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A. significant decrease in the radon concentration of artesian groundwater was observed at a well‐established observation site in Northeast Japan about two days before an M6.0 earthquake occurred about 200 km from the site. On the basis of a concurrent in situ (−126m) temperature drop of the groundwater, we infer that precursory changes in regional stress were locally amplified near this site and resulted in the temporary mixing of cooler, radon‐poor groundwater into the normal artesian flow.
Wakita et al. (Mon,) studied this question.