Coseismic hydrogeologic response to the 2019 Ridgecrest Earthquake in the fault system at Devils Hole, Nevada

Abstract:

Hydrogeologic responses to earthquakes are well-documented in the geoscientific literature. Devils Hole, a fluid-filled fault cavern located in Death Valley National Park (Nevada), is documented as one of the most sensitive water levels in the world to far-field earthquakes. In July 2019, the M_w6.4 and 7.1 Ridgecrest, California earthquakes occurred less than 150 km from Devils Hole. In this study, we analyze water level response data to the earthquakes at both Devils Hole (DH) and an adjacent shallow monitoring well (AM-5). The water level at Devils Hole is connected to a fractured carbonate rock aquifer while the AM-5 well is completed in the Valley Fill Aquifer unit, which is a subunit of the Basin Fill Aquifer. A persistent, coseismic groundwater decline of about 0.3 ft is observed at Devils Hole through July 2020. This could indicate a persistent change in hydrogeologic parameters at Devils Hole. No coseismic water level offset was identified at the shallow monitoring well. This might indicate strong heterogeneity in hydrogeologic properties between the fault-zone system at Devils Hole and the AM-5 monitoring well. Analysis of the tidal response of both water levels shows positive phase shifts (leading) of about 12 to 15°, suggesting possible vertical leakage of the aquifers. Temporal fluctuations in phase shift might indicate variations in degrees of leakage in the fault-system. A significant drop of phase shift is observed after the Ridgecrest Earthquake at the AM-5 well. On the contrary, no significant change in phase shift was observed at Devils Hole. We suspect the difference in groundwater observatory setting between the fractured carbonate rock aquifer and the Valley Fill aquifer is producing the heterogeneous hydrogeologic response to earthquakes observed in the Devils Hole region.