On August 23, 2011, a magnitude 5.8 earthquake hit the Piedmont region of the U.S. state of Virginia at 1:51:04 p.m. EDT. The epicenter, in Louisa County, was 38 mi (61 km) northwest of Richmond and 5 mi (8 km) south-southwest of the town of Mineral. It was an intraplate earthquake with a maximum perceived intensity of VIII (Severe) on the Mercalli intensity scale. Several aftershocks, ranging up to 4.5 in magnitude, occurred after the main tremor.
The quake was felt across more than a dozen U.S. states and in several Canadian provinces, and was felt by more people than any other quake in U.S. history. No deaths and only minor injuries were reported. Minor and moderate damage to buildings was widespread and was estimated by one risk-modeling company at $200 million to $300 million, of which about $100 million was insured.
The earthquake prompted research that revealed that the farthest landslide from the epicenter was 150 miles (240 km), by far the greatest landslide distance recorded from an earthquake of similar magnitude. Previous studies of worldwide earthquakes indicated that landslides occurred no farther than 36 miles (58 km) from the epicenter of a magnitude 5.8 earthquake. The Virginia earthquake study suggested that the added information about East Coast earthquakes may prompt a revision of equations that predict ground shaking.
Geology
The earthquake occurred in the Virginia seismic zone, located in the Piedmont region. The Virginia Piedmont area was formed originally as part of a zone of repeated continental collisions that created the ancestral Appalachian Mountains, a process that started during the Ordovician period with the Taconic orogeny and finished during the Carboniferous Period with the Alleghanian orogeny. The reverse faults formed during the various orogenies were partly reactivated as normal faults in extension during the Mesozoic Era as the supercontinent Pangaea broke apart. During the Cenozoic Era, compression from the opening and spreading of the Atlantic has caused some of these structures to be reactivated in a reverse sense.
The earthquake's epicenter and most of the aftershocks lie between the surface traces of two structures, the Spotsylvania Fault, a southeast dipping zone of high ductile strain, and the Chopawamsic Fault, a thrust fault. The earthquake's focal mechanism shows reverse slip faulting on a north to northeast striking fault plane. The spatial distribution of aftershocks show that the causative fault dips to the southeast at 50–55°. There was no surface faulting associated with the earthquake. The size of the rupture is as yet uncalculated, but similar quakes have been caused by slippage along fault segments that are 5 to 15 km (3 to 9 mi) long.
After the earthquake, several websites speculated about whether hydraulic fracturing (fracking) for natural gas production could have caused or contributed to the quake. There were not any fracking operations in Virginia at the time of the quake. The nearest fracking was occurring in the Marcellus shale in West Virginia.
Aftershocks
Numerous aftershocks occurred after the main tremor. The first four (of magnitude 2.8, 2.2, 4.2 and 3.4) occurred within 12 hours of the main shock. A 2.5-magnitude shock occurred just after midnight on August 25, followed at 05:07 UTC by the strongest, a magnitude 4.5 aftershock that woke many residents in Northern Virginia and Washington, D.C., and was felt as far away as New England, Georgia, and Illinois.
Research
Scientists have known that the difference between seismic shaking in eastern North America versus western North America is due in part to the geologic structure and rock properties that allow seismic waves in the East to travel farther without weakening, but during November 2012, the USGS announced that recent research showed that earthquake shaking in the eastern United States can travel much farther and cause damage over larger areas than previously thought. USGS scientists found that the Virginia earthquake caused landslides at distances four times farther—and over an area 20 times larger—than previous research had shown. USGS Director Marcia McNutt said, "Scientists are confirming with empirical data what more than 50 million people in the eastern U.S. experienced firsthand: this was one powerful earthquake. Calibrating the distance over which landslides occur may also help us reach back into the geologic record to look for evidence of past major earthquakes from the Virginia seismic zone."
The USGS found that the farthest landslide from the 2011 Virginia earthquake was 150 miles (240 km) from the epicenter, along the Blue Ridge Parkway near Virginia's border with North Carolina. This is by far the greatest landslide distance recorded from any other earthquake of similar magnitude. Previous studies of worldwide earthquakes indicated that landslides occurred no farther than 36 miles (58 km) from the epicenter of a magnitude 5.8 earthquake. The study suggested that the added information about East Coast earthquakes may prompt a revision of equations that predict ground shaking. Study author Randall Jibson said, "what makes this new study so unique is that it provides direct observational evidence from the largest earthquake to occur in more than 100 years in the eastern U.S. Now that we know more about the power of East Coast earthquakes, equations that predict ground shaking might need to be revised."
The earthquake, along with a magnitude-5.8 quake on the border of New York and Ontario in 1944, is the largest to have occurred in the U.S. east of the Rocky Mountains since an 1897 quake centered in Giles County in western Virginia whose magnitude has been estimated as 5.8 or 5.9.
In addition to the great landslide distances recorded, the landslides from the 2011 Virginia earthquake occurred in an area 20 times larger than expected from studies of worldwide earthquakes. Scientists plotted the landslide locations that were farthest out and then calculated the area enclosed by those landslides. The observed landslides from the Virginia earthquake enclose an area of about 33,400 km2 (12,900 sq mi), while previous studies indicated an expected area of about 1,500 km2 (580 sq mi) from an earthquake of similar magnitude.
Effect
It is estimated that approximately one-third of the U.S. population might have felt the earthquake, more than any other earthquake in U.S. history. People in certain areas of Pennsylvania, however, did not feel the earthquake at all despite being relatively close to the epicenter. About 148,000 people reported their ground-shaking experiences caused by the earthquake on the USGS "Did You Feel It?" website. Tremors from the Virginia earthquake were felt as far south as Atlanta, Georgia; as far north as Quebec City, Quebec; as far west as Illinois and as far east as Fredericton, New Brunswick, with damage reported as far away as Brooklyn, New York.
Although earthquakes in the central and eastern U.S. are substantially less frequent than in the western U.S., they are typically felt over a much broader region. East of the Rocky Mountains, an earthquake can be felt over an area as much as ten times larger than a similar magnitude earthquake on the west coast. Western rock is relatively young, therefore it absorbs much of the shaking caused by earthquakes. Hence, western earthquakes result in intense shaking close to the epicenter, but fade more quickly the farther the earthquakes travel. In the eastern United States, the rock is much older, and earthquake energy can therefore spread farther and have a greater impact. A magnitude 5.5 eastern U.S. earthquake can usually be felt as far as 300 mi (500 km) from its origin, and sometimes causes damage as far away as 25 mi (40 km). The relatively shallow depth of this earthquake also contributed to its widespread effects.
United States
Soon after the earthquake, the U.S. Federal Aviation Administration ordered a ground stop along the East Coast, causing some flight delays. The air traffic control tower at John F. Kennedy International Airport was evacuated. Flights were delayed at several airports, including Charlottesville–Albemarle Airport, John F. Kennedy International Airport, Newark Liberty International Airport, Ronald Reagan Washington National Airport, Dulles International Airport, Richmond International Airport, and Philadelphia International Airport. At Ronald Reagan National Airport, ceiling tiles fell in one terminal, and flights were halted.
A huge increase of cell-phone calls immediately after the event congested the AT&T, Verizon Wireless, Sprint Nextel, T-Mobile USA, and Frontier Communications networks in the Mid-Atlantic region, causing disruptions and loss of service for as much as an hour after the earthquake.
Virginia
The epicenter of the earthquake was in Louisa County, Virginia, where damage was greatest and several minor injuries occurred. The town of Mineral, located 5 mi (8 km) north-northeast from the earthquake's epicenter, reported the collapse of two buildings, as well as minor damage to several other structures, including the collapse of the ceiling in its Town Hall. Only minor injuries were reported, including the hospitalization of several people reporting chest pains related to the stress of the experience. Fallen chimneys and other structural damage to buildings was also reported in Louisa, the county seat. The Gilboa Christian Church, in Cuckoo, built in 1832, was heavily damaged and rendered unusable; while its cemetery's vertical standing tombstones were toppled and razed to the ground. At Louisa County High School, cinderblocks fell in classrooms, and cracks were seen in walls. Six students had minor injuries. Louisa County schools were closed on August 24 while engineers assessed damage to school buildings. The high school and Thomas Jefferson Elementary were closed for the remainder of the school year. Inspections revealed that 65 homes sustained major or severe damage and 125 homes experienced mild to moderate damage. Damage in Louisa County was estimated at $80.6 million, of which $63.8 million was from damage to public school buildings and $14.7 million was from damage to residences. On August 25, county officials declared a state of local emergency in order to allow them to request state aid.
Multiple reactor sensors at the North Anna Nuclear Generating Station, located 10 mi (16 km) northeast of the epicenter, detected a slight power reduction as a result of vibrations in the reactor or monitoring devices. This caused the two nuclear reactors to shut down automatically seconds before off-site power was lost. Amid public fears that the earthquake could cause a nuclear accident, prompted in part by the Fukushima disaster which had occurred six months prior, the Nuclear Regulatory Commission sent additional inspectors to the Virginia plant after preliminary measurements suggested that the ground shook more than the two reactors were designed to handle. The damage was minimal and the NRC advised that further inspections should not be interpreted to mean that the plant was not safe. After a $21 million inspection, engineers stated that they only found cosmetic damage. On November 11, 2011, the NRC gave its approval for restarting the reactors.
In Charlottesville, about 27 mi (43 km) west of the epicenter, a gas leak closed several streets, including West Main Street.
In Spotsylvania County, the August 24 opening of public schools was delayed while damage to buildings was assessed. Six patients were treated at the Spotsylvania Regional Medical Center for minor injuries resulting from the earthquake.
Several buildings in Culpeper, about 37 mi (60 km) north of the epicenter, sustained structural damage. The brick façade of the Levy Building, built during 1848, collapsed and the building was condemned and demolished. The walls of St. Stephen's Episcopal Church, constructed during 1821, buckled and were deemed unstable by town officials. Another church, Culpeper Baptist Church, built during 1894, lost its chimney. Schools in Culpeper County delayed their scheduled August 24 opening to assess damage to buildings. Two minor earthquake injuries were reported by the Culpeper Regional Hospital. Damage in Culpeper was estimated at $10 million.
In Fredericksburg, about 37 mi (60 km) northeast of the epicenter, the Dickinson Building on the campus of Germanna Community College was deemed unusable for the rest of the semester, and the whole college was closed until alternative classrooms could be found, re-opening on September 6. Also in Fredericksburg, a gas leak led to the evacuation of homes and businesses in a two-block radius. Officials estimated the damage total at around $711,000.
In Prince William County, approximately 59 miles (95 km) north of the epicenter, damage to the dam for Lake Jackson was documented by officials in May 2012 though local residents had noticed lower water levels in the lake in 2011. Most of the water in the lake was released to relieve pressure. The dam will be rehabilitated at a cost of approximately $900,000.