For a few minutes from Georgia to Maine, the question rang out: What was that? The answer - a rare East Coast earthquake, magnitude 5.8 - was far down on the list for most not used to the earth shaking beneath them.
In Washington and New York, their nerves still raw, thoughts instantly turned to terrorism. In small towns and rural areas near the epicenter and elsewhere, guesses ran the gamut: A truck crash or train derailment. A plane breaking the sound barrier. Worse, a nuclear reactor exploding.
There ended up being no known deaths or serious injuries, but cracks appeared in the Washington Monument and the National Cathedral, which had three capstones break off its tower. Windows shattered and grocery stores were wrecked in Virginia, where the quake was centered. The White House and Capitol were evacuated.
With the 10th anniversary of the Sept. 11 terrorist attacks just weeks away, people were shaken physically and emotionally and poured from high-rises like the Empire State Building in New York, sick with mental images of planes and bombs.
"I ran down all 60 flights," accounting office worker Caitlin Trupiano said. "I wasn't waiting for the elevator."
Chris Kardian, working in his garage in suburban Richmond, Va., not far from the epicenter, opted for the more prosaic and plausible: He blamed the shaking on two of his children in the overhead playroom.
"I just thought they were running around and being really loud," he said. "After about 15 seconds, it didn't stop and I thought, `I don't have that many kids in the house!"'
Most needed just minutes to discover what it really was: A temblor the U.S. Geological Survey said was centered 40 miles northwest of Richmond, traveling 3.7 miles beneath the earth's surface and momentarily jarring as many as 12 million people.
The most powerful earthquake to strike the East Coast in 67 years shook buildings and rattled nerves. There were no reports of deaths, but fire officials in Washington said there were at least some injuries.
The U.S. Park Service evacuated and closed all monuments and memorials along the National Mall. The Pentagon, the White House, the Capitol and federal agencies in and around Washington were evacuated. Roads out of the city were clogged with commuters headed home.
Stressed-out D.C. mother of four Marion Babcock, who spent two hours traffic instead of her normal 25 minutes, did the only sensible thing for her frazzled, frightened kids: "I treated their post-traumatic stress with copious amounts of chocolate mint and cookie dough ice cream."
Between cell phones and social networks, news of the quake seemed to travel faster than the temblor itself.
Jenna Scanlon of Floral Park, N.Y., ended a phone call with someone in McLean, Va., and announced to her office colleagues there had been an earthquake. Seconds later, 7 World Trade Center began to shake.
The scope of the damage - or lack of - also quickly became clear on social networks. Instead of collapsed freeways, people posted images of toppled lawn chairs and yogurt cups, broken Bobbleheads, picture frames askew on walls.
On Facebook, people joked with posts such as "S&P has downgraded earthquake to a 2.0," a swipe at the rating agency that recently lowered the federal government's creditworthiness. Another suggested New Jersey Gov. Chris Christie, a large man, had just "jumped into" the presidential race.
A 5.8-magnitude quake releases as much energy as almost eight kilotons of TNT, about half the power of the atomic bomb dropped on Hiroshima, Japan, during World War II.
Still, those along the West Coast who are used to the earth moving couldn't help but take a jab or two.
"Really all this excitement over a 5.8 quake??? Come on East Coast, we have those for breakfast out here!!!!" wrote Dennis Miller, a lifelong California resident whose Pleasanton home sits on a fault line.
A 5.8, he said, wouldn't even wake him from his sleep.
"We were laughing," said 26-year-old San Francisco resident Stellamarie Hall, "but we definitely understand that New York and certain metropolitan areas are not designed around earthquakes."
The earthquake that devastated Japan released more than 60,000 times more energy than Tuesday's, but there was real damage. At the majestic Washington National Cathedral, at least three of the four top stones on the central tower fell off, and cracks appeared in the flying buttresses at the cathedral's east end, the oldest part of the structure. The top of the Washington Monument has a crack.
Ceiling tiles fell to the floor at Reagan National Airport. The gothic-style Smithsonian Castle, built in 1857, had minor cracks and broken glass. And vigorous shaking left a crack and hole in the ceiling at historic Union Station when a chunk of plaster fell near the main entrance.
The steeple and bell tower at St. Patrick Catholic Church in Baltimore were badly damaged, and the building was closed as a precaution.
In Reading, Pa., highway inspectors shut down a bridge after spotting cracks in an abutment, but later said they might have been there before.
In West Virginia, environmental regulators sent engineers to inspect massive coal slurry dams that could wipe out entire communities if they were to fail and release billions of gallons of wastewater.
Amtrak said trains along the Northeast Corridor between Baltimore and Washington were operating at reduced speeds and crews were inspecting stations and railroad infrastructure before returning to normal.
Earryl Reevey, who has had three heart attacks and was traveling home for his brother's funeral, was visibly shaken after getting off a train from Baltimore at Penn Station in New York.
"Suddenly, I felt like somebody grabbed me and was shaking me like a baby, for about three seconds," recalled the man from Long Branch, N.J. "I heard some people scream and I thought we were going to derail. And then, everything went normal."
Even those who knew what was happening had braced for worse, some remembering the Indian Ocean quake that triggered a tsunami and a nuclear disaster in Japan.
"I knew it was an earthquake, but my first thought was, `Oh my God, something's going to happen to the power plant," said 21-year-old Whitney Thacker in Mineral, Va., a town near the epicenter where the sidewalks were littered with fallen stones, masonry and broken glass. "It was scary."
Dominion Virginia Power shut down its two-reactor nuclear power plant within 10 miles of the quake's epicenter, but said there was no evidence of any damage to the decades-old North Anna Power Station.
In a news release, the utility company said off-site power to the nuclear plant was restored Tuesday night and it was no longer relying on backup generators. The utility didn't say when the plant's two reactors would be restarted.
By the standards of the West Coast, where earthquakes are much more common, the Virginia quake was mild. Since 1900, there have been 40 of magnitude 5.8 or greater in California alone. Forty-three have been of magnitude 6 of greater.
But quakes in the East tend to be felt across a much broader are, the waves traveling "pretty happily out for miles," said U.S. Geological Survey seismologist Susan Hough.
More than 12 million people live close enough to the quake's epicenter to have felt shaking. They were still feeling minor aftershocks up to 4.8 in strength.
The last quake of equal power to strike the East Coast was in New York in 1944. The largest East Coast quake on record was a 7.3 that hit South Carolina in 1886.
The fear in some places was real.
Michael Leman had been mowing a neighbor's lawn in Mineral when bricks began falling from a chimney and the earth heaved a large propane tank about a foot off the ground.
"I thought that tank was about to explode," he said, "and I ran for dear life."
This program aired on August 24, 2011. The audio for this program is not available.