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Groundwater-Level Response to Virginia Earthquake, August 23, 2011
A magnitude 5.8 (Mw) earthquake occurred at 1:51 pm EDT (17:51 UTC) on August 23, 2011, at a depth of about 3.7 miles (6 kilometers) beneath the town of Mineral, Virginia, located approximately 27 miles (45 km) east of Charlottesville. The event is among the largest earthquakes recorded in the eastern United States.
How do Earthquakes Affect Groundwater Levels?
Groundwater-level responses to earthquakes have been investigated for decades, and have been documented close to and far from earthquake epicenters. The most common groundwater-level response is a water-level oscillation. This occurs frequently, but often is not recorded because water-level measurements typically are not recorded frequently enough with today’s electronic instrumentation to record the water-level response. An instantaneous water-level offset, or step, is more commonly recorded. These step changes can be large enough to make a well flow at land surface, or to cause a well to go dry near an earthquake. Typically, however, the water-level changes are several feet or less. Recovery to the pre-earthquake water level can be nearly instantaneous, or it may take as long as days or months, or may not recover at all.
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); scientists also are investigating water-level changes observed before an earthquake (preseismic), though research is needed to explain these phenomena.
Step changes in groundwater levels occur 'near field' of an earthquake because the earthquake subjects the earth's crust, including its aquifer systems, to stress and permanent strain (deformation). This deformation process results in altered fluid pressure within the aquifer systems, and consequently, a steplike change in water level would be expected. Changes can be up or down, because the stress and strain effects on the system vary. For upward steps in shallow wells, compaction of overlying sediments may raise the groundwater level. Alternately, in a fractured rock aquifer, the fractures providing water to the well may be widened, unclogged, or sealed by the wave train of an earthquake. New water-bearing fractures may even be created. Water levels in these systems can permanently increase or decrease as a result.
Sample graphs of groundwater levels (hydrographs) have been selected from USGS wells as examples of the response of measured groundwater levels to the August 2011 earthquake:
The maximum water level change recorded in the USGS real-time groundwater network due to the August 23, 2011, Virginia earthquake was approximately two feet in a well in Pennsylvania. Most water level changes were less than 0.5 feet.
The hydrographs presented here are produced from wells with near real-time instrumentation. Data are provided automatically via satellite in near real time. The USGS monitors other wells in these areas, but measurements from those wells are not available until they are retrieved from the field and loaded into the USGS National Water Information System.
Slide Show of Selected Hydrographs
The six hydrographs below present groundwater levels measured in selected wells monitored by the USGS (all data are from the National Water Information System web site). These graphs were selected as examples of locations where measured groundwater levels responded to the August 23, 2011, Virginia earthquake. The red arrow indicates the approximate time of the mainshock of the earthquake near Mineral, Virginia.
Eastern Seaboard Map
The points on the map below indicate approximate locations of selected USGS groundwater monitoring wells where measured groundwater levels responded to the August 23, 2011, Virginia earthquake. Click on a well location to view a graph of the groundwater level measured at that location.
For More Information
For more information on the effect of earthquakes on wells, streams, and springs, visit the following online resources:
Additional questions can be directed to Rod Sheets (614-430-7710), USGS Water Science Field Team Groundwater Specialist.
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