The earthquake which struck the state of Virginia on 23 August, 2011 was the second largest ever recorded in the state. At a magnitude of 5.8, it fell just short of the tremor which struck Giles County in 1897, which is reported by the United State Geological Survey (allowing for the changes in earthquake measuring techniques) to have been M 5.9 (USGS, Historic Earthquakes).
The 2011 Virginia Earthquake
Initial data from the USGS show that the earthquake, which had its epicentre between the towns of Mineral and Louisa and some 84 miles south west of Washington DC, was shallow, occurring at a depth of less than 4 miles. At the time of writing, no casualties had been reported, although the AP news agency reported damage to both the Washington Monument and the National Cathedral.
Although there was little physical damage, the tremor was felt widely across the eastern USA (USGS notes that such tremors in the central and eastern part of the continent are typically felt over a much larger area than those to the west of the Rockies). Jill Stefko, a Pennsylvania resident, told Decoded Science that ‘it was like a giant hand shaking the room’ and described how television and internet connections were interrupted.
The Central Virginia Seismic Zone
The Virginia earthquake differed from the majority of major seismic events (in America and elsewhere) in that it did not occur at a boundary between two of the Earth’s tectonic plates. Such boundaries are characterised by seismic activity. Most of North America is located on the North American Plate, bounded on the east by the Mid Atlantic Ridge and the west by the San Andreas Fault Zone.
Within the Earth’s tectonic plates, however, the geology is complex and often unstable, and many earthquakes (typically smaller than those at plate boundaries) occur in association with zones of folded and faulted rock. Although detailed geological studies have yet to be undertaken, the August 2011 earthquake is known to have been the result of movement along faults in a complex known as the Central Virginia Seismic Zone.
The Seismic History of Virginia
The CVSZ is a complex which, according to the USGS, ‘has produced small and moderate earthquakes since at least the 18th century’. Recorded earthquakes in Virginia are not uncommon: although rigorous seismic recording has been in place for little more than a century, other written and oral sources indicate that, within the region, ‘moderate earthquakes cause slight local damage…about twice a decade on the average’ (Tarr and Wheeler).
A list of significant earthquakes in the state produced by Stover and Coffman indicates that Virginia experienced 15 notable seismic events between 1568 and 1989 (though no magnitudes are assigned to them). Given this historic seismicity and the geological setting, the August 2011 earthquake, though large, was by no means an atypical event within the state.
AP news agency. Puzzled East Coasters: An earthquake? No way. 24 August 2011. Accessed 24 August, 2011.
Carl W. Stover and Jerry L. Coffman. Seismicity of the United States 1568-1989 (revised). USGS Professional Paper 1527. Accessed 24 August, 2011.
Arthur C. Tarr, and Russell L. Wheeler. Earthquakes in Virginia and Vicinity 1774 – 2004. U.S. Geological Survey Open-File Report 2006–1017. Accessed 24 August, 2011.
United States Geological Survey. Earthquake Hazards Program: Magnitude 5.8 – VIRGINIA. 2011 August 23. Accessed 24 August, 2011.
United States Geological Survey. Historic Earthquakes: Giles County, Virginia. Accessed 24 August, 2011.
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