The Response Of Well-aquifer System To Periodic Loadings And Earthquakes

As one of the widely distributed fluids in the crust,groundwater shows sensitive to crustal strains and solid deformation.Studying the process of groundwater response to earthquakes may provide insight into the interaction between hydrogeological processes and tectonic activities in the shallow crust at spatial and temporal scales.Up to now,earthquake-induced hydrology responses have been documented globally.Most of them are related to the earthquake-induced permeability changes in the shallow crust.More observations can prompt efforts to better understand the relationships between earthquakes and groundwater.Meanwhile,such earthquake-induce hydrological responses are seldom uniform in space or time.Factors and mechanisms that determine the sensitivity of these changes remain poorly understood.Therefore,based on the records of earthquake-groundwater monitoring wells in China,we investigated the characteristics of water level response to periodic loadings in frequency domain and analyzed changes in response behavior after earthquakes.Furthermore,we investigated the changes in well-aquifer system after earthquakes and evaluated the potential causal factors that determine the sensitivity of earthquake-induced changes.Main insights are listed as following:(1)The major diurnal and semidiurnal periodic components in well water levels reflect the responses to periodic loadings such as barometric pressure and earth tide.Wavelet transform is a better technique to transform water level time series into time-frequency space,which is useful for capturing the possible effects induced by earthquakes on the well-aquifer system as showing in Zuojiazhuang well-aquifer system.(2)Permeability changes in the aquifer and aquitard caused by a large remote earthquake were quantitatively evaluated based on the tidal and barometric response analysis of Zuojiazhuang well-aquifer system.Our results showed that the transmissivity of the aquifer increased by a factor of 6(from 0.3 to 1.8 m2/d)and the diffusivity of the aquitard doubled(from 0.69 to 1.4 m2/s)following the 2011 Tohoku earthquake.(3)Meanwhile,the evolution of chemical elements following 2018 Tonghai earthquake by weekly water sampling from Jiangchaung well were documented.Combined with water level records,we found that the earthquake caused changes in both water levels and trace elements but caused no significant changes in the stable isotope and major ions.We considered that earthquake-induced shaking caused a breach in the hydraulic barrier of the isolated reservoir that led to the mixing of water with low concentrations of trace elements as the mechanism to explain such response.(4)Two closely wells(10 m)that show no response to seismic waves and respond differently to barometric pressure and earth tide were analyzed,which indicated that the response sensitivity to periodic loadings determined by hydrogeological settings.Based on the different sensitivities of water level and hydrogeological properties variations in response to four earthquakes in Dongchuan and Xuanwei well-aquifer systems,three major factors that may control the sensitivity of the earthquake-induced hydrogeological responses were proposed as:seismic factor-peak ground velocity(PGV),hydrogeological factors-aquifer confinement,and well location relative to local faults.