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Date(s) - 27 novembre 2025
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27 novembre 2025, 13h15 amphithéâtre du CRPG | Yuji Sano (Center for Advanced Marine Core Research, Kochi University)
Groundwater helium anomaly related to inland earthquake in Japan
Earthquakes in Japan are classified into two types: subduction-type earthquakes and inland earthquakes. The former earthquakes occur at the boundary between subducting oceanic plate and overriding continental plate and generally large in magnitude. On the other hand, latter earthquakes are on land and can cause major disasters when they occur near densely populated areas. Geochemical studies of groundwater associated with earthquakes have been conducted in Japan since the late 1970s. Increases of He/Ar ratio in groundwater were observed prior to the earthquake close to the sampling site [1]. To explain these observations, rock fracture experiment was conducted where He was released quantitatively as granite was compressed [2]. At the time of 1995 Kobe earthquake, a decrease in the 3He/4He ratio and an increase in the 4He/20Ne were observed [3]. The anomaly in the groundwater was interpreted to be the release of He with low 3He/4He ratios produced by radioactive decay of U and Th in the aquifer rocks. Subsequent studies suggested that the anomaly was in response to changes in the geotectonic strain of the area [4,5]. In this work, I discuss that helium in groundwater acting as an effective strain gauge in the 2024 Noto earthquake based on the 3He/4He and 4He/20Ne ratios.
Reference [1] Sugisaki, 1978, Nature 275, 209-211. [2] Honda et al., 1983, Earth Planet. Sci. Lett. 59, 429-436. [3] Sano et al., Chem. Geol. 150, 171-179. [4] Sano et al., 2016, Scientific Reports 6, 37939. [5] Onda et al., 2018, Scientific Reports 8, 4800.