Abstract
Global navigation satellite system data from Hakone volcano, central Japan, together with GEONET data from the Geospatial Information Authority of Japan, were used to investigate the processes associated with the volcanic activity in 2015, which culminated in a small phreatic eruption in late June 2015. Three deep and shallow sources, namely spherical, open crack, and sill, were employed to elucidate the volcanic processes using the observed GNSS displacements, and the MaGCAP-V software was used to estimate the volumetric changes of these sources. Our detailed analysis shows that a deep inflation source at 6.5 km below sea level started to inflate in late March 2015 at a rate of ~ 9.3 × 104 m3/day until mid-June. The inflation rate then slowed to ~ 2.1 × 104 m3/day and ceased at the end of August 2015. A shallow open crack at 0.8 km above sea level started to inflate in May 2015 at a rate of 1.7 × 103 m3/day. There was no significant volumetric change in the shallow sill source during the volcanic unrest, which is evident from interferometric synthetic aperture radar analysis. The inflation of the deep source continued even after the eruption without a significant slowdown in inflation rate. The inflation stopped in August 2015, approximately 1 month after the eruption ceased. This observation implies that the transportation of magmatic fluid to a deep inflation source (6.5 km) triggered the 2015 unrest. The magmatic fluid may have then migrated from the deep source to the shallow open crack. The phreatic eruption was then caused by the formation of a crack that extended to the surface. However, steam emissions from the vent area during and after the eruption were apparently insufficient to mitigate the internal pressure of the shallow open crack.
Highlights
Phreatic eruptions are the result of perturbations of the volcanic hydrothermal system (e.g., Barberi et al 1992)
The rapid inflation from late March implies the transportation of magmatic fluid into a mid-crustal magma chamber, as inferred from the contemporaneous deep low-frequency seismic events
The inflation that initiated from mid-May in the shallow source region may be due to the injection of hydrothermal fluid into the hydrothermal system above the magma chamber
Summary
Phreatic eruptions are the result of perturbations of the volcanic hydrothermal system (e.g., Barberi et al 1992). Recent phreatic eruptions in the Japanese Islands have caused significant human casualties, with 61 fatalities during the September 27, 2014, Mt. Ontake eruption and one fatality during the January 23, 2018, Mt. Kusatsu Shirane eruption. Hakone volcano, located in the central part of Honshu Island, central Japan, recently generated a small phreatic eruption on June 29, 2015, at the Owakudani fumarole area, which is the largest fumarolic area on the volcano (Fig. 1). There were no fatalities, the volcanic activity caused serious human safety concerns since the area is an internationally famous sightseeing location in Japan, with many visitors to the fumarole area year-round. The elevated fumarolic activities caused serious damage to some of the steam wells that are used to make hot springs for the surrounding hotels, and these hotels were forced to temporarily stop providing hot springs to their visitors, resulting in significant economic losses to the area. The phreatic eruption was not a large event, its impact raised an important question regarding how intensively
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