Structure of Shallow Hydrothermal System in Hakone Volcano, Japan, Inferred from Surface Displacements

Since a phreatic eruption is caused by ruptures in hydrothermal systems beneath volcanoes, detecting and monitoring a hydrothermal system can play an important role in predicting such an eruption. Interferometric Synthetic Aperture Radar (InSAR), which detects ground deformations over a large area, may be a key technology for use in various fields, as shown from the exponential growth of recent studies in terms of number and quality. The present contribution reviews surface deformations caused by the hydrothermal system of Hakone volcano, as detected by InSAR before, during, and after the 2015 eruption. The opening of the NW-SE-trending crack and localized uplift in the Owakudani fumarole area were captured by InSAR analyses during the 2015 unrest at Hakone volcano. Moreover, an InSAR time series analysis showed steady subsidence on the west side of the Owakudani fumarole area. Based on models explaining these surface displacements, the shallow hydrothermal system of Hakone volcano is characterized by NWSE to WNW-ESE-trending crack-shaped fluid supply paths and pocket-shaped fluid reservoirs. During the 2015 and previous phreatic eruptions, it is probable that fluid was supplied using the same crack-like path, implying that fluid was repeatedly supplied using the same structure. Therefore, in order to predict the occurrence of phreatic eruptions at Hakone volcano, it is necessary to monitor volcanic activity by taking into account these structures. The activity of Hakone volcano, including formations of these NW-SE to WNW-ESE-trending cracks, is dominated by a regional stress field. This stress field is caused by shear deformation due to plate motion occurring in this region; that is, the subducting Philippine Sea Plate, and the colliding Izu Peninsula.