報告書番号

N08053

タイトル（和文）

長期間の火山活動に基づく噴火ハザード評価－北海道恵山火山を事例として－

タイトル（英文）

Eruptive hazard assessment in long-term volcanism-an example from the Esan Volcano, Northern Japan-

概要 （図表や脚注は「報告書全文」に掲載しております）

国内外の社会的重要構造物に関わる火山活動評価ガイドライン・規格をレビューし，あわせて，北海道の恵山火山において，数万年に及ぶ長期間の火山活動評価の事例検討を行った．その結果，長期的な噴火ハザード評価における以下のような課題と留意点が得られた．
文献調査からは，汎用的規格・基準による数値的評価が火山活動では容易ではないこと，最も脅威となる噴火現象は，大規模火砕噴火と新火口の形成と考えられることから，火山噴火予知の要素のうち，場所・規模・様式の情報を取得することが重要であること，さらに，対象火山の噴火史の特性に合わせた，適切な調査組合せと調査範囲の設定を行うことが重要であること，などの成果が得られた．
恵山火山の調査からは，噴火堆積物の誤認は大きな問題となること，大噴火と大噴火の間の中小規模噴火を精度良く調べる必要があること，さらに，長期間の評価では，年代測定手法の選択に制約を受けること，火山活動が長期間に渡ると，火口分布パターンが改変されることがあることなどの成果が得られた．したがって，噴火史を詳細に調べ，噴火規模・様式とマグマの時空変化を適切に把握する必要があると考えられる．

概要 （英文）

A scientific assessment of eruptive hazard due to long-term volcanism is an important issue for avoiding a serious damage to public facility. A precise and appropriate methodology for the prediction is highly necessary. We have reviewed the safety guidelines for the evaluation of volcanic hazard onto nuclear facility. Application of the present methodology is performed at the Esan volcano, northern Japan, which is one of the best volcanoes in order to reveal practical problems. The following important issues are clarified in terms of long-term volcanism through those examinations. (1) Because a volcanic activity varies through many eruption events, it is difficult to establish a particular numerical threshold that applies to every volcano in the same way. This is the characteristic of the volcanic hazard and is distinctive from the seismic hazard assessment. (2) Effective parameters at the site evaluation of critical facilities are location, volume and style (kind) of the volcanic eruption. (3) Most likely threats of volcanic eruption to the facility are the large-scale explosive eruption and the crater migration. (4) The crater migration can be determined by both tectonic and local stress fields. Modification in the migration pattern may occur at volcano with long-term activity. (5) Careful observations on spatial and temporal change of magma property are useful in order to predict the change in volume and style of future eruption. (6) It is important to correctly interpret volcanic deposits. The misinterpretation may result in significant mistake in evaluation of volcanic history and associated hazards. (7) Moderate- and small-sized eruptions between large eruption events can be recovered by using the borehole data. (8) Accuracy of radioactive age is largely different between 14C method and other dating methods. This in-homogeneity requests us to pay attention to the method used for different range of radioactive age. (9) Choosing an appropriate set of surveys is required for the evaluation of long-term volcanism over a few ten thousand years.

報告者

キーワード