A quantitative method using the precursory radon decline as a tracer to estimate fracture porosity is presented with the help of case studies in Japan and Taiwan. In fractured aquifers of limited recharge, the in-situ volatilization of dissolved radon could cause a decline of radon in groundwater precursory to an earthquake. A mathematical model has been developed to correlate the radon decline with fracture porosity and volumetric strain change in the aquifer rocks based on the mechanism of in-situ radon volatilization. The anomalous decline in groundwater radon prior to the Japan 1978 M = 7.0 Izu-Oshima-kinkai earthquake has been used to estimate the fracture porosity at the Nakaizu SKE-1 well for 0.0000426. The anomalous decline in groundwater radon prior to the Taiwan 2003 Mw 6.8 Chengkung earthquake has been used to estimate the fracture porosity for 0.0001117 at the Antung D1 well.

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