You're sitting at your desk, or in your kitchen, and all of a sudden the building begins to wobble. To some people, the 5.8-magnitude earthquake in central Virginia Tuesday felt like a gentle rolling. To others, closer to the epicenter, it was more violent.
And then it was over. People from New England to the Carolinas were left asking, "What was that?"
If you live in California or southern Alaska, you probably know what to do in an earthquake. But Easterners don't often feel tremors, and may not know how to react in a major emergency. The Northridge earthquake in Los Angeles in 1994 had a magnitude of 6.7, and at least 33 people died.
Here are some pointers from FEMA, the Federal Emergency Management Agency if you are caught in a major earthquake:
If You're Indoors:
Drop to the ground and take cover. Get under a sturdy table or other piece of furniture, and hold on until the shaking stops.
If you can't get under something, cover your face and head with your hands and crouch in an inside corner.
If you're in bed, stay there. Cover your head with a pillow.
Doorways are not great places for shelter, even though emergency managers used to recommend them. They're a goood option if debris is falling and you can't get anywhere else.
Stay put until the shaking stops. FEMA says most injuries occur when people try to move to another place.
Don't use elevators if you've been in a major quake. And don't be surprised if power goes out or sprinklers are activated.
If You're Outdoors
Stay there. Stay away from buildings, power lines, streetlights and other things that could fall on you.
People are rarely injured by the actual shaking of an earthquake. Instead, falling debris is the greater danger.
If you're in a car, try to ease to a stop, preferably in an open area away from buildings, trees or overpasses.
Major Eastern Earthquakes Rare
Most people who felt Tuesday's earthquake did not need such advice; the quake was not violent enough. But brick and masonry buildings did sustain damage -- more than one would see in California with its stricter building codes. And the quake was a reminder than even in the East, even between fault lines, there can be risks.
The Cape Ann, Mass., earthquake of Nov. 18, 1755, felt from Halifax, Nova Scotia, south to the Chesapeake Bay, violently shook Cape Ann and Boston, where much of the damage occurred in landfill around the city wharfs,. The quake, estimated to have had a magnitude of 6.0 to 6.3, knocked down chimneys and stone fences. People aboard a ship 200 miles off the coast of Cape Ann felt the quake and feared they had run aground.
A magnitude 5.4 earthquake struck western Ohio on March 9, 1937, cracking a schoolhouse in Anna, Ohio, breaking chimneys and walls, reducing the output of oil and gas wells and creating new springs where old springs had run dry. The quake could be felt as far away as tall buildings in Chicago and Milwaukee and in Toronto. It also was felt in Kentucky, Michigan, Missouri, Pennsylvania and West Virginia.
Many people on the Eastern Seaboard on Tuesday admitted they were freaked out by the surprise earthquake. But many took it in good humor.
"My wife and I were married 36 years ago at the same hour, even to the minute, of the earthquake," said Dr. John Messmer, a professor at Penn State College of Medicine. "I told her our love made the earth move once again in honor of our anniversary."
ABC News' Jane E. Allen and Roger Sergel contributed information for this story.