A study for enhancement of predicting volcanic eruptions and revealing properties of a host medium of magmatic activities in Usu volcano, Japan

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Volcanic phenomena observed in four eruptions during the 20th century were compiled and characteristics of precursory seismic activities of these eruptions were compared, in order to enhance short term prediction of volcanic eruptions. We showed similarities of earlier stage of the precursory activities between the summit and flank eruptions, while there exist unique activities only before the flank eruptions. For long term prediction, seismic activities and ground deformations in dormant period were investigated. The seismic activities beneath the summit are caused by subsidence of the 1977-82 crypto-dome. Beneath Usu volcano, the basement layer gets shallower toward the north. Northward and southward migrating seismic activities before the 2000 eruption occurred along the dipping basement, suggesting the activities were constrained by the subsurface structure. Asymmetric patterns of ground deformations caused by lava and crypto-domes were related to the subsurface basement structure. Furthermore, subsurface velocity structure of Hokkaido-Komagatake volcano was revealed for future comparative studies, and a high velocity region was detected beneath the summit.