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August 23, 2011 1:51pm; 5.8 Magnitude Earthquake
Louisa County, Virginia

Virginia and much of the East Coast experienced a widely-felt earthquake at 1:51 p.m. eastern daylight time on Tuesday, August 23, 2011. According to the U.S. Geological Survey, the epicenter of the quake was located near Cuckoo, in Louisa County. With a magnitude of 5.8, this is the largest Virginia earthquake recorded by seismometers. 26 aftershocks have been reported by the USGS and the area is currently being monitored by geophysicists from several leading science institutions.

The U.S. Geological Survey is now reporting that this is the most widely-felt earthquake in U.S. history. Photos from the hazard assessment team can be found here.

Map of all recorded earthquakes and known faults in Virginia (Zoom to adjust map, click on earthquakes and faults to get more information)

Central Virginia Seismic Zone

The epicenter of the 5.8M earthquake falls within the Central Virginia Seismic Zone, a cluster of dozens of earthquakes that have occurred within the past 120 years, centered about halfway between Richmond and Charlottesville. Several known faults are present in the area: the Chopawamsic Fault, the Lakeside Fault, and the Spotsylvania Fault. These are old faults, related to plate tectonic events that closed and then reopened the Atlantic Ocean about 200 million years ago. Even though these faults are quite old and considered to be inactive, occasional earthquakes continue to occur.

Generalized Map representing the Central Virginia Seismic Zone (courtesy of the USGS)

 

Geologic structures within the Central Virginia Seismic Zone (David Spears)

 

Interstate 64 Seismic Line Interpretation

Virginia Division of Geology and Mineral Resources Publication 66 is an interpretation of the seismic profile line along Interstate-64 in Central Virginia. This publication proves helpful in showing the fault structure of the Central Virginia Seismic Zone. Interpretation by geologists on the image below shows several scallop faults in an overthrust regime. Using two way travel time, the small red bounding box shows the proposed location 3.5 miles souwthwest along-strike of the 5.8M earthquake.

Interpreted 2-D seismic line along I-64 across the Central Virginia Seismic Zone ( DGMR Publication 66 ) (click on image to view full size line)

 

DGMR-USGS Intensity Map

Geologists from the Virginia Division of Geology and Mineral Resources and the USGS gathered immediately following the earthquake to record intesity mearurements across the area. From the data collected the DMGR has created an preliminary intensity map (updates to the map will be made in a timely manner). Download PDF of Preliminary Intensity Map

Preliminary intensity map for the 5.8M earthquake (click on image to view full size PDF)

 

East Coast Earthquakes

Earthquakes in the eastern U.S. are different from the earthquakes that occur in more seismically active areas, such as California. California is located on the boundary between two large blocks of the earth’s crust, the North American and Pacific tectonic plates. As these plates grind past each other, stresses build up and periodically release catastrophically. Virginia, however, is located in the middle of the North American plate; the nearest tectonic plate boundary is the Mid-Atlantic Ridge, in the middle of the Atlantic Ocean. Earthquake activity occurring away from plate boundaries is known as “intraplate seismicity.” Such earthquakes are generally less severe and less damaging than those occurring at plate boundaries, although occasional large earthquakes, such as the 5.8M in central Virginia, do occur.

For more information on earthquakes in Virginia From DGMR Earthquake Information Page
DGMR Earthquakes Brochure (pdf)
Virginia Minerals - On Earthquakes (February 1977) (pdf)
Virginia Minerals - The Carters Bridge, Virginia, Earthquake of February 27, 1977 (August 1977) (pdf)
Virignia Minerals - Siesmic Hazard in Virginia (November 1978) (pdf)

 

Additional Links

Virginia Tech Seismological Observatory (Seismographs)