Toward a classification of volcanic eruptions into the magmatic or phreatic events by infrasound observations

2021 Fiscal Year Final Research Report

• Project/Area Number: 19K14819
• Research Category: Grant-in-Aid for Early-Career Scientists
• Allocation Type: Multi-year Fund
• Review Section: Basic Section 17040:Solid earth sciences-related
• Research Institution: Kyoto University
• Principal Investigator: Yamada Taishi 京都大学, 防災研究所, 助教 (60804896)
• Project Period (FY): 2019-04-01 – 2022-03-31
• Keywords: 火山噴火 / 空気振動 / 噴煙

Outline of Final Research Achievements

The present study found an empirical relation between erupted tephra mass (m, kg) and infrasound energy (Einf, J) as m/Einf=0.02-0.1 at Sakurajima (Minamidake), Kuchinoerabujima, and Shinmoedake volcanoes. With the possible initial ash temperature range, we convert the relation into F/Einf=0.3-3.5, where F is the initial buoyant force of the eruption cloud. We investigated the F/Einf relation by estimating F from the maximum eruption cloud height, focusing on the difference between magmatic (mainly Vulcanian) and phreatic eruptions. Since latent heat of water vaporization in the erupted materials contributes to the eruption cloud buoyancy, one expects that phreatic eruption has a greater F/Einf value than magmatic eruptions. Average values of F/Einf are obtained for magmatic (478 events) as 20.5 and phreatic eruptions (8 events) as 18.9. Water content in the atmosphere can also play a role in the eruption cloud dynamics, which will be the next research target.

Free Research Field

火山物理学

Academic Significance and Societal Importance of the Research Achievements

マグマ噴火と水蒸気噴火の識別は、火山活動の評価、見通しを議論する上で重要である。本課題の成果だけでは不十分であるものの、現地調査に依らない観測量に基づく識別方法を議論したという点では一定の意義があるものと考える。また本課題が見出した放出火山灰量と空気振動エネルギーの関係は、火山噴火に伴う噴出物量の即時的な評価を可能にするものである。火山噴火の規模、誘発される噴火災害のポテンシャルを迅速な評価を可能にするという観点において社会的意義があり、今後も研究を進める。