Radon anomalies precursory to the 2003 M_w = 6.8 Chengkung and 2006 M_w = 6.1 Taitung earthquakes in Taiwan

Contrary to the normally observed increase in groundwater radon that occurs prior to earthquakes, we have measured anomalous decreases in radon concentration prior to the 2003 M_W = 6.8 Chengkung and 2006 M_W = 6.1 Taitung earthquakes that occurred within a 55 km radius from the Antung D1 monitoring well in eastern Taiwan. The v-shaped pattern of radon anomalies recognized at Antung is valuable for detecting the aseismic strain precursory to potentially disastrous earthquakes in a fractured aquifer surrounded by ductile aquitard in seismotectonic environments in this area.