The Ups and Downs of Groundwater Levels after the July 2019 Ridgecrest, CA Earthquakes
Effects from the two July 2019 Ridgecrest, CA earthquakes were observed in several USGS continuous groundwater-level monitoring sites in California, Nevada, and Arizona.
The two large Ridgecrest, CA, area earthquakes were felt across much of the state over the July 4th holiday, and the seismic waves travelled across the entire United States, including Alaska. The first large earthquake was a magnitude 6.4 that occurred on July 4th at 10:33 AM PDT. The second, larger earthquake was a magnitude 7.1 that occurred on July 5th at 8:19 PM PDT. Not only was the shaking substantial enough to be felt across the southwestern states, but there were surface-fault ruptures that geologists have been closely studying. Observations show that the magnitude 7.1 event caused a maximum of 6 to10 feet of right-lateral offset along about 30 miles of rupture. Rarely is the surface rupture for large earthquakes expressed as a single, clear break, and in this case the rupture is particularly broadly distributed along part of its length.
Groundwater-level response to the Ridgecrest earthquakes
Geologists aren't alone in conducting post-quake assessments. Effects from the two earthquakes were observed in several USGS continuous groundwater-level monitoring sites in California, Nevada, and Arizona. The map in figure 1 shows the locations of the earthquake epicenters and five USGS groundwater-monitoring sites with corresponding graphs of groundwater levels over time (hydrographs).
Groundwater-level responses to earthquakes have been investigated for decades. Groundwater-level responses most often occur as the earthquake’s seismic wave train arrives (coseismic), though responses have been observed after the wave train passes (postseismic) (Sneed, Galloway, and Cunningham, 2003); scientists also are investigating groundwater-level changes observed before an earthquake (preseismic), though research is needed to explain these phenomena. Seismic waves have two main types of effects on groundwater levels: oscillations, and "step" offsets.
The most common aquifer response to a seismic event is a repeated rise and fall (oscillation) in groundwater levels. Seismic waves cause expansion and contraction of the aquifer tapped by the well, in turn causing oscillatory pressure changes (Cooper and others, 1965; Liu and others, 1989). These oscillations are rarely recorded because groundwater-level measurements typically are not taken frequently enough to capture the groundwater-level response. Examples of groundwater-level oscillations in a well near Christiansburg, Virginia, caused by numerous earthquakes worldwide, can be seen at https://va.water.usgs.gov/earthquakes/.
An instantaneous groundwater-level offset, or step, is a more commonly recorded response to earthquakes. These step changes can be large enough to make a well flow at land surface, or to cause a well to go dry after an earthquake. Typically, however, the groundwater-level changes are several feet or less. Recovery to the pre-earthquake groundwater level can be nearly instantaneous, or may take as long as days or months, or the groundwater levels may not recover at all.
Offsets in groundwater levels occur near the earthquake rupture because the earthquake subjects the earth's crust, including its aquifer systems, to stress and permanent strain (deformation). This deformation alters fluid pressure within the aquifer systems, and consequently, a step-like change in groundwater level results (Sneed, Galloway, & Cunningham, 2003). Offsets can be up or down, because the earthquake compresses some locations and stretches others. New permeability pathways may even be created, causing groundwater levels to permanently increase or decrease. Such responses were seen within hours after the Pymatuning earthquake in Pennsylvania where the earthquake caused wells on a hill to drain due to increased vertical permeability and other wells to show sudden rises in groundwater levels and even start flowing immediately after the event.
Disturbances in the groundwater levels in the USGS wells shown in figure 1 occurred in response to the Ridgecrest earthquakes. The groundwater levels that show step-like changes generally recovered immediately following the events, but at two sites (hydrographs at 4 and 5), there are longer-term changes in slope or permanent shifts in altitudes of the groundwater levels. Responses to the magnitude 6.4 earthquake (<0.5 foot) are generally smaller than the second, stronger event that had a magnitude of 7.1 (between 0.1 and 3 feet).
Hydrogeologic responses to earthquakes have important scientific implications with regard to our earth’s intricate plumbing system, The exact mechanisms linking hydrogeologic changes and earthquakes are not fully understood, but monitoring these changes improves our insights into the earthquake mechanisms and hydrogeologic properties and processes.
For additional information, please visit https://earthquake.usgs.gov/learn/topics/groundwater.php.
References
Cooper, H. H. Jr, J. D. Bredehoeft, I.S. Papadopulos, R. R. Bennett, 1965, The response of well-aquifer systems to seismic waves. Journal of Geophysical Research, v. 70, p. 3915-3926.
Liu, L., Roeloffs, E., & Zheng, X., 1989, Seismically induced water level fluctuations in the Wali Well, Beijing, China. Journal of Geophysical Research, v. 94, no. B7, p. 9453-9462. doi:10.1029/jb094ib07p09453
Sneed, M., Galloway, D. L., & Cunningham, W. L., 2003, Earthquakes-Rattling the Earth's Plumbing System: U.S. Geological Survey Fact Sheet 096-03, 4p.
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