Volcano-geological approach towards high-resolution reconstructions of explosive eruption phenomena and their utilization to the volcanic hazard mitigations

2018 Fiscal Year Final Research Report

• Project/Area Number: 15K01245
• Research Category: Grant-in-Aid for Scientific Research (C)
• Allocation Type: Multi-year Fund
• Section: 一般
• Research Field: Natural disaster / Disaster prevention science
• Research Institution: Ibaraki University
• Principal Investigator: FUJINAWA Akihiko 茨城大学, 理工学研究科(理学野), 教授 (10143140)
• Co-Investigator(Kenkyū-buntansha): 伴 雅雄 山形大学, 理学部, 教授 (50208724) ; 大場 司 秋田大学, 国際資源学研究科, 教授 (10272014) ; 長谷川 健 茨城大学, 理工学研究科(理学野), 准教授 (00574196)
• Project Period (FY): 2015-04-01 – 2019-03-31
• Keywords: 火山噴火 / 水蒸気爆発 / カルデラ噴火 / マグマ / 火砕流 / 火砕サージ / 火砕物

Outline of Final Research Achievements

High-resolution reconstruction approach revealed 7 distinct phreatic explosion events that occurred during recent 800 years at Zao volcano. Each bed of the phreatic explosion origin contains considerable amounts of juvenile materials with varying juvenile/non-juvenile ratios, showing commonly temporal transition from phreatic to magmatic eruption phases. Such transitions are recognized at Nasu 1408 eruptions and also at Azuma-Kofuji volcano about 5000 years ago. Thus, we collected intensively the high-resolution data on the phreatic explosions that invited magmatic eruptions. Lahar deposits show rather distinctive grain-size distributions than the other concurring deposits of the common phreatic origin.
Similar approaches have been made for several caldera-forming explosions. The deposits once regarded as the product of single eruption event are newly subdivided into several facies resulted from distinct eruption modes with different transportation and/or deposition mechanisms.

Free Research Field

火山地質学，岩石学

Academic Significance and Societal Importance of the Research Achievements

高解像度の噴火復元を蓄積した事で，詳細な噴火様式や爆発規模の時間変遷が解明できた．これは噴火現象の具体的描像を動画的に再現でき，分解能をあげた噴火現象の議論を可能にする．具体的には，溜まりから火口に至る移動発泡現象を実験，理論物理的に解明する研究にも直接対応する記載データが提供できる．水蒸気噴火のメカニズム考察にも，本研究の成果は物理現象とリンクした地質学的証拠を提示しつつある．
社会的意義は噴火予知に対する具体的な描像の提供にある．目撃情報と堆積物対比に基づく水蒸気爆発の復元は高解像度，高信頼度の噴火変遷を，カルデラ噴火では現代人がほぼ未体験の破局噴火の具体的描像を，それぞれ具現化できた．