title: Groundwater helium anomaly reflects strain change during the 2016 Kumamoto earthquake in Southwest Japan.
abstract: Geochemical monitoring of groundwater and soil gas emission pointed out precursor and/or coseismic anomalies of noble gases associated with earthquakes, but there was lack of plausible physico-chemical basis. A laboratory experiment of rock fracturing and noble gas emission was conducted, but there is no quantitative connection between the laboratory results and observation in field. We report here deep groundwater helium anomalies related to the 2016 Kumamoto earthquake, which is an inland crustal earthquake with a strike-slip fault and a shallow hypocenter (10 km depth) close to highly populated areas in Southwest Japan. The observed helium isotope changes, soon after the earthquake, are quantitatively coupled with volumetric strain changes estimated from a fault model, which can be explained by experimental studies of helium degassing during compressional loading of rock samples. Groundwater helium is considered as an effective strain gauge. This suggests the first quantitative linkage between geochemical and seismological observations and may open the possibility to develop a new monitoring system to detect a possible strain change prior to a hazardous earthquake in regions where conventional borehole strain meter is not available.
authors.affiliation: Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa, Chiba 277-8564, Japan.
authors.forename: Yuji
authors.initials: Y
authors.lastname: Sano
authors.affiliation: Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa, Chiba 277-8564, Japan.
authors.forename: Naoto
authors.initials: N
authors.lastname: Takahata
authors.affiliation: Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa, Chiba 277-8564, Japan.
authors.forename: Takanori
authors.initials: T
authors.lastname: Kagoshima
authors.affiliation: Institute for Geothermal Sciences, Kyoto University, Beppu, Oita 874-0903, Japan.
authors.forename: Tomo
authors.initials: T
authors.lastname: Shibata
authors.affiliation: Department of Earth and Environmental Sciences, Kumamoto University, Kurokami, Kumamoto 860-8555, Japan.
authors.forename: Tetsuji
authors.initials: T
authors.lastname: Onoue
authors.affiliation: Department of Geophysics, Tohoku University, Sendai, Miyagi 980-8578, Japan.
authors.forename: Dapeng
authors.initials: D
authors.lastname: Zhao