International Workshop on the Mechanism of Phreatic Eruption

Relationship between phreatic eruptions and the active hydrothermal
system of Kusatsu-Shirane volcano, JapanAkihiko TeradaKusatsu-Shirane Volcano Observatory, Volcanic Fluid Research Center, School of Science,
Tokyo Institute of Technology
E-mail: terada(at)ksvo.titech.ac.jp

Mount Kusatsu-Shirane, central Japan, is an active andesitic volcano that is associated withfumaroles, hyper-acidic hot waters, and other signs of vigorous thermal activity (Ohba et al.,
2000; Terada and Hashimoto, 2017). Yugama crater lake is located at the top of the volcano
and forms part of the beautiful scenery of the area. The lake water has high concentrationsof Cl– and SO42–, which change markedly in response to volcanic activity. The thermal activity
is most obvious at the famous spa town of Kusatsu, where the flow rate of hot water is~32,000 L min–1. Four million people visit the town of Kusatsu each year to see the volcano
and enjoy the hot spring, so understating the hydrothermal system associated with phreaticeruptions is critical.
Geochemical data indicate that the hot springs are supplied by a tow-phase reservoir of hotwater produced by the condensation of magmatic gases. The fumarolic gases arecharacterized by high H2S and CO2 concentrations, and separated from the reservoir (Ohbaet al., 2008). Magnetotelluric surveys show clay layers below Yugama crater lake, and thesecontrol the subsurface flow of hot water (Nurhasan et al., 2006; Ogawa et al., in prep.).
In 2014, unusual volcanic activity included changes in seismic activity, ground deformation,and the compositions of fumarolic gases and lake water, providing insights into thehydrothermal system. It is inferred that a horizontal body of a source of over-pressure exists
1,000 m below the bottom of Yugama crater lake. Precise measurements of the hypocentersof microearthquakes show that they are distributed from depths to the bottom of a bellshaped impermeable clay layer that underlies Yugama crater lake. A source of low-frequency
earthquakes (Nakano et al., 2003) and demagnetization/magnetization (Takahashi and Fujii,2014) is located close to the over-pressure source. On the basis of these observations, I inferthat a hydrothermal reservoir exists below the clay layer that underlies Yugama crater lake.
Phreatic eruptions are related to the growth of fractures that connect the reservoir to theground surface or the bottom of the lake.
Identification of the precursors to phreatic eruptions at Kusatsu-Shirane volcano remainchallenging (Terada, 2018). Geophysical and geochemical parameters have been monitored

International Workshop on the Mechanism of Phreatic Eruption

continuously at Kusatsu-Shirane volcano since the 1970s, providing an effective record ofchanges in volcanic activity. Geochemical observations were used to predict a phreaticeruption in 1976 (Ossaka et al., 1980), however, such proxies for volcanic activity are notcorrelated with ejecta mass, location, or timing of eruptions. Although obvious changes ingeochemical and geophysical data were detected around Yugama crater lake in 1990 and2014, then no eruptions occurred at Yugama.

Fig. 1. Schematic illustration of water flow and storage near Yugama crater lake at KusatsuShirane volcano.
Main References
Nurhasan, Ogawa, Y., Ujihara, N., Tank, S. B., Honkura, Y., Onizawa, S., Mori, T. and Makino,M., 2006, Two electrical conductors beneath Kusatsu-Shirane volcano, Japan, imaging
by audiomagnetotellurics and their implications for hydrothermal system. Earth Planet.
Space, 58, 1053–1059, doi: 10.1186/BF03352610
Terada, A. and Hashimoto, T., 2017, Variety and sustainability of volcanic lakes: Response
to subaqueous thermal activity predicted by a numerical model. Jour. Geophys. Res.,
122, 6108–6130, doi: 10.1002/2017JB014387
Terada, A., 2018, Kusatsu-Shirane volcano as a site of phreatic eruptions, Jour. Geol. Soc.
Japan, 124, 251-270, doi: 10.5575/geosoc.2017.0060 (In Japanese with Englishabstract and figures)