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Dynamic stresses induced groundwater reductions observed within a fractured aquifer near Ash Meadows, Nevada associated with the 2019 Ridgecrest earthquake

Ruei-Jiun Hung, & Matthew Weingarten

Submitted September 11, 2022, SCEC Contribution #12143, 2022 SCEC Annual Meeting Poster #022

Groundwater observations in Ash Meadows, Nevada are dedicated to protect its unique ecosystem and to manage groundwater resources. The groundwater levels are sensitive to earth tides and earthquakes, allowing us to study dynamic hydrologic properties. During the 2019 M6.4 and M7.1 Ridgecrest earthquakes, groundwater levels declined in the fractured carbonate aquifer by 7-9 cm, whereas no coseismic groundwater level response was observed in the overlying (~280 m) alluvial aquifer, suggesting a local hydrologic heterogeneity. Here, we start with groundwater tidal analysis for the time period between 2017 and 2020, covering the Ridgecrest earthquake occurrence, to better understand the hydrologic properties. The groundwater tidal phase shows a leading phenomenon to the earth tidal one, likely suggesting vertical groundwater flows were more predominated in the aquifer systems. Using a vertical groundwater flow model, we estimate an average hydraulic diffusivity ~0.25 m2/s for the carbonate aquifer and ~0.05 m2/s for the alluvial aquifer. The specific storage for the both aquifers are ~ 10-6, which is reasonable for the aquifers in the Ash Meadows regions. 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. We also observe and analyze the effect of static and dynamic stresses produced by the earthquake, and found static stresses should have produced only coseismic water level increases in the Ash Meadows region. Meanwhile, dynamic stresses were large enough to induce coseismic water level response and are likely the driving force for groundwater level reductions in the carbonate aquifer. A subsequent 2-year observation of water levels indicates a persistent decline at a rate of 1.7-4.6\times{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. The observation of coseismic groundwater variations to the tidal and earthquake-induced dynamic stresses in Ash Meadows is important, as the long-term decline of water levels may adversely impact long-term aquifer storage and the ecological communities relying on the aquifer system.

Key Words
Ash Meadows, Devils Hole, groundwater tidal analysis, coseismic groundwater response, Ridgecrest earthquake

Hung, R., & Weingarten, M. (2022, 09). Dynamic stresses induced groundwater reductions observed within a fractured aquifer near Ash Meadows, Nevada associated with the 2019 Ridgecrest earthquake. Poster Presentation at 2022 SCEC Annual Meeting.

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