Abstract
We successfully monitored the ground deformation of an eruption center during the 2015 phreatic eruption of Hakone volcano, Japan, using ground-based interferometric synthetic aperture radar (GB-InSAR). GB-InSAR has been developed and applied over the past two decades and enables the frequent (< 10 min) aerial monitoring of surficial deformation of structures and slopes. We installed a GB-InSAR 4 days before the eruption of Hakone volcano on June 29, 2015, and monitored the ground deformation of an area where uplift was detected by a satellite InSAR. The ground deformation observed by the GB-InSAR began suddenly on the morning of June 29 almost coincident with the intrusion of hydrothermal fluid that was inferred by other geophysical observations. The hydrothermal crack is considered to have caused the eruption, which was known by an ash fall 5 h later. The GB-InSAR results indicated a significant uplifted area which is approximately 100 m in diameter, and new craters and fumaroles were created by the eruption in and around the area. The displacement reached up to a total of 45 mm until the evening of June 29 and continued at least until the morning of July 1. During our observation, the displacement rate decreased twice, and the timing of each decrease seemed to correspond to the formation of new conduits as implied from geophysical observations.
Highlights
A phreatic eruption is a volcanic explosion caused by confined steam and gas beneath the ground surface without the direct involvement of magma (Barberi et al 1992)
The GB-InSAR enables frequent (< 10 min) areal monitoring of ground deformation with resolutions ranging from several mm to tens of mm depending on the monitoring distance and atmospheric conditions
We installed a GB-InSAR to monitor the ground deformation of an eruption center area, which was implied by the surface deformation analysis using satellite-based InSAR (Doke et al 2018)
Summary
A phreatic eruption is a volcanic explosion caused by confined steam and gas beneath the ground surface without the direct involvement of magma (Barberi et al 1992). We installed a GB-InSAR instrument such that the scanned area covers the central Owakudani area (COA) just 4 days before the 2015 eruption of Hakone volcano and continuously monitored the ground deformation associated with the phreatic eruptions until July 3 (Fig. 1). That are similar in principle to those of a satellite-based InSAR, are used in the software (Guardian 3.0) to remove the atmospherically induced phase variation prior to the computation of the relative displacements. Monitoring procedures and results The instrument was installed near the Owakudani ropeway station, approximately 200 m NW of the eruption center (Fig. 1). The time variations of the relative displacement of each area are shown in Fig. 4 together with the earthquake magnitudes and the cumulative counts of seismic events since June 25. Visual observations of the displacements in this time interval imply that the residual atmospheric effect is approximately 1–2 mm which is 3–4 times the STDEV
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