Kusatsu-Shirane volcano as a site of phreatic eruptions

Abstract

This paper reviews the hydrothermal systems of Kusatsu-Shirane volcano, Japan, which are associated with phreatic eruptions. The existence of hydrothermal systems at this volcano is easily explained: hot springs are derived from unique thermal water that results from condensation of magmatic gas. Kusatsu-Shirane also exhibits fumaroles characterized by high H2S and CO2 contents, which are separate from the condensation of magmatic gas. Clay layers composed of smectites control the subsurface flow of thermal water. Hypocenter distributions of microearthquakes approach from depth to a bellshaped impermeable clay layer underlying the Shirane pyroclastic cone, indicating the clay layer’s role in storing thermal water supplied from depth. Sources of low-frequency earthquakes, ground deformation, and demagnetization/magnetization are located around the bell-shaped impermeable clay. These observations indicate that a hydrothermal reservoir exists under the clay layer. Phreatic eruptions seem to result from the growth of cracks connecting the reservoir to the surface. Precursory changes in volcanic activity precede phreatic eruptions at Kusatsu-Shirane in most cases; however, the contents of such precursors do not correlate with the ejecta mass, locations, and lead times of eruptions. Kusatsu-Shirane has been continuously monitored since the 1970s. The phreatic eruption of 1976 was predicted based on geochemical observations, but no precursor warning was detected before the onset of a series of phreatic eruptions in 1982–1983. Microearthquake swarms that occurred in 1989–91 and 2014 were followed by demagnetization and changes in the chemical composition of the water in Yugama Crater Lake and the fumaroles. These changes were similar to precursors of past phreatic eruptions at Kusatsu-Shirane, but no phreatic eruption occurred at Yugama Crater within 2–3 years of either set of changes. Multiparameteric monitoring, including geophysical and geochemical observations, is a powerful tool for detecting changes in volcanic activity, but it is difficult to identify precursors of phreatic eruptions.