The 2003 Chengkung earthquake of magnitude (M) 6.8 on December 10, 2003 was the strongest earthquake near the Chengkung area in eastern Taiwan since 1951. The radon-monitoring station at the Antung hot spring as located 20 km from the epicenter. Approximately 65 days prior to the 2003 Chengkung earthquake, precursory changes in the groundwaterís radon concentration were observed. The radon anomaly was a decrease from a background level of 780 pCi/L to a minimum of 330 pCi/L. Observations at the Antung hot spring suggest that the groundwater radon, when observed under suitable geological conditions, can be a sensitive tracer for strain changes in the crust preceding an earthquake. The Antung hot spring is situated in a tuffaceous-sandstone fractured block inside mudstone. Under such geological conditions, the dilation of rock masses was produced at a rate faster than the recharge rate of pore water and gas saturation developed in rock cracks preceding the earthquake. Meanwhile, the radon in groundwater volatilized and partitioned into the gas phase. Our understanding of geology and radon phase-behavior would help select radon-monitoring sites in the future.

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