There are no immediate reports of any serious injuries.
Reports of damage were minor.
The tremor cracked a window at the Blue Cross Building at 19th and Market Streets. In Cherry Hill, an official said a "structural crack" was visible on the 13-story Towers of Windsor apartment complex.
A gas leak was reported on Craig Drive in Deptford, prompting the evacuation of some residents.
The quake triggered an early evening rush hour and police mobilized for an exodus by commuters from Center City.
The Federal Building and U.S. Courthouse at 6th and Market Streets and city's Criminal Justice Center closed ahead of schedule.
The Comcast tower, the city's tallest skyscraper, reported no damage but sent its workers home early as a precaution.
Just before 3 p.m., Mayor Nutter reported that all city buildings appeared safe and ordered city workers back to their work locations.
"We got an assessment through department heads and commissioners, all their facilities, everything appears to be fine," Nutter said. "We are contacting private building owners throughout Philadelphia, they need to check their buildings with their own engineers." He said city inspectors from Licenses and Inspections would be available to help any owners with concerns.
The mayor said there had been some reports of people smelling gas in a couple of locations, but the situations appeared to be minor and it was unclear if they had anything to do with the earthquake.
None of the city's roads, highways or bridges appeared to have been affected by the quake, Cutler said.
At 3:45 p.m., SEPTA placed speed restrictions of 30 mph on the Broad Street Line, Market Frankford Line and trolley lines until visual or on-foot inspections of all track, tunnels, bridges and overpasses were made. Service resumed at 4:15. Regional Rail trains are operating at normal speeds, but passengers should expect delays of at least 60 minutes throughout the evening rush. Shuttle buses are being used on the Broad Street Line between Walnut-Locust and AT&T Stations while track inspections take place.
Rina Cutler, the deputy mayor responsible for transportation and utilities, said the Broad Street Subway Line was closed south of Walnut-Locust because inspectors had noticed a crack in the ceiling near Broad and Snyder Streets. It was unclear whether the crack resulted from the earthquake, she said, service south of Walnut Street resumed after 4 p.m.
The PATCO High Speed Line suspended all service for about two hours.
Philadelphia International Airport instituted a "ground stop" under which flights to Philadelphia were held at their departing cities until crews inspected the runways. Similar ground stops were reported up and down the East Coast until flights were allowed to resume around 3 p.m.
PECO reported no problems.
Citizens Bank Park was evacuated, but members from both the Phillies and Mets teams remained inside. The game will be played as a scheduled, the Phillies announced.
The U.S. Geological Survey says the temblor measured an estimated 5.9 on the Richter scale and was centered 34 miles northwest of Richmond, Va.
The shaking was felt from North Carolina to Ohio and New York City and even Martha's Vineyard, where President Obama and his family are vacationing.
It was probably the strongest quake in central Virginia since a 4.8 temblor in 1875.
Two nuclear reactors near the epicenter were taken off line, officials said.
All the nuclear plants in this region continue to operate but a number have declared what are called "unusual events" due to seismic activity. An "unusual event" is the lowest of the emergency declarations, according to Diane Screnci of the Nuclear Regulatory Commission.
"Our chairs were shaking, trembling," said Arlene Henry, who works in the city human resources department on the 15th floor of the Municipal Services Building. "I thought it was just me. Then I thought, I've got to get out of here."
"I was scared. I left my cellphone and everything," said Charles Gilmore, who works in the city Finance Department on the 13th floor of the building and walked down the stairs.
Zack Stalberg, president of the Committee of Seventy government watchdog group, was heading up to his office on the 10th floor of 8 Penn Center when the elevator car he was in began shaking.
When the doors opened on his floor, before he could exit, frightened people began running into the car to get out of the building.
"That's was my first clue that something unusual was happening," Stalberg said.
Wyncote author Charles Fishman was at the Philadelphia International Airport, ready to board a flight for Dallas, when the concrete walls and pillars began to shake noticeably.
"It took my brain a moment to realize," he said. But as soon as it did, and he began to think he shouldn't be sitting where he was, the shaking stopped. "It couldn't have lasted more than two to three seconds," he said.
He turned to the flight attendant and said, "That was an earthquake."
She answered, "It was? I thought they were just moving baggage."
No one panicked, but everyone started talking. "People had that wide-eyed, surprised look," he said.
Virginia Wilks, a resident of the Richard Allen Homes in North Philadelphia, said she thought her cat was clawing at her blankets from under the bed.
"Then I saw the TV shaking," said a rattled Wilks.
At the Jersey Shore, the sand "felt like jello" said Al Battaglia, coming off the beach at Dorset Ave in Ventnor.
"The lifeguards jumped out of their stands because the stand was shaking," he said. "We thought out son snuck up behind me and was shaking our chairs."
Chris Megella, 9, of Spring City, was in the surf when he felt the sand moving beneath him. "I fell on my knees," he said.
On the Florida Avenue beach in Atlantic City, Amber Cobb, 15, of Mullica, was awakened from a nap. She had the same sensation voiced by so many: Is this just me? "I felt like I was going crazy," she said. "Everybody looked up, it was like, oh my God. Then the text messages started."
In Atlantic City, casino guests on high floors said they felt the buildings sway. "We were looking out the window and the floor was moving," said Janice Ceriello of Glen Cove, N.Y., who had just checked in to the 32d floor of Caesars. "I felt seasick.
"Then we went to lunch."
At the slots at Bally's, gamblers took it 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."
Hundreds of state employees were evacuated from the state Capitol in Harrisburg when the earthquake struck, shaking windows and floors in the century-old building and causing papers to fall off shelves in some offices.
Among the evacuees was Auditor General Jack Wagner who had scheduled 2 p.m. news conference in his second floor office to announce an immediate audit of Philadelphia school chief Arlene Ackerman's contract buyout deal.
"I was sitting in my office and sensed the walls moving and then the table where I was sitting started to vibrate and I realized something serous was happening," said Wagner, standing in a park outside the Capitol complex looking at the 90-year-old state finance building. "It tells us how potentially vulnerable we are."
The heavy, ornate brass chandeliers in House Majority leader's office Mike Turzai's office just off the Capitol Rotunda started swayed as if someone was swinging from them, witnesses said.
"You could feel vibrations on floor and the chandelier is big they don't move on their own," said Trisha Graham, deputy press secretary. "At first I didn't know what do expect I'd never felt one before but when vibrations subsided it was apparent it was an earthquake. It almost felt you were rollercoaster sitting down."
Workers were later allowed to return to the Capitol building.
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