Recurrent groundwater radon anomalies were observed at the Paihe spring (P1) in southwestern Taiwan prior to the Mw 6.3 Jiasian and Mw 6.4 Meinong earthquakes that occurred on March 4, 2010 and February 5, 2016, respectively. Specifically, the concentration of groundwater radon decreased from background levels of 144 ± 7 and 137 ± 8 pCi/L to minima of 104 ± 8 and 97 ± 9 pCi/L prior to the 2010 Jiasian and 2016 Meinong earthquakes, respectively. The Paihe spring (P1) is located 46 km and 45 km, respectively, from the epicenters of the 2010 Mw 6.3 Jiasian and 2016 Mw 6.4 Meinong earthquakes. The above radon anomalies observed at the Paihe limestone spring corroborated that a small fractured aquifer can be used as an effective natural strain meter by applying radon as a tracer for earthquake warning in southwestern Taiwan. There are scientific difficulties and uncertainties in earthquake prediction. Nonetheless, a long-term monitoring of precursory declines in groundwater radon can provide useful data for forecasting local disastrous earthquakes.
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