Very Local Subsidence Near the Hot Spring Region in Hakone Volcano, Japan, Inferred from InSAR Time Series Analysis of ALOS/PALSAR Data

Abstract

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 30 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.

Volcanic Unrest at Hakone Volcano after the 2015 phreatic eruption — Reactivation of a Ruptured Hydrothermal System?

“…The seismic swarm beneath the northern (2017; A in Figure 4b) and western (2019; B in Figure 4c) caldera rim during the unrest episodes could be the manifestation of uid injection from depth to a separated part of the hydrothermal system that caused a pore pressure rise and uid migration as observed in the previous unrest (Yukutake et al 2011), although a detailed analysis remains yet to be done (Figure 12d). Doke et al (2020) detected a contraction source in the west of Owakudani using InSAR time series analysis, which is distinct from the vapor pocket beneath the eruption center. This result implies that the hydrothermal system beneath the central cone of Hakone volcano is not a single large expanse as implied from the resistivity structure (Yoshimura et al, 2018), which is highly interconnected.…”

Editorial for Special Issue “Monitoring Land Subsidence Using Remote Sensing”

“…Land subsidence rates were compared also with the geological information of the analyzed area: they suggest that the land subsidence (with maximum displacement rate that reaches up to −6 ± 0.46 mm/y in the LOS direction and only in localized deformation zones) occurred due to relatively weak subsurface layers (hazard zone reported as an artificial fill area) that either was affected by loading of new constructions or by hydro-compaction. Doke et al [15] performed an InSAR time series analysis of the Hakone Volcano (Japan) from 2006 to 2011 using the SBAS method and ALOS-PALSAR scenes (24 from the ascending and 22 from the descending orbits, respectively). The authors corrected the obtained InSAR displacements using the available GNSS data.…”

“…The PSI and SBAS approaches can be successfully applied over many different ground targets such as buildings, infrastructures, outcrops, base soils, low-vegetated area, etc. [11], and in several research fields, such as tectonics and volcanology [15], landslides [10,11,16], clay deposits deformations [5,17], and groundwater/oil/natural gas depletion [9,18].…”

“…Despite the large advantages of the multi-temporal DInSAR methodology, the acquisition of ground-based data, mainly from leveling and GNSS techniques, is crucial for validation and integration of the satellite measurements. Nowadays, the land subsidence monitoring mostly relies on the integration between available and reliable InSAR data and sparse, but highly accurate, GNSS measurements, which are often compared to other geodetic observation methods [9][10][11][12]15,19,20].…”

Additional Reference Height Error Analysis for Baseline Calibration Based on a Distributed Target DEM in TwinSAR-L

“…These advantages make it possible not only to generate high-precision digital terrain model with less influence of vegetation, but also better to realize the global measurement of forest height and related biomass with PolInSAR and multi-baseline tomography techniques [6][7][8]. It can also fulfill the measurement of deformations and motions on the Earth with accuracy down to centimeters or even millimeters using repeat-pass interferometry [9][10][11]. Therefore, many countries, such as Germany, Japan, and China, are vigorously developing L-band space-borne SAR.…”