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.
