Ten-year helium anomaly prior to the 2014 Mt Ontake eruption.
Mt Ontake in central Japan suddenly erupted on 27(th) September 2014, killing 57 people with 6 still missing. It was a hydro-volcanic eruption and new magmatic material was not detected. There were no precursor signals such as seismicity and edifice inflation. It is difficult to predict hydro-volcanic eruptions because they are local phenomena that only affect a limited area surrounding the explosive vent. Here we report a long-term helium anomaly measured in hot springs close to the central cone. Helium-3 is the most sensitive tracer of magmatic volatiles. We have conducted spatial surveys around the volcano at once per few years since November 1981. The (3)He/(4)He ratios of the closest site to the cone stayed constant until June 2000 and increased significantly from June 2003 to November 2014, while those of distant sites showed no valuable change. These observations suggest a recent re-activation of Mt Ontake and that helium-3 enhancement may have been a precursor of the 2014 eruption. We show that the eruption was ultimately caused by the increased input of magmatic volatiles over a ten-year period which resulted in the slow pressurization of the volcanic conduit leading to the hydro-volcanic event in September 2014.
1] Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa, Chiba, Japan [2] Department of Geoscience, National Taiwan University, Taipei, Taiwan.
Yuji
Y
Sano
Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa, Chiba, Japan.
Takanori
T
Kagoshima
Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa, Chiba, Japan.
Naoto
N
Takahata
Research and Education Faculty, Kochi University, Nankoku, Kochi, Japan.
Yoshiro
Y
Nishio
CEGA, Departamento de Geologia, Universidad de Chile, Santiago, Chile.
Emilie
E
Roulleau
GEOTOP &Département des sciences de la Terre et de l'atmosphère, Université du Québec à Montréal, Montréal, Canada.
Daniele L
DL
Pinti
Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico, USA.
Tobias P
TP
Fischer