Geophysical comparison of the three eruptions in the 20th century of Usu volcano, Japan

Usu volcano has erupted seven times since 1663. The last three eruptions occurred during the 20th century (1910, 1943 and 1977) and were observed by standard instruments. Although the three eruptions displayed different surface manifestations, they were, similarly, volcanisms derived from dacitic magmas. In this paper, the three eruptions are compared mainly from the viewpoint of magma movements before and during the eruptions on the basis of geophysical data, i.e. explosive, seismic and thermal activities, ground deformations, and level changes in aquifers. Post-eruption activities are also comparatively reviewed. Some phenomena related to the three eruptions are reinterpreted: for the 1910 eruption, the cryptodome model of the upheaval is rejected, and the tectonic structure of the craterlet line is proposed; for the 1943 and the 1977 eruptions, the ratios between seismic energy release and deformation are discussed; for the 1977 eruption, a tilt model of the sector deformation is proposed and relationships between anomalous changes in the aquifer level and ground upheavals are discussed. Magnitudes of the three eruptions are estimated. Tentatively, the 1943 and 1977 eruptions are roughly of the same order of magnitude and the 1910 eruption is one order of magnitude smaller.