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Major Earthquake 2011

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.

Earthquake

Virginia Division of Geology and Mineral Resources, DMME

 

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

The interactive map above shows all earthquakes in the state of Virginia (recorded by instruments and reported by USGS) and mapped faults (red lines) (from the Virginia State Geologic Map, 1993). The white bullseye is the epicenter of the 5.8 earthquake on August 23, 2011, the large red dots with black centers are earthquakes that have occurred within the last year (from October 2011). The smaller dots are all other earthquakes. All earthquake information is collected from ANSS (Advanced National Seismic System).

 

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 of Central Virginia Seismic Zone

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

 

Geologic structures of the Central Virginia Seismic Zone

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 southwest along-strike of the 5.8M earthquake.

Generalized Map of Central Virginia Seismic Zone

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)

 

 

Damage Intensity Map

Geologists from the Virginia Division of Geology and Mineral Resources and the USGS gathered immediately following the earthquake to record intensity measurements across the area. From the data collected the DMGR has created a damage intensity map.

Damage Zone Intensity map for the 5.8M Earthquake

Damage Zone Intensity map for the 5.8M Earthquake (click on image to view full size line)

 

Virginia Earthquake Interpreted Mercalli Intensity

August 23, 2011 Virginia Earthquake Interpreted Mercalli intensity (click on image to view full size line)

 

 

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.

 

References

Earthquakes

Major Virginia Earthquakes

DGMR Earthquakes Brochure

Virginia Minerals - On Earthquakes (February 1977)

Virginia Minerals - The Carters Bridge, Virginia, Earthquake of February 27, 1977 (August 1977)

Virginia Minerals - Seismic Hazard in Virginia (November 1978)

Virginia Tech Seismological Observatory (Seismographs)