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. After large earthquakes, the properties of aquifers can change, leading to changes in water levels in wells. We study changes in water levels that we attribute to changes in the properties of fractures or the permeabilities of aquifers that connect to the wells. We do so by monitoring how deformation caused by tides leads to changes in water levels in two wells. The deeper well in a less permeable formation has larger changes. The larger the ground motion from earthquakes and the larger the amplitude of the coseismic water level changes, the more likely and larger the changes in permeabilities. The larger the changes, the slower the recovery to pre-earthquake properties. The shallower well in a more permeable formation has smaller changes and no long-term changes in water level and hydrogeological properties, which we attribute to more uniform pore-pressures around the well. high permeability aquifers are less susceptible to disruption by seismic waves hydraulic property changes depend on the amplitude and frequency of ground motion larger water level changes accompany larger changes in fracture orientation and irreversible changes in aquifer properties