Precursory changes in the radon concentration of groundwater were observed by Wakita et al. (1980) prior to the 1978 Izu-Oshima-Kinkai earthquake of magnitude 7.0. Mechanisms for interpreting the anomalous radon decrease are examined in this paper. The SKE-1 well is situated in a volcanic-rock fractured aquifer of limited recharge. Given these geological conditions, the dilation of brittle rock mass occurred at a rate faster than the recharge of groundwater and gas saturation developed in newly created cracks preceding the earthquake. Radon volatilization into the gas phase can explain the anomalous decrease of radon precursory to the 1978 earthquake. To support the hypothesis, vapor-liquid two-phase radon-partitioning experiments were conducted at formation temperature (14 °C) using formation water from the SKE-1 well. Experimental data indicated that the decrease in radon concentration from 483 ± 3 count/min to 439 ± 7 count/min required a gas saturation of 2.35% developed in rock cracks through the dilatancy process.
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