Long- and Short-Term Effects of Seismic Waves and Coseismic Pressure Changes on Fractured Aquifers

Two adjacent groundwater wells on the North China Platform are used to study how earthquakes impacted aquifers. We use the response of water level to solid Earth tides to document changes after earthquakes and how aquifer and fracture properties recovered to pre-earthquake properties. We consider two models for the phase and amplitude of water level response to the lunar diurnal (O₁) and semidiurnal (M₂) tides: a leaky aquifer model, and a model in which fracture orientation determines the response. In the leaky aquifer model, changes arise from changes in permeability and storage; in the fracture model, changes are due to changes in apparent orientation of transmissive fractures. Responses in one well are best explained by the leaky aquifer model, and can explain the large amplitude coseismic water level and permeability changes and the non-recoverable changes after the largest earthquake. Responses in the other well are consistent with the fracture model and show little coseismic change in water level but changes in apparent fracture orientation. Larger ground motions lead to larger coseismic water level changes and longer recovery times. We propose that the well in the more permeable and shallow aquifer has less variable pore-pressures around the well. Larger coseismic strains from water level changes may enable longer-lasting changes in aquifer properties. We conclude that relatively high permeability aquifers are less susceptible to impacts from seismic waves, and thus have small changes in water levels and hydrogeological properties.