著者

大野 希一 国方 まり 鈴木 正章 西村 裕一 長井 大輔 遠藤 邦彦 千葉 達朗 諸星 真帆

出版者

特定非営利活動法人日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.47, no.5, pp.619-643, 2002-11-29

被引用文献数

4

---

The volcanic activities of the Usu 2000 eruption were monitored and reported by many scientists and the mass media. Summarizing these observation results, most of relatively large explosion events occurred during March 31 to April 7, 2000. Around the Nishiyama and Kompira crater groups, the pyroclastic deposits with multi units can be divided into 19 layers on the basis of their visible color, grain size and sedimentary structure; from Layer A to Layer S in ascending order. The eruptive dates of each layer inferred from the wind directions, the eruptive sequence, and the distribution of deposits are summarized as follows; the Layer A, characterized as the light gray color ash fall deposit including in some pumice layers, was generated by March 31 phreatomagmatic explosions occurred at Nishiyama crater group. The Layer B, composed poorly sorted breccia and ash layer with gray color, was generated on March 31 p.m. at Nishiyama craters. The Layer C to the Layer G, dark brown-gray aggregate ash, were derived from the volcanic eruptions occurred on April 1 to 2 in Nishiyama and Kompira crater group. The Layer H to Layer M and Layer O, mainly consist with gray and reddish brown aggregate ash including in lithic fragments, were generated during April 3 and 4 in Kompira crater group. The Layer N, which distributes around N19 crater, generated on April 4. The Layer P, massive ash with gray color, was generated on April 6 in Kompira crater group. After April 7, the Layer S, characterized as light brown aggregate ash, has been generated from the recent minor activities around limited craters. The amount of Layer A fallen in the range from the source to Toyako Onsencho Town is estimated at 1.2×10^8 kg, and total amount of Layer A including in the distal area is 2.4×10^8 kg. On the other hand, amount of other deposits generated during April 1 to 6 (e.g. Layer B, N, and Q) is an order of 10^6-10^7 kg. Total amount of the pyroclastic deposits erupted from the Usu 2000 eruption is more than 6.4×10^8 kg.