Reconstructing past volcanic plume activity of Kusatsu–Shirane volcano, Japan, using historical bird’s-eye views of the Kusatsu spa area

Kusatsu–Shirane volcano is one of the most active volcanoes in Japan and is characterized by frequent phreatic eruptions. Understanding its history of plume activity, including minor phreatic eruptions and temporal variations in fumarolic activity, is critical for evaluating its future eruptive potential. In this study, we focused on historical bird’s-eye view (BEV) of the Kusatsu spa area, located at the foot of the volcano. Numerous BEVs are still preserved because they were frequently published and widely circulated as souvenirs from the 18th to the early 20th centuries, whereas a major fire in the Kusatsu spa area in 1869 destroyed many historical documents. Based on analyses of 69 BEVs, 5 historical documents, and two pictorial maps, we show that volcanic plume activity has persisted for the past 200 year, with the exception of a quiescent period during 1849–1881 AD. The heat discharge rate is estimated to have been on the order of around 50–100 MW, which is comparable to, or possibly greater than, that of the present-day activity. Preceding the 1882 eruption, which was one of the two largest eruptions of Kusatsu–Shirane volcano in historical times, volcanic plumes and hot crater lakes showed a temporary decline for ca . 30 year (1849–1881 AD), probably due to the sealing of subsurface fluid pathways that led to the subsequent major eruption in 1882. Since the 1941 eruption, thermal activity has changed from persistent plume emissions to a hot crater lake. The onset of lower-enthalpy fluid emissions was likely caused by a change in the heat and mass balance of the underlying hydrothermal system. For the first time, historical documents describing the activity of the Motoshirane pyroclastic cone have been identified. These indicate that transient, vigorous fumarolic activity or a single eruptive event similar to the 2018 eruption at the Motoshirane pyroclastic cone occurred during 1815–1830.