セント・ヘレンズ火山1980年岩屑なだれおよび御嶽火山1984年岩屑なだれへの動圧モデルの適用

概要

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A rigid body sliding on a powder layer floats at above a critical velocity as the dynamic pressure sustains the sliding body. The critical velocity for floatation, ν_f is related to the pressure of the rigid body, P and the thickness of the rigid body, D by the following equation: ν_f^2=2P/ρ_p=2ρ_rgD/ρ_p where ρ_p and ρ_r are densities of powder layer and rigid body, and g is acceleration of gravity. In order to explain the unexpectedly long run-out distance of debris avalanche, this dynamic pressure model is applied to the two cases of debris avalanche occurred upon the 1980 eruption of St. Helens volcano, USA, and the 1984 collapse of Ontake volcano, Japan. Estimated from the missing volume at the source amphitheaters and the thickness of the terminal deposits, the ν_f for the St. Helens case is 90-45 m/s and that for the Ontake case is 40-24 m/s. The observed velocities are 70-48 m/s for the St. Helens case and 66-16 m/s for the Ontake case. The estimated values of ν_f are equal to or slightly smaller than the observed velocities. The dynamic pressure model is thus to work as a plausible mechanism of debris avalanche, although it is necessary to find direct connection of this working model with the debris-avalanche deposits and the substrates.
特定非営利活動法人日本火山学会の論文
2002-03-05