UPDATE No. 19: Looks like the Washington Monument was damaged after all.
The National Park Service found a crack in the stones near the top of the monument after completing a secondary inspection Tuesday, the Associated Press reports. Engineers will take another look at the damage Wednesday to determine the best way to repair it before reopening the monument to the public.
UPDATE No. 18 at 6:12 p.m. It looks like the news reports have slowed to a trickle, so we're going to bring this quasi-live blog to an end for now. But we'll be back with any major developments. Experts warn that aftershocks could occur anytime in the next few hours or even days. And if that's not enough to keep everyone's attention there's always Hurricane Irene, which is on track to reach the Carolinas this weekend.
UPDATE No. 17 at 5:13 p.m.: Slate's David Weigel has a look at how Washington pundits responded to the earthquake. (Spoiler alert: Turns out, they're not as funny as they think they are.)
UPDATE No. 16 at 5:01 p.m.: The Capitol building has reopened to allow staff to collect their personal belongings but the visitor's center is still closed, according to ABC 7 in D.C.
UPDATE No. 15 at 4:45 p.m.: Here's some raw video of the White House and the Capitol building during the quake (via the AP):
UPDATE No. 14 at 4:37 p.m: Despite what Twitter may be telling you, the Washington Monument is not leaning, the National Parks Service says. Agency spokesman Jeffrey Olson tells the AP that there was "absolutely no damage" to the monument, the Lincoln Memorial or other tourist destinations long the Mall.
In the immediate aftermath of the quake, Fox News tweeted that a D.C. police officer told one their producers that the Washington Monument "may be tilting following" the quake. Twitter and the Internet then did what it does...
UPDATE No. 13 at 4:22 p.m.: Slate's Will Oremus has a primer on East Coast fault lines here.
UPDATE No. 12 at 3:53 p.m.: The U.S. Geological Survey has downgraded the earthquake's magnitude to 5.8, which was its original assessment in the immediate aftermath of the event. The agency had elevated it to a 5.9 briefly.
UPDATE No. 11 at 3:48 p.m.: The D.C. fire department says that there are numerous injuries as a result of the earthquake, but so far there are no reports of serious injuries or deaths, the AP reports.
In addition to the National Cathedral, there have also been reports of damage to a number of buildings in D.C., including the Ecuadorian embassy and a handful of schools.
UPDATE No. 10 at 3:35 p.m.: The White House and its surrounding federal buildings have been given the all clear and people are being allowed back into the building, CNN reports.
UPDATE No. 9 at 3:29 p.m.: The first aftershock has been reported in roughly the same area as the original quake. The U.S. Geological Survey reports that there was a 2.8 aftershock 5 miles SSW of Mineral, Virginia.
UPDATE No. 8 at 3:26 p.m.: The U.S. Geological Survey is warning that aftershocks with 2.0 to 3.0 magnitude are likely to hit the area at some point in the future.
UPDATE No. 7 at 3:24 p.m.: Quick PSA: Emergency officials are urging people not to call 911 unless they have an actual emergency to report.
UPDATE No. 6 at 3:21 p.m.: The first reports of structural damage are coming in. The Washington National Cathedral appears to have been one of the harder hit buildings in D.C. A spokesman for the cathedral says that at least three of the four pinnacles -- the top stones on the cathedral's towers -- have fallen off and the central tower appears to be leaning.
UPDATE No. 5 at 3:16 p.m.: The earthquake forced an early end to a press conference on the DSK dismissal. ABC News has the video.
UPDATE No. 4 at 3:11 p.m.: The Associated Press reports that the earthquake knocked out power at Dominion Virginia Power's North Anna nuclear plant but the companys says that the utility maually shut down both nuclear reactors without incident and no damage apparent.
UPDATE No. 3 at 3:04 p.m.: Government officials are warning people to be prepared for potential aftershocks. "People should be expecting [them], especially over the next hour or two," U.S. Geological Survey seismologist Lucy Jones told CNN.
UPDATE No. 2 at 2:58 p.m.: Seven nuclear plants on the East Coast declared unusual events, according to the Nuclear Regulatory Commission. That declaration is the least serious of the four emergency notifications, the Wall Street Journal reports.
Jim Steets, a spokesman for the company (Entergy) that owns the Indian Point nuclear plant 50 miles north of New York City, reported "no issues" there after the quake, the paper reports.
UPDATE No. 1 at 2:39 p.m.: So far no reports of structural damage in D.C. or anywhere else.
Here was the scene in the Pentagon, via msnbc.com: "[A] low rumbling built and built to the point that the building was shaking. People ran into the corridors of the government's biggest building and as the shaking continued there were shouts of 'Evacuate! Evacuate!'"
POSTED at 2:09 p.m.: An earthquake in Virginia rocked much of the East Coast Tuesday afternoon, with early reports of shaking being felt all the way from Charlotte, N.C., to New York City and Ohio.
The USGS says that the quake had a magnitude of 5.9 and that its epicenter was 4 miles south of Mineral, Virginia, and 83 miles southwest of Washington, D.C. The federal agency had originally assigned the quake a preliminary magnitude of 5.8 in the immediate aftermath of the quake. The earthquake occurred roughly 0.6 miles below the earth's surface.
News of the earthquake quickly dominated Twitter, with users suggesting that they felt the quake up and down much of the Atlantic Coast and even as far away as Toronto.
Reuters reports that both the Pentagon and the U.S. Capitol building were being evacuated shortly after the shaking started.
Here's the map from USGS: