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
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 has more faults that diffuse the energy
earthquakes are felt at greater distances in the eastern part of North America, because bedrock in the western states
(especially in Los Angeles/San Franciso 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
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.

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)
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. The Washington Monument cracked, closing it for several years and requiring a $15 million repair project. 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. In Louisa County, 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
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
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
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. 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 Spottsylvania high-strain zone
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
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.

map of Central Virginia Seismic Zone
Source: USGS Earthquake Hazards Program - Quaternary Fault and Fold Database

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 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 - and 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

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

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

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. 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.10

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

recording from seismometer

Links

world quakes, 1963-98

References

1. This Dynamic Earth - Understanding Plate Motions, USGS, pubs.usgs.gov/gip/dynamic/understanding.html, and assuming the shift in location of the Yellowstone "hot spot" ovver the last 16 million years reflects the movement of the North American Plate, as described by Robert B. Smitha, and Lawrence W. Braile, "The Yellowstone hotspot," Journal of Volcanology and Geothermal Research, Volume 61, Issues 3-4, pp.121-129, 135-187, July 1994, http://www.sciencedirect.com/science/article/pii/0377027394900027 (last checked August 23, 2011)
2. "Earthquakes," Virginia Department of Mines, Minerals, and Energy, http://www.dmme.virginia.gov/dgmr/pdf/EARTHQUAKES.pdf (last checked May 16, 2013)
3. personal communications, "Manassas finds itself on terra not-so-firma," Washington Post, September 30, 1997, http://www.washingtonpost.com/wp-dyn/content/article/2010/07/16/AR2010071602617.html (last checked May 16, 2013)
4. "Another aftershock reported in Virginia," Washington Post, August 23, 2011, http://www.washingtonpost.com/blogs/post_now/post/earthquake-reported-along-the-east-coast/2011/08/23/gIQAozdEZJ_blog.html; "Historic Earthquake Strikes Historic Virginia Buildings," Fredericksburg Free Lance-Star, August 25, 2011 http://www.fredericksburg.com/News/FLS/2011/082011/08252011/647482; "Louisa schools symbols of recovery," Richmond Times-Dispatch, August 23, 2012, http://www2.timesdispatch.com/news/2012/aug/23/louisa-schools-symbols-recovery-ar-2149981/; "Washington Monument scaffolding topped off for next stage of earthquake repairs," Washington Post, May 4, 2013, http://www.washingtonpost.com/local/washington-monument-scaffolding-topped-off-for-next-stage-of-earthquake-repairs/2013/05/14/985668e8-bc06-11e2-89c9-3be8095fe767_story.html (last checked May 16, 2013)
5. Personal communications, August 23, 2011; J. W. Horton, R. A. Williams, "The 2011 Virginia Earthquake: What Are Scientists Learning?," American Geophysical Union, EOS Transactions, Volume 93 Number 33 (August 14, 2012), http://www.agu.org/pubs/pdf/2012EO330001_rga.pdf (last checked May 16, 2013)
6. David B. Spears, Brent E. Owens, and Christopher M. Bailey, "The Goochland-Chopawamsic Terrane Boundary, Central Virginia Piedmont" in Geology of the National Capital Region - Field Trip Guidebook, US Geological Survey Circular 1264, http://pubs.usgs.gov/circ/2004/1264/; Christopher Bailey, Brent Owens, Catherine R. Shirvell, "Northern ancestry for the Goochland terrane as a displaced fragment of Laurentia: Comment and Reply," Geology, January 2005 v. 33, no. 1; p. e70, http://geology.geoscienceworld.org/cgi/content/full/33/1/e70; Leah C. Cheek, Kyle J. Grimsley, Erin E. Fenlon, Katie E. Luciano, "Identity Crisis: From where did the Goochland Terrane Originate?" in Geology of Virginia, http://web.wm.edu/geology/virginia/whats_new/goochland.php; J. W. Horton, R. A. Williams, "The 2011 Virginia Earthquake: What Are Scientists Learning?," American Geophysical Union, EOS Transactions, Volume 93 Number 33 (August 14, 2012), http://www.agu.org/pubs/pdf/2012EO330001_rga.pdf (last checked May 16, 2013)
7. "Giles County Earthquake of May 31, 1897," Virginia Tech Seismological Observatory, http://www.geol.vt.edu/outreach/vtso/Giles-Intensity.html; "Earthquakes Recorded in Virginia: A Primer," Virginia Tech Seismological Observatory, http://www.geol.vt.edu/outreach/vtso/quake.html (last checked May 16, 2013)
8. "Earthquakes in Virginia Fact Sheet," Virginia Department of Mines, Minerals, and Energy, http://www.dmme.virginia.gov/dgmr/pdf/earthquakes.pdf (last checked May 16, 2013)
9. "Magnitude 2.3 Earthquake in Virginia," US Geological Survey, May 15, 2013, http://www.usgs.gov/blogs/features/usgs_top_story/magnitude-2-3-earthquake-in-virginia/; "2.3 magnitude earthquake recorded in Louisa," Richmond Times-Dispatch, May 15, 2013, http://www.timesdispatch.com/news/state-regional/article_8f8a817c-bd5a-11e2-93d8-001a4bcf6878.html (last checked May 16, 2013)
10. "Public Assistance Program Approved for Louisa County," Federal Emergency Management Agency news release, November 12, 2011, http://www.fema.gov/news-release/2011/11/12/public-assistance-program-approved-louisa-county; "Virginia - Earthquake, FEMA-4042-DR," Major Disaster Declaration declared on 11/04/2011, Federal Emergency Management Agency, http://www.fema.gov/ar/disaster/4042 (last checked May 16, 2013)

compared to other parts of the United States, Virginia has relatively low risk of damaging earthquakes
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)


Rocks and Ridges
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