東台灣成功地區於2003年12月10日發生地震矩規模(Mw) 6.8之地震，為1951年來最強烈的一次。自2003年7月起在花蓮玉里安通溫泉區建立地下水水氡(Rn-222)監測站。此監測站位於2003年成功地震震央24公里處鄰近池上斷層，為現今歐亞板塊及菲律賓海板塊之交界處。2003年成功地震發生前約65天，地下水水氡(Rn-222)濃度有異常下降變化，濃度由780 pCi/L下降至最低值330 pCi/L。

安通溫泉區地質構造是由含破碎裂隙玄武岩及砂岩所組成，被泥岩包圍。地震前岩體膨脹產生新裂隙，地下水補注到新裂隙的速度遠低於新裂隙產生的速度，引起溶解於地下水中的氡氣逸出水相至新裂隙中的氣相。氡分配到氣相的行為可使用來解釋在2003年成功地震前，地下水中氡濃度異常下降的現象。為了支持這假設，進行氡的氣液兩相分配試驗。使用現地溫泉水在水溫60 ℃狀態下，探討氣體飽和度(gas saturation)與水氡濃度下降之關係。由試驗數據指出，地下水水氡濃度由780 pCi/L到330 pCi/L之異常下降是需要岩體裂隙增加並產生10％氣體飽和度(gas saturation)。

The 2003 Chengkung earthquake of magnitude (Mw) 6.8 on December 10, 2003 was the strongest earthquake near the Chengkung area in eastern Taiwan since 1951. The Antung radon-monitoring station was located 24 km from the epicenter. Approximately 65 days prior to the 2003 Chengkung earthquake, precursory changes in the radon concentration of ground-water were observed. The radon anomaly was a decrease from a background level of 780 pCi/L to a minimum of 330 pCi/L.

Mechanisms and geological conditions for interpreting anomalous decreases in ground-water radon prior to earthquakes are seldom discussed in the literature. The Antung hot spring is situated in a basaltic fractured block inside mudstone. Under such geological conditions, we made a hypothesis that 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. Radon partitioning into the gas phase may explain the ground-water radon anomalous decrease precursory to the 2003 Chengkung earthquake. To support the hypothesis, vapor-liquid two-phase radon-partitioning experiments were conducted at formation temperature (60 ℃) using formation brine from the Antung hot spring. Experimental data indicated that the anomalous decrease of radon concentration from 780 pCi/L to 330 pCi/L required a developed 10 % rock cracks relative to water volume.

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