Very local subsidence near the hot spring region in Hakone Volcano, Japan, inferred from InSAR time series analysis of ALOS/PALSAR data

Monitoring of surface displacement by satellite-based interferometric synthetic aperture radar (InSAR) analysis is an effective method for detecting land subsidence in areas where routes of leveling measurements are undeveloped, such as mountainous areas. In particular, InSAR-based monitoring around well-developed hot spring resorts, such as those in Japan, is useful for conserving hot spring resources. Hakone Volcano is one of the major hot spring resorts in Japan, and many hot spring wells have been developed in the Owakudani fumarole area, where a small phreatic eruption occurred in 2015. In this study, we performed an InSAR time series analysis using the small baseline subset (SBAS) method and ALOS/PALSAR scenes of the Hakone Volcano to monitor surface displacements around the volcano. The results of the SBAS-InSAR time series analysis show highly localized subsidence to the west of Owakudani from 2006–2011 when the ALOS/PALSAR satellite was operated. The area of subsidence was approximately 500 m in diameter, and the peak rate of subsidence was approximately 25 mm/year. Modeling using a point pressure source suggested that the subsidence was caused by a contraction at approximately 700 m above sea level (about 300 m below the ground surface). The rate of this contraction was estimated to be 1.04 × 104 m3/year. Hot spring water is collected from a nearby well at almost the same depth as the contraction source, and its main dissolved ion component is chloride ions, suggesting that the hydrothermal fluids are supplied from deep within the volcano. The land subsidence suggests that the fumarole activity is attenuating due to a decrease in the supply of hydrothermal fluids from deeper areas.