The epicenter was in Mineral, Virginia.
"It's one of the largest that we've had there," said U.S. Geological Survey seismologist Lucy Jones.
Aftershocks were a concern, she said. "People should be expecting (them), especially over the next hour or two," she added.
The quake was felt in Philadelphia, Pennsylvania; New York City and on Martha's Vineyard, Massachusetts, where President Barack Obama is vacationing. Cell phone service has been disrupted in New York City part of which has been evacuated.
Traders in the New York Stock Exchange felt the shaking and shouted to each other, "Keep trading!" CNN's business correspondent Alison Kosik reported from the floor at 2:20 p.m. E.T.
"Everybody was told to take the stairs," Wolf Blitzer said, speaking live at 2 p.m. E.T. from CNN's D.C. bureau.
The Pentagon has been evacuated, CNN's Barbara Starr reports. "When the building began shaking rather violently, hundreds of people began streaming out," she said, because many people thought that the building was under attack. Starr was standing in the Pentagon's press office when the roof started to shake.
Twitter traffic suggests the quake was felt all over the East Coast.
In Philadelphia, HunterPence3 tweeted, "Wow Earthquake just shook the entire locker room!"
In Cleveland, "tribeinsider" wrote "I'm no expert but i think we just had an earthquake here."
And even in Toronto, Canada, tweets said that the shaking could be felt for minutes.