Sustained Changes in Well Water Levels Following a Large Earthquake: Possible Evidence of Permeability Decreases in a Shallow Groundwater System

Observation of earthquake‐induced changes in well water levels provides an opportunity to study the effects of seismic activity on the groundwater system. In this study, we used data from a well network in the alluvial Canterbury Plain of New Zealand’s South Island to document sustained and complex changes in well water levels following the 2010 Darfield earthquake. Sustained increases in well water levels in midstream areas, as well as sustained decreases in downstream areas, resulted from decreases in permeability. These decreases in permeability were possibly caused by consolidation of liquefied sediments following the earthquake. Plain Language Summary The Mw 7.1 Darfield earthquake occurred in the eastern part of New Zealand’s South Island on September 4, 2010. It generated widespread hydrological responses in the Canterbury Plain, such as the formation of new springs and changes in well water levels. Although many studies have focused on coseismic hydrological responses to earthquakes, few have studied the sustained effects of large earthquakes on water well levels. In this study, we analyze data from a well network and propose a model to explain the sustained changes observed in the well water levels. In the model, a decrease in permeability of the shallow groundwater system was induced by seismic sediment consolidation in the Canterbury Plain. This in turn resulted in sustained changes in the hydraulic gradient of the shallow groundwater system, which increased groundwater levels in midstream areas but decreased levels in downstream areas. Key Points Sustained changes occurred in well water levels in the Canterbury Plain after the Darfield earthquake These sustained changes suggest widespread permeability decreases in the shallow groundwater system These permeability decreases could be attributed to the consolidation of sediment in the Canterbury Plain