Exploring the relationship between soil degassing and seismic activity by continuous radon monitoring in the Longitudinal Valley of eastern Taiwan

Donghua (DH) continuous soil gas monitoring station is located on the suture zone in eastern Taiwan. The variations in radon concentration are synchronized to variations in carbon dioxide (CO2), which is considered as the companion gas for radon (Rn). Results reveal that semi-diurnal variation of soil-gas, probably generated by the solar tide effect, achieve values of ~ 500 Bq/m3 for Rn and 0.4% for CO2. The variations exceeding the threshold by statistical analysis can be used to identify events associated with geodynamic processes such as earthquakes. A significant increase in soil-gas concentrations was recorded 2 months before the Rueisuei earthquake (ML = 6.4, October 31, 2013) and recurrent anomalies were observed 3 weeks before the Fanglin earthquake (ML = 5.9, May 21, 2014). Both earthquakes occurred in the Longitudinal Valley area of eastern Taiwan. This study suggests that longer periods of soil gas anomalies are positively correlated with impending earthquake of larger magnitude. From the high precision GPS data showed an appreciable decrease in extension rate was also observed approximately 4 months before the Rueisuei earthquake. It is worthy to have a continuous monitoring on multiple parameters (soil radon, carbon dioxide, atmospheric pressure, humidity, temperature and precipitation) at DH station.