Aug. 23, 2011 -- The largest East Coast quake in memory rattled nerves and buildings from Martha's Vineyard to to North Carolina, prompted the evacuation of Congressional buildings, slowed rail and air traffic, and forced two nuclear reactors offline.
The earthquake, estimated to be a 5.8 magnitude, sent people pouring out of office buildings, hospitals, the Pentagon and the State Department when it struck at 1:51 p.m. The pillars of the capitol in Washington, D.C., shook. Alarms sounded in the FBI and Department of Justice buildings, and some flooding was reported on an upper floor of the Pentagon as a result of the quake.
Parks and sidewalks in Washington were packed with people who fled their buildings. All of the monuments along the National Mall have been closed. Police on horseback kept people a safe distance from the Washington Monument and the new Martin Luther King, Jr. Memorial.
National Parks Service Spokesman Jeffrey Olson told the Associated Press that there was "absolutely no damage" to the Washington Monument, Lincoln Memorial or other tourist destinations along the Mall.
The National Cathedral suffered damage to at least three of the cathedral's pinnacles, Dean of the Cathedral Samuel Lloyd said. The cathedral has been cordoned off with yellow police tape as a precaution.
Officials inspected Congressional buildings before members of Congress and their staff were allowed to return to their offices.
A number of small aftershocks are likely, says Dale Grant, a geophysicist with the U.S. Geological Survey. "We would not expect anything bigger, but it cannot be completely ruled out.
"The earth does what it does," he added.
The quake was felt as far north as New Hampshire and in Martha's Vineyard where President Obama and his family are vacationing. It was felt as far south as South Carolina and as far west as Cleveland, Ohio.
Over the last 10 years, earthquakes have been felt in every state, said geophysicists at the U.S. Geological Survey at a press conference late this afternoon. Today's quake was felt in 25, an event David Wald called "rather unique."
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The East Coast gets earthquakes from time to time, but rarely of a magnitude to make skyscrapers sway.
Paul Segall, a Stanford geophysicist who studies the structure and development of earthquake faults, called today's shaker "a significant earthquake for that part of the world. It could do significant damage."
"I can't remember an event that large on the East Coast," he said.
No significant damage or fatalities have been reported. Some injuries have been reported in Washington D.C., the fire department spokesman told the Associated Press. In New York City, the fire department said that they received a surge in calls.
Authorities in New York and Washington said cell phone traffic was so heavy that it hampered their ability to respond to emergencies. A spokesperson for the Federal Emergency Management Agency urged people to email and use text messaging instead of their cell phones for their next few hours to ease the congestion.
The epicenter of the quake was near Mineral, Va., 39 miles from Richmond, Va., and 83 miles from the nation's capital. The quake was .6 miles deep.
It was centered in the Central Virginia Seismic Zone. "The Applachians were once bigger than the Rocky Mountains, and they've worn down over time, and we do still see seismic activity going up into Canada," said Grant. But the last quake of this size in that zone was most likely in 1875. In December 2003, a magnitude 4.5 earthquake in that zone produced minor damage.
According to convertalot.com, a web site that compiles measurements and calculators for a variety of statistics, the magnitude 5.8 earthquake released energy equivalent to the explosion of 10,676 tons of TNT.
Amanda Reidelbach, office manager and spokeswoman for the Louisa County Department of Emergency Services in Mineral, Va., said that the town has felt "at least a half dozen or so" aftershocks since the initial quake struck.
"There were pretty serious aftershocks," she said. "We walked out onto the street and felt the ground just rumbling."
There have been reports of structural damage to some residences in town, Reidelbach says, but no reports of significant injuries. Mandatory evacuations were put in place shortly after the quake with all non-essential government and county personnel were sent home for the day. Schools were also closed.
The epicenter of the quake is very close to two Dominion Power nuclear power plants, North Anna 1 and 2.
Elizabeth Stuckle, spokesperson for the Nuclear Regulatory Commission, said that the reactors were "automatically and safely shut down." The plant declared an "unusual event" which is the lowest category of four emergency classifications. Back-up generators automatically kicked in to keep the reactors cool, the NRC said.
Nine other nuclear plants on the East Coast declared an "unusual event," but were none shut down.
The tremblor affected travel in the region.
Amtrak said it was running at reduced speed and was checking tracks and terminals for damage. The Washington Metropolitan Area Transit Authority said the Metro is moving at 15 mph as inspectors check all tracks.
Flights at Dulles International Airport in Virginia, JFK International in New York and in Philadelphia were temporarily halted. Control towers at JFK and Newark International airports were temporarily evacuated. The delay will cause slow air traffic throughout the region, the Federal Aviation Administration warned. In addition, about a dozen flights were diverted from JFK to Boston.
A woman who works at Mineral Barber Shop in Mineral, Va. said that the inside of her shop is a mess but there doesn't appear to be any major damage outside the town square.
In Richmond, Va., a woman who works on the 18th floor of a 20 story building said she and her co-workers left the building when the shaking first began.
"At first I thought it was someone jumping on floor above me, but then it was really loud and shaky," she said.
People in The New York Times building on 42nd street in Manhattan said they felt the entire building shift, and watched office furniture move. As the tall buildings in New York swayed, people ran out into the street.
The New York City Criminal Court in lower Manhattan was also evacuated.
In Baltimore, artist Lisa Lewenz was working in her basement studio when she began to feel movement under her feet.
"Everything started trembling, with a big boom sound coming up from the ground. I've lived in L.A. long enough to know this drill, so rushed upstairs, and found the glassware still shuttering for about a minute. Couldn't get through by the phone to friends, and there was no news online, so I started worrying my house was collapsing," Lewenz said.
Rare East Coast Earthquake Reaches 5.8 Magnitude
Since there were no serious injuries, some saw the lighter side in the unexpected quake.
Michelle Mittelstadt said, "My first earthquake! What's next: Plague of locusts?"
Another woman who works with the Federal Aviation Administration said, "If you have to be evacuated for an earthquake, the National Mall is a nice spilling-out point!"
The earthquake felt along the eastern corridor follows an earthquake felt Monday in Colorado. That 5.3 magnitude earthquake struck near Trinidad, Colo. The USGS's Wald said he saw "no evidence" the two were related physically; they were simply "coincident in time."
The United States' Geological Survey said that earthquakes have been felt in the central Virginia area since 1774.
ABC News' Jane E. Allen, Christina Caron, Troy McMullen, Jack Cloherty, Jim Sciutto, Aaron Katersky and Dennis Powell contributed to this report.