Giles County and Central Virginia seismic risk areas in Virginia
Source: US Geological Survey (USGS), Earthquakes in Virginia and Vicinity 1774 - 2004
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
North American Plate
Source: US Geological Survey (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 are felt at greater distances in the eastern part of North America, because bedrock in the western states (especially in Los Angeles/San Francisco area) has more faults that diffuse the energy
Source: US Geological Survey (USGS) Magnitude 2.3 Earthquake in Virginia
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.
potential for earthquakes are greater in Central Virginia and Southwestern Virginia
Source: US Geological Survey (USGS) Virginia Seismic Hazard Map
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 Culpeper two months earlier. Near the southern edge of the Culpeper Triassic basin, a magnitude 3.2 earthquake rattled Charlottesville in 2001.
probability of earthquake greater than 4.75 magnitude within next 100 years (southwest Virginia)
Source: USGS Earthquake Probability Mapping
The 1997 earthquake in Manassas was a tiny 2.5 magnitude tremor, but was clearly felt in the Northern Virginia area. One local person's description was drawn from James Bond novels, with a report of being "shaken, not stirred" after hearing what sounded like an unusually large sonic boom. She checked to see if a tree had fallen on the roof, before realizing she had felt an earthquake.
Another resident, whose house was above the epicenter, was awoken from a deep sleep. For years she had avoided traveling to California from a fear of earthquakes, but had returned the previous day from San Francisco. She did experienced an earthquake, but it occurred while napping in her own bed in Manassas.
As reported in the Washington Post the next day:3
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 August 23, 2011 earthquake, 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 11 miles away at Lake Anna shut down automatically.
Buildings on the Fairfax campus of George Mason University were closed, urban workers in Washington DC took the rest of the afternoon off, and a classic traffic jam developed on Northern Virginia highways. Gargoyles fell from the National Cathedral in Georgetown. The Washington Monument cracked, closing it for several years and requiring a $15 million repair project. In 2016, park rangers were still trying to repair the elevator.
In Culpeper County, the chimneys at the historic Salubria house cracked. The Levy Building in the town of Culpeper, built in the 1800's, was damaged so severely that it was demolished and replaced by a small park.
In Louisa County near the epicenter of the quake, the chimney at Cuckoo collapsed and the quake caused $80 million in damage. Thomas Jefferson Elementary School and Louisa High School were damaged so heavily they had to be razed and replaced.4
Community Internet Intensity Map - how people in different areas reported feeling the 2011 quake
Source: US Geological Survey Earthquake Hazards Program, M5.8 – Virginia
Water as well as land is affected by the earth shaking. When the August, 2011 quake hit, fish jumped out of Lake Jackson in Prince William County, and later that day at least one homeowner near Manassas reported that water from a private well had turned brown. As seismic waves moved through bedrock, compressing and then relaxing, groundwater levels changed in wells monitored by the US Geological Survey over 350 miles away. In most cases, water levels moved up and down about 6 inches, but the rise and fall was over 25 inches in one Pennsylvania well.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 plastered onto the edge of an expanding continent.
geologic structures within the Central Virginia Seismic Zone include various terranes that were accreted onto the edge of the North American tectonic plate in the Paleozoic era, with fault lines between the terranes
Source: Virginia Department of Mines, Minerals and Energy, Major Earthquake 2011
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 Chopawamsic Terrane. The suture line where different chunks of crust came together is a fault zone dipping 45° to the southeast, revealed by the depth of aftershocks subsequent to the 2011 Mineral quake.6
location of Spotsylvania 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.
Source: USGS Earthquake Hazards Program - Quaternary Fault and Fold Database
Oklahoma is experiencing a swarm of mild earthquakes triggered by drilling for oil/natural gas and/or by deep injection of fluids left over after oil/gas well development. In Virginia, the story is different.
There has not been a sudden increase in the number of earthquakes in Virginia due to drilling, because there has not been a major increase in drilling. Virginia has only a handful of oil wells, and most natural gas wells produce methane from layers of coal too deep to mine economically. There is potential for fracturing ("fracking") sedimentary layers in the Taylorsvile Basin northwest of Fredericksburg, but commercial development of hydrocarbon resources by fracking has not occured there yet.
drilling for oil and gas, and injection of drilling fluids, has lubricated faults in Oklahoma and triggered earthquakes
Source: US Geological Survey (USGS), Oklahoma Struck by Series of Quakes
Nonetheless, the number of earthquakes in Virginia is "increasing" - because geologists are getting better at recording earth tremors. New sensors have been installed to identify the smaller quakes.
In the 25 years between 1978-1993, there were over 160 recorded earthquakes in Virginia, but only one-two per year (16% of those recorded by seismographs) were felt. Now more seismographs have been installed, so the percentage of earthquakes felt-by-people vs. recorded-by-scientists is even smaller. Until the 1970's, almost all recorded quakes in Virginia were just the ones strong enough to be felt. Prior to the coal and timber boom in the 1880's, there were not that many people in western Virginia to provide reports from personal experience from the Giles County Seismic Zone.8
The old wives tale that "lightning does not strike twice in the same place" is not true for lightning, and certainly not true for earthquakes. Louisa County, where the 5.8 magnitude quake struck in 2011, experienced a magnitude 2.3 earthquake again on May 15, 2013. Similar to the 2011 quake, that particular aftershock was triggered by a shift in the crust 3-4 miles deep. By May 15, 2013, seismographs had recorded 450 aftershocks, including 50 that were felt by people and 38 aftershocks that were at least 2.3 magnitude.9
probability of earthquake greater than 4.75 magnitude within next 100 years (central Virginia)
Source: USGS Earthquake Probability Mapping
Homeowner insurance policies typically exclude coverage for an earthquake. When a house in Virginia is damaged by an earthquake, homeowners must pay for repairs out of their own pocket - unless the President declares a Federal disaster. After the 2011 quake, the Federal Emergency Management Agency (FEMA) recommended against Federal disaster relief, using the Preliminary Damage Assessment to determine that local/state governments could absorb the costs of recovery. However, Governor McDonnell appealed the FEMA recommendation.
bricks were displaced from homes and garages in Louisa County during the 5.8 magnitude quake in 2011, and at least one carport collapsed
Source: Virginia Tech, Virginia Tech researchers offer aid after Virginia earthquake
In November, 2011, President Obama authorized Individual Assistance to provide grants to private homeowners, of which 98% lacked insurance for earthquake damage. The president soon expanded his disaster declaration to authorize Public Assistance, which provided Federal funding to rebuild public facilities such as Louisa County schools. The Federal Emergency Management Agency ended up paying out $19 million in Louisa County, and another $13 million in nine nearby jurisdictions.10
greater ground movement near a quake's epicenter results in greater damage to structures
Source: Virginia Department of Mines, Minerals and Energy, Major Earthquake 2011
Much of the damage in Northern Virginia was not covered by insurance. The Lake Jackson Dam in Prince William County began leaking in 2012. It may have cracked slightly during the quake, but damage was not obvious for almost a year. The county spent close to $1 million to repair the dam.11
the 2011 earthquake may have cracked the foundation of Lake Jackson Dam in Prince William County, nearly 60 miles away from the epicenter
A monument marking 600 Confederate graves at the Warrenton Cemetery was also affected. Maintenance workers realized in 2013 that the 2,500 pound statue known as "Lady Virginia" had moved several inches from its centered position on top of a 40-foot high granite shaft. The local chapter of the United Daughters of the Confederacy raised funds for the repair.12
the 2011 earthquake caused the statue known as Lady Virginia to wobble on top of its shaft at Warrenton Cemetery, 60 miles from the epicenter in Louisa County
the chimneys of the historic home at Cuckoo in Lousia County had to be repaired after the 2011 quake
compared to other parts of the United States, Virginia has relatively low risk of damaging earthquakes
Source: US Geological Survey (USGS) 2008 National Seismic Hazard Maps
(showing showing 5% probability of exceeding 5Hz/0.2 Seconds Peak Ground Acceleration in 50 Years within Conterminous U.S)
seismic zones are defined by the frequency of earthquakes
Source: Virginia Department on Mines, Minerals, and Energy, Mapping Seismic Hazards in Virginia