Virginia Earthquakes Virginia and the eastern side of the North American continent are in the middle of a tectonic plate. The North American Plate is one of the 15 or so major "chunks" of crust that float on top of the hot mantle. The plate includes both continental crust and heavier (iron- and magnesium-rich) oceanic crust. The eastern coast of the United States marks the boundary between continental and oceanic crust, but the North American Plate includes both continental and oceanic crust. The Eastern Shore/Virginia Beach are at the edge of the continent, but are not located at the edge of the continental plate. Instead, the eastern edge of the North American Plate is in the middle of the Atlantic Ocean at the Mid-Atlantic Ridge. At that ridge, magma rises slowly and pushes Virginia (and the rest of the North American Plate) towards China, at the rate of about 2-3 centimeters/year or about 14 miles every million years.1 Virginia is located far from the edge of the North American Plate. In contrast, California/Oregon/Washington are at the edge of the North American Plate. In Southern California, the western edge of the North American Plate rubs against the eastern edge of the Pacific Plate. That boundary is marked by the San Andreas Fault and many other named/unnamed faults. The North American and Pacific plates are moving in different directions, and the rocks bend under the strain - up to a point. Earthquakes occur when the pressure to move exceeds the capacity of the rocks to resist motion. Intermittently, the North American/Pacific plates break free of each other, and the edges spring into a new alignment. As the land moves into the new alignment, everything shakes nearby in the earthquake - and buildings, highway bridges, etc. may collapse. Since Virginia is in the relatively-stable inner portion of a plate, Virginia does not experience the large-magnitude earthquakes that affect Los Angeles, Alaska, Haiti, Japan, Chile, or other places that are on a tectonic plate's edge. As noted by the Virginia Department of Mines, Minerals, and Energy:2 Virginia is located near the center of the North American plate and, thus, experiences a much lower rate of seismicity than California. Another difference is that California earthquakes often break the ground surface, while earthquakes in Virginia usually occur on faults at depths of from 3 to 15 miles. Thus, the earthquakes felt in the Commonwealth today generally have no relationship with faults seen at the surface.

North American Plate Source: USGS World Plate Boundaries Animation

The United States east of the Mississippi has many fewer earthquakes than does the west, and western quakes are stronger. However, the less-intense eastern earthquakes can cause damage further away from their origin. In the east the underlying bedrock is pretty well-connected (like a concrete slab). Waves from eastern earthquakes can travel farther that in the west, where the underlying topography is so chopped-up (like a brick patio) that the energy of a quake is dissipated closer to the epicenter.

Earthquakes in Virginia are rare in the Coastal Plain, but are not restricted to just one region. Two zones in Virginia are more susceptible to earthquakes than others, and can be identified by the rivers which follow those faults. The James River follows the Central Virginia Seismic Zone between Charlottesville and Richmond, while the New River follows the Giles County Seismic Zone from Radford to the West Virginia border.

Virginia is pretty stable, but just about any place in the state can experience an earthquake. Manassas was surprised by a 2.5 magnitude tremor in 1997, and an equivalent earthquake was felt in Culpepertwo months earlier. Near the southern edge of the Culpeper Triassic basin, a magnitude 3.2 earthquake rattled Charlottesville in 2001.

The 1997 earthquake in Manassas was tiny one, but was clearly felt in the Northern Virginia area. If you're a James Bond fan, you'll appreciate one local person's description as having been "shaken, not stirred" after hearing what sounded like an unusually large sonic boom. (He did check to see if a tree had fallen on the roof.) Another resident, whose house may have been right above the epicenter, was "stirred" from a deep sleep. Ironically, she had just returned the previous day from San Francisco. For years she had avoided traveling there, from a fear of earthquakes. Sure enough, after visiting California, she experienced an earthquake... while sleeping in her own bed in Manassas. As reported in the Washington Post the next day:³

"...befuddled residents wandered outside their dwellings and workplaces and flooded emergency dispatchers with telephone calls asking what had happened. Fire and rescue officials rushed around in a vain attempt to find an explosion to explain the event, checking with everyone holding blasting permits in Prince William County."

epicenters of "significant" - felt by people - earthquakes in Virginia
(up to 2009, does not show 2011 quake)
Source: USGS National Atlas

The most recent large earthquake in Virginia occurred on August 23, 2011, when a magnitude 5.8 quake was centered near Mineral, Virginia (followed by a 4.5 aftershock) in the Central Virginia Seismic Zone. In 2003, a 4.5 magnitude quake had struck nearby in Goochland County.

In the 2011 earthquake, the rumbling/shaking lasted 10-15 seconds and triggered buildings to be evacuated between New York City to Richmond. Modern technology is fragile, and the two nuclear power plants at Lake Anna shut down automatically. Buildings on the Fairfax campus of George Mason University were closed, the Washington Monument cracked, urban workers in Washington DC took the rest of the afternoon off, and a classic traffic jam developed on Northern Virginia highways. In Culpeper County, the chimneys at the historic Salubria house cracked. In Louisa County, near the epicenter of the quake, the chimney at Cuckoo collapsed.⁴

Water as well as land is affected by the earth shaking. When the quake hit, fish jumped out of Lake Jackson in Prince William County, and later that day at least one house near Manassas reported that water from the private well had turned brown.⁵

a rare image: "Latest Earthquakes in the USA - Last 7 days" shows August 23, 2011 earthquake Source: US Geological Survey

location of Mineral, in Louisa County - and Central Virginia Seismic Zone Source: USGS National Atlas

The 2011 Mineral quake appeared to be associated with the Spotsylvania high-strain zone, the boundary between the Goochland and Chopawamsic terranes. Those terranes are chunks of continental crust that were squeezed onto the North American continent as the Iapetus Ocean closed during the Paleozoic era. The edges of different terranes are zones of weakness, and potential sites of future earthquakes that relieve stresses that build up within the bedrock of Virginia as the North American Plate drifts westward.

The Chopawamsic Terrane was a volcanic island arc that formed in the Ordivician period. Before the island arc was scrunched into Virginia, it was similar to Japan or the Aleutian Islands today. During the Taconic orogeny, as the Iapetus Ocean closed, the Chopawamsic Terrane was pushed by tectonic motion and platered onto the edge of an expanding continent. The Goochland Terrane (including the State Farm gneiss) is a much older chunk of continental crust, formed originally prior to the Grenville orogeny. Before the Iapetus Ocean began to close, the Goochland Terrane was located near Manhattan (and perhaps further to the northeast). As the continental plates moved, the Goochland Terrane migrated southwest and merged into Virginia on the eastern side of the Choppawamsic Terrane.⁶

location of Spottsylvania high-strain zone Source: US Geological Survey Circular 1264, Geology of the National Capital Region - Field Trip Guidebook

intensity of ground motion in 2011 Mineral quake Source: US Geological Survey, Shakemap us082311a

The other "big one" in Virginia (about a 5.8-6.0 on the Richter scale) was on May 31, 1897, in Pearisburg, the county seat of Giles County. The judge in the courthouse adjourned a trial, jumped over the railing, and fled outside with everyone else as the courthouse rattled, brick walls cracked, and chimneys fell over.⁷ It was one of Virginia's most powerful recorded earthquakes - but our recorded memory extends back only a few centuries, while the geologic history of the state extends back hundreds of millions of years. In 1959, Giles County was shaken again by a 3.8 temblor and windows were broken in the 1975 Veterans Day earthquake in Blacksburg.

Quaternary fault zones in Virginia
Source: USGS Earthquake Hazards Program - Quaternary Fault and Fold Database

Source: USGS Earthquake Hazards Program - Quaternary Fault and Fold Database

About 300 total earthquakes have been recorded since Virginia became a state. Since 1977, in about the last 25 years, there have been about 150-200 earthquakes in Virginia.⁸ Don't assume that we're getting more earthquakes all of a sudden. We're just getting better at recording earthquakes, since sensors have been installed to identify the smaller quakes in the last 25 years. Until seismographs were installed, the only recorded quakes were the ones strong enough to be felt - and prior to the coal and timber boom in the 1880's, there were not that many people in western Virginia to provide reports...

Giles County and Central Virginia seismic risk areas in Virginia
Source: USGS Earthquakes in Virginia and Vicinity 1774 - 2004

probability of earthquake greater than 4.75 magnitude within next 100 years (central Virginia)
Source: USGS Earthquake Probability Mapping

probability of earthquake greater than 4.75 magnitude within next 100 years (southwest Virginia)
Source: USGS Earthquake Probability Mapping

Links

• Advanced National Seismic System (ANSS)
• Recent Earthquakes in Central US
• Science - The Giles County, Virginia, Seismic Zone (March 4, 1983)
• US Geological Survey
• USGS Earthquake Hazards Program - Virginia
• Data for Quaternary faults, liquefaction features, and possible tectonic features in the Central and Eastern United States, east of the Rocky Mountain front, Open-File Report 00-260, with information on the Central Virginia seismic zone, Everona fault-Mountain Run fault zone, Lindside fault zone, Old Hickory faults, Pembroke faults, and Stanleytown-Villa Heights faults
• National Earthquake Information Center (NEIC)
• Earthquakes in Virginia and Vicinity 1774 - 2004
• Virginia Department of Mines, Minerals, and Energy
• Earthquakes
• Virginia Tech Seismological Observatory
• Earthquakes recorded in Virginia: A Primer
• December 9, 2003 light earthquake in central Virginia
• Virginia's Largest Earthquakes (including the Giles County Earthquake of May 31, 1897)

References