Anomalous change of groundwater radon concentration monitored at Nakaizu well in 2011

•A counter-flow extractor is useful for continuous radon observation.•Groundwater radon anomalies were recorded before the 2011 Tohoku earthquake.•A modified volatilization can interpret radon concentration anomalies.

An anomalous increase in radon concentration was measured at the Nakaizu observatory on the Izu Peninsula prior to the 2011 Tohoku earthquake using a custom-made radon counter. Since the increase was more than three times the standard deviation of radon concentration variations over 35 years of recorded data, it is considered likely that it is a precursor to the earthquake. Following the earthquake, the radon concentration decreased exponentially to the background level. The anomalous increase is explained using a modified volatilization model containing three important aquifer parameters: the groundwater saturation ratio, the fracture surface area per unit volume, and the porosity. The modified model can also explain the radon concentration behavior prior to the 1978 Izu-Oshima-Kinkai earthquake.