Assesment of the response of the meteorological/hydrological parameters on the soil gas radon emission at Hsinchu, northern Taiwan: A prerequisite to identify earthquake precursors

摘要

The present study is an attempt to assess and quantify the influence of the meteorological (atmospheric temperature and pressure) and hydrological (rainfall and ground water head-GWH) parameters on the soil gas radon emission at Hsinchu, northern Taiwan. The quasi-periodic variations corresponding to diurnal and semi diurnal periods were estimated and eliminated by decomposing the time series for the period of September 16, 2009 to March 5, 2010 to singular spectrum analysis. The reconstructed non-periodic variations, which reproduce the salient feature of recorded time series, were searched for meteorological/hydrological influences in radon emission. The combined response of barometric pressure and atmosphere temperature are found to be small when compared to the total variability in radon. The influence of rainfall on radon is found to be strongest. At the onset of rainfall, radon shows a step-jump that attains peak with a time lag of 12-15 h. This enhancement is attributed to entrapment of soil gas in the top soil cover as increased soil moisture prevents escape of radon into the atmosphere (capping effect). The decay of radon after the recession of rainfall is approximated by double exponential decay terms, one corresponding to the natural decay of radon with half life of 3.84 days and second representing slow weakening of capping effect. The third effect related to internal loading due to rise and fall of groundwater modulates the propagation of radon in overlying strata, accounting for the long term variations in radon. The rainfall inflicted changes in radon look strikingly similar to earthquake related precursory or co-seismic perturbations, inferred by long term synotopic observations. It is surmised that unless radon variations are corrected for meteorological/hydrological contamination, some precursory signals are masked on one hand while on the other hand some anomalies are falsely viewed as earthquake precursors.