2022 Fiscal Year Final Research Report

Ovservational study of hydrothermal system of Aso Volcano with respect to the phreatic eruption mechanism

Research Project

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Project/Area Number	20H02006
Research Category	Grant-in-Aid for Scientific Research (B)
Allocation Type	Single-year Grants
Section	一般
Review Section	Basic Section 17040:Solid earth sciences-related
Research Institution	Kobe University
Principal Investigator	Minami Takuto 神戸大学, 理学研究科, 助教 (90756496)
Co-Investigator(Kenkyū-buntansha)	寺田暁彦東京工業大学, 理学院, 准教授 (00374215) 宇津木充京都大学, 理学研究科, 助教 (10372559) 風間卓仁京都大学, 理学研究科, 助教 (20700363)
Project Period (FY)	2020-04-01 – 2023-03-31
Keywords	火山 / 阿蘇 / 水蒸気噴火
Outline of Final Research Achievements	As initially predicted, a phreatic eruption occurred at Aso volcano in October 2021 during the project period, and valuable geophysical observation data was obtained before and after the eruption. Particularly in electromagnetic surveys utilizing artificial electric currents, the three-dimensional analysis of the obtained data revealed that the electrical conductivity, a measure of how easily electrical currents flow in the medium, was low before the phreatic eruption in October 2021 and gradually increased after the eruption, in the region up to several hundred meters below the first crater of Nakadake, Aso volcano. This indicates that the amount of subsurface hot water is changing in conjunction with the phreatic eruption. This study provides observational evidence that the underground electrical conductivity structure changes over time during periods including phreatic eruptions.
Free Research Field	火山学
Academic Significance and Societal Importance of the Research Achievements	水蒸気噴火は大規模なマグマの移動を必要とせず、水蒸気圧の増加が非破壊的に進む場合には、地震観測による噴火予測が難しい。一方で、2014年の御嶽山水蒸気噴火でも知られる通り、水蒸気噴火は甚大な被害をもたらす可能性があり、非破壊な前兆現象を観測的に捉える手法の確立が、火山防災の観点からも重要である。本研究では、活発な状態が続く阿蘇山において、2021年10月の水蒸気噴火を含む期間において、電磁探査を断続的に実施した。その結果、水蒸気噴火の前に地下の比抵抗が大きく変動していたことを明らかにした。この結果は、水蒸気噴火のメカニズムの理解、及び、水蒸気噴火の予測に、電磁探査が有用であることを示している。