Inflation of Izu-Oshima volcano has been observed since soon after the latest middle-scale eruption occurred in 1986. Such deformation is understood as re-storage of magma beneath the volcano. Because more than 230 years have passed since the last large-scale eruption in 1777, Izu-Oshima is overdue for an average recurrence interval of large-scale eruptions, 100 to 150 years. Preparing for the forthcoming eruption, we aime to improve the resolution of volcanic activities in historic time. Archives on historical eruptions, as well as topography, and stratigraphy of the volcano are reviewed. The results are summarized as follows. (1) Based on historical archives as well as stratigraphy of the volcano, we reviewed sequences of Y₃ (1552 Ten'bun), Y₂ (1684 Jokyo) and Y₁ (1777 An'ei) eruptions. All these three eruptions occurred inside the caldera and proceeded in the order of scoria fallout-lava effusion-ash fall, which is the typical sequence in Izu-Oshima. Duration of main eruption stage, start from basal scoria eject to lava effusion, ranges from 1 to 2 weeks in Y₃ and Y₂ eruptions, to 14.5 months in Y₁ eruption which is bigger than the former two. Those of magma-withdrawal stage, ash-fall stage, lasted 6 years in Y₂ and 9 years in Y₁, respectively. (2) In Y₁ eruption (1777-1792), the local government of Izu grasped the sequence of volcanic activities by frequent reports from the island and by inspections of government officers. The local government submitted reports and mitigation programs to the central government. (3) In case of future large-scale eruption occur in caldera, accurate information of magma-head level is indispensable for the prediction of eruption behavior. (4) We pointed out the significance of topographic lows on the northwestern and the northern flank to predict the flow of lavas.

View full abstract