T154 Central Virginia Earthquakes of 2011: Geology, Geophysics, and Significance for Seismic Hazards in Eastern North America

Tuesday, 6 November 2012
8:00 AM - 12:00 PM
207D

GROUNDWATER-LEVEL CHANGES CAUSED BY THE 2011 MW 5.8 MINERAL VIRGINIA EARTHQUAKE

Paper number: T154-4
Program: Technical Sessions
Time: 8:45 AM – 9:00 AM

Authors

Abstract

Water levels in wells, measured by the USGS and others, were affected as far as 560 km from the August 23, 2011 Mw 5.8 Mineral, Virginia earthquake. The earthquake-induced offsets - both rises and drops - were as much as 0.35 m and approached their maximum excursions within less than 1 hour to about 24 hours. Recovery rates differed among wells, though most are obscured by larger responses to rainfall accompanying Hurricane Irene four days later. Based on modeling, the earthquake fault offset imposed areal strains of approximately 10-6 within a 20-km radius of the epicenter, but water-level changes within this radius exceeded plausible ranges of poroelastic changes due to calculated coseismic strain. Strong ground motion, or “seismic shaking”, is an alternate explanation for the excess water-level deviation. Observed groundwater-level steps farther from the epicenter are not caused by near-field strain from fault slip, so we attribute these to seismic shaking. Consistent with this explanation, the groundwater-level changes occurred within the area where earthquake-induced shaking reached Mercalli intensity IV or greater. Earthquakes of the same magnitude in the western U.S. have not caused groundwater-level changes over this large an area.

The distribution of groundwater responses has a directional component that generally follows the trend of the Appalachian orogenic front, is subparallel to the strike of the earthquake fault, and generally corresponds to the distribution of ‘felt’ reports. Like the seismic ground motions and felt reports, the groundwater level changes may have been influenced by the SW-to-NE rupture propagation during the main shock. The locations of wells exhibiting water-level changes trends more westerly south of the transition between the central and southern Appalachians near 38° N latitude. No network wells south of this transition responded to the earthquake. In contrast, wells responded as far north as upstate New York.

Several possible mechanisms that may explain how strong ground motion changes groundwater levels will be discussed. The Christiansburg, Virginia well is a particularly promising target for enhanced data collection to constrain these mechanisms as it consistently responds to worldwide earthquakes.