The quake, which caused no serious damage Friday, had a magnitude of 5.2, which is considered moderately strong, said John Filson, coordinator of the U.S. Geological Survey's earthquake hazards program in Reston, Va.
"But moderate is about as big as you will get" in Pennsylvania, Filson said.
Because the quake was so rare and so strong, the U.S. Geological Survey has dispatched teams of seismologists to the region to measure aftershocks. The survey reported that the largest aftershocks would be most likely to occur within a week and probably would have a magnitude of no more than 4.0.
The survey would never monitor a region in the West so closely after a similar-sized quake, geophysicist Jim Taggart told the Pittsburgh Post-Gazette.
Only 52 earthquakes with a magnitude of 2.0 or greater have been recorded in Pennsylvania in the past two centuries, and most of them occurred in Berks, Lancaster and Lebanon counties. Six were in northwestern Pennsylvania.
Friday's quake hit around 4 p.m. and shook buildings and rattled windows as far away as Buffalo, N.Y. Its epicenter was in West Salem Township, west of Greenville.
Geologists said northwestern Pennsylvania is at the edge of a weak area of the earth's crust that extends from southern Ontario, Canada, to the central Mississippi River valley. Jacobo Bielak, an engineering seismologist at Carnegie Mellon University, noted that oil fields are often associated with earthquake zones, and the production of oil requires tremendous pressure.
Filson said a stronger earthquake is unlikely, but he conceded that seismologists don't know that for sure. "The work we do is like standing outside of a sports stadium listening to the roar of the crowd inside and trying to figure out what game is being played and what the rules are," Filson said.