Sensors in the Delaware Groundwater Monitoring Network wells detected small changes in water pressure around the time of the Nov. 30, 2017 4:47 PM earthquake. The wells, located just east of Little Creek 2.2 miles southwest of the epicenter of the 4.1 magnitude earthquake, showed that water pressures show a step increase between readings collected just before the quake at 4:45 PM and just after the quake at 5:00 PM. Pressures remained higher than pre-quake values for many hours. As shown in the accompanying chart, the greatest change in pressure occurred in the Piney Point aquifer, a confined aquifer found at depths between 350 and 450 feet below land surface. Smaller positive changes in pressure occurred in the Federalsburg and Cheswold aquifers. A small negative change in pressure occurred in the shallow unconfined aquifer. Pressure changes that could be attributed to the earthquake were not observed in any other Delaware Groundwater Monitoring Network wells.
The fact that groundwater in Delaware responds to distant earthquakes is not new (see http://www.dgs.udel.edu/sites/default/files/publications/OFR11.pdf). The groundwater pressure response seen this past November is similar to pressure changes observed elsewhere after other earthquakes, including pressure changes associated with the 2011 Mineral, Virginia event (https://water.usgs.gov/ogw/eq/VAquake2011.html). Given the relatively small magnitude of the Delaware quake and the presence of weakly consolidated, underlying Coastal Plain geologic units, it is possible that the stress caused by the nearby earthquake slightly reduced the volume of the aquifers by reducing porosity or by slightly shifting the positions of the aquifers and confining beds. Because the sensors operate on a fixed 15-minute schedule, the high-frequency response of groundwater pressures to the earthquake was not recorded and more detailed analysis of the earthquake impacts on the aquifers cannot be done.
The Delaware Groundwater Monitoring Network provides data to water managers, scientists, engineers, and the public to determine water conditions, water availability, and water quality. The DGS runs the network in cooperation with the Department of Natural Resources and Environmental Control and other State and UD groups. The Survey’s annual appropriation as well as supplemental funds from the State capital budget and federal sources support network infrastructure and operations. Additional information and data products can be found at http://www.dgs.udel.edu/water-resources.
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