Groundwater dynamic changes induced by earthquakes in an aquifer-aquitard system from well monitoring in Southwest China

Groundwater level, permeability and chemical components can be affected by earthquakes, however there are few comprehensive investigations on the combination of long-term continuous monitoring data and multiple strong earthquakes. In this study, continuous two-year dataset of groundwater levels and chemical compositions of groundwater (Ca2+, Mg2+ and HCO3-) in well #32 were collected to analyze the groundwater dynamic changes induced by earthquakes in the aquifer-aquitard system. The groundwater level appeared co-seismic rise change induced by Yangbi MW 6.1 earthquake and Luding MW 6.6 earthquake. The vertical permeability, estimated by the tidal response model, exhibited decrease changes during the period of Yangbi MW 6.1 earthquake and Luding MW 6.6 earthquake. Meanwhile, the continuous two-year chemical compositions showed that Ca2+ and HCO3- concentrations decreased, and Mg2+ concentrations increased during the two earthquakes period. The correlation between the vertical permeability and chemical compositions showed that there was a significant negative correlation between the vertical permeability and Mg2+, and a significant positive between the vertical permeability and Ca2+, HCO3-. A possible mechanism for observed fluctuations in some chemical compositions during earthquakes periods was that the reduction of mixing effect of different groundwater caused by permeability decreased. The flow of groundwater richened in Ca2+ and HCO3- from the overlying aquifer to the observation aquifer has been reduced. Meanwhile, due to the weakening of dilution effect, the Mg2+ concentration of the observation aquifer increased. This study can enhance understanding of the groundwater dynamic changes induced by earthquakes.