened and the inflation stopped during the next 1 year period from 2016 to 2017. During the period from 2017 to 2020, the crack began closing, probably because of the increase in emissions of volcanic fluid or gas with the formation of a new crater at the western side of Jigokudani valley during the period from 2017 to 2018.Graphical