Centered near the town of Mineral, Va., the quake rocked communities from Georgia to Canada. With an estimated 5.8 magnitude, it marked the most powerful temblor on the East Coast in 114 years and the third-strongest on record.
To quake veterans, it may have been little more than a stiff breeze: buildings did not collapse, roads did not buckle, no serious injuries were reported.
But it was a sense of the unfamiliar that shook so many and sent tens of thousands of people streaming into the afternoon sun.
The Comcast Center dismissed its employees for the day. Phillies executives briefly fled Citizens Bank Park. Thousands of workers filed from government complexes in Doylestown, Media, and Camden. Parts of the White House, Capitol, and Pentagon were evacuated.
In Harrisburg, Trisha Graham, deputy press secretary for Pennsylvania House Majority Leader Mike Turzai, had been sitting in her state Capitol office when she saw the enormous brass chandeliers swaying as if ghosts were riding them.
Then the floor started vibrating.
"It almost felt [as if] you were on a roller coaster sitting down," Graham said.
About 180 miles away, customers at a Wawa in Sea Isle City clung to store shelves to steady themselves. Along the coast, the sand wiggled like Jell-O.
"The lifeguards jumped out of their stands because the stand was shaking," Al Battaglia said as he left a Ventor beach.
Not everyone felt it, or believed.
"We were at the top of the Rocky steps at the Art Museum and didn't feel a thing," said Giovanni Tecce, an executive accounting firm worker who was taking his relatives on a tour of the city.
Wyncote author Charles Fishman said he was at the Philadelphia International Airport, ready to board a flight for Dallas, when the concrete walls and pillars began to shake noticeably. He turned to the flight attendant.
"That was an earthquake," Fishman told her.
"It was?" she said. "I thought they were just moving baggage."
The airport briefly delayed Philadelphia-bound flights that had not yet taken to the air, and SEPTA and Amtrak suspended train service on many lines. Area nuclear plants activated their emergency-response systems but remained at a low level.
The Delaware County emergency center logged 412 calls in the half hour after the quake, nearly six times as many as during the same span a day earlier, said Chad Brooks, operations chief.
Across the region, cellphone traffic surged, stifling some callers for a time. "It's like the highways," said Verizon spokesman Tom Pica. "Only so many people can fit on the highway. The roads are built for a certain capacity."
Initial reports suggested only slight damage.
"I think we're talking about a couple of broken windows," Mayor Nutter said.
A water main burst along the heavily traveled Germantown Pike in Whitemarsh Township. Cherry Hill officials were examining a "structural crack" in the 13-story Towers of Windsor apartment complex. A few municipalities responded to complaints of gas leaks, including one at Gloucester County College.
And Pennsylvania transportation officials pledged a quick review of bridges and roadways statewide.
Repairing the frayed nerves was probably a bit easier.
About 40 minutes after the quake, a local Red Cross spokesman had issued an e-mail checklist about earthquake preparedness. It began: "That was sure exciting, wasn't it?"
By late afternoon, the tension had melted away. Friends and coworkers - those not fortunate enough to be sent home - were sharing anecdotes about the quake, even joking about it.
In Marlton, N.J., Debby Hyland said she and others had been at a luncheon at the Little Mill Country Club after a funeral for her 100-year-old mother-in-law, Myra Hyland, when the chandeliers began shaking.
"It's Myra, trying to talk to us," she quipped.
And at the slots at Bally's in Atlantic City, gamblers, naturally, took the interruption in stride.
"The chairs were shaking," said Tracy Currie, 46, of North Carolina. "We were looking at each other saying, Did you feel that? I got up and looked out the window to see if there was any tsunami. Then we went on playing."
Contact staff writer John P. Martin at 215-854-4774 or at firstname.lastname@example.org.