Late Holocene Tephrostratigraphy at Chokai Volcano, Northern Japan, and Contribution to Hazard Assessment

抄録

History and pattern of explosive eruptions at Chokai volcano, Japan, in the last 2500 years were investigated from tephra survey and accelerator mass spectrometry (AMS) radiocarbon dating. The tephrostratigraphy was established based on observations at eight hand-dug trenches and three outcrops. The well-correlated tephra layers were dated at c. 2.5 ka, 2.1–1.9 ka, 1.8 ka, and 1.6 ka, indicating major eruptions occurred at these ages. The tephra from the documented 871 CE eruption was also identified. Componentry analysis of ash was carried out for these five eruption deposits. The changes in tephra facies and ash components within an unbroken series of tephra layers indicate a shift from hydrothermal-dominant phreatic or phreatomagmatic eruption to magma-dominant eruptions in a single episode. Common eruption sequences were identified based on the combination of tephra facies variation and records of witnessed eruptions. Every volcanic activity begins with precursory activity of seismicity, fumaroles, and snow melting for weeks to months, then onset hydrothermal-dominant eruption happens. Then, the eruption evolves to a magma-dominant eruption, or alternatively, the hydrothermal-dominant eruption persistently continues until cessation. The eruption sizes are VEI 2 or more minor. Lahar can occur at any stage of the eruption, resulting in damage to the residential area at the base of the volcano. The eruption patterns and the extent of hazard risks elucidated by this study will be utilized to hazard mitigation plans.