JMA reported that after the 23 January eruption near Motoshiranesan (the highest peak belonging to the Kusatsu-Shiranesan complex) seismicity, characterized by volcanic earthquakes and tremor, was elevated; it decreased the next day. Minor but elevated seismicity continued through 30 January, punctuated by periods of tremor. The eruption occurred from a fissure oriented E-W, located just inside the N rim of the northernmost Kagamiike Kitahi craters. JMA noted no juvenile material in the eruption deposits. The Alert Level remained at 3 (on a 5-level scale).

Geological Summary. The Kusatsu-Shiranesan complex, located immediately north of Asama volcano, consists of a series of overlapping pyroclastic cones and three crater lakes. The andesitic-to-dacitic volcano was formed in three eruptive stages beginning in the early to mid-Pleistocene. The Pleistocene Oshi pyroclastic flow produced extensive welded tuffs and non-welded pumice that covers much of the E, S, and SW flanks. The latest eruptive stage began about 14,000 years ago. Historical eruptions have consisted of phreatic explosions from the acidic crater lakes or their margins. Fumaroles and hot springs that dot the flanks have strongly acidified many rivers draining from the volcano. The crater was the site of active sulfur mining for many years during the 19th and 20th centuries.

Sources: Japan Meteorological Agency (JMA), The Mainichi Daily News