Persistent Groundwater Reduction Induced by Dynamic Stresses From the 2019 Ridgecrest Earthquake Observed Within A Fractured Aquifer Near Ash Meadows, Nevada, USA.

The fractured carbonate aquifer system of Ash Meadows, Nevada is sensitive to time-varying stresses such as earth tides and earthquakes. Most recently, the 2019 M6.4 and M7.1 Ridgecrest earthquakes, both located more than 100 km from the Ash Meadows aquifer system, produced sustained groundwater level declines of 7-9 cm in the deep fractured carbonate aquifer. No coseismic groundwater level response was observed in the overlying (~280 m thickness) alluvial aquifer. Here, we study the dynamic hydrologic processes of the fractured rock aquifer system to better quantify both driving forces and long-term impacts. First, we analyze the effect of static and dynamic stresses produced by the earthquakes, finding that static stresses should have produced only coseismic water level increases in the Ash Meadows region. Dynamic stresses, however, were large enough to induce coseismic water level response and are likely the driving force for groundwater level reductions in the carbonate aquifer. Second, long-term groundwater tidal analysis both before and after the Ridgecrest event shows a groundwater tidal phase that leads the earth tide phase, suggesting vertical groundwater flow predominates the aquifer system. Using a vertical groundwater flow model, we estimate an average hydraulic diffusivity ~0.25 m2/s for the fractured carbonate aquifer and ~0.05 m2/s for the alluvial aquifer. The estimated specific storage for both aquifers is ~ 10-6 m-1. Although tidal analysis does not show an observable long-term change in hydraulic parameters, we observed a temporary hydrologic enhancement in the 6 days following the M6.4 foreshock. A subsequent 2-year observation of water levels indicates a persistent decline at a rate of 1.7 - 4.6×10-5 m/day, which is in direct contrast to a generally increasing water level before the earthquake. As of August 2021, water levels in the carbonate aquifer have declined between 1.4-3.7 cm since the earthquake. This study highlights the importance of aquifer monitoring that can detect earthquake perturbations, as they may produce profound short-term and long-term impacts on sensitive aquifer systems.