Mention major earthquake zones in North America, and San Francisco, Anchorage and Mexico City likely come to mind.

How about Salunga, Pa.?

Not so much.

But residents of the Salunga-Landisville area in northwestern Lancaster County were jolted by a fairly significant (by East Coast standards) earthquake early last Saturday.

With a magnitude of 3.3, the quake was among the most powerful in Pennsylvania in recent memory.

Then, on New Year's Eve, a 2.2 quake struck near Dillsburg, York County, about 42 miles west of Salunga. It was the third earthquake in that area since October, and residents there have been rattled by dozens of smaller aftershocks.

Scientists are uncertain if the York and Lancaster County earthquakes are related, but agree that the Lancaster region, including York and Berks Counties, is an earthquake "hot spot" of sorts.

"Lancaster County is an active earthquake area," said Kevin Furlong, a professor of geosciences at Pennsylvania State University. "We've had a bunch of 2.5 to 3.0 quakes in the last 50 years. The Lancaster quake is on the larger end of what we typically have."

Earthquakes that register in the 3-to-4-magnitude range are hardly powerful by world standards. There may be some fairly vigorous shaking and cracks in the 4-magnitude range, but scientists say no real damage occurs until a quake reaches magnitude 5.

The biggest earthquake in modern history in Pennsylvania hit in the northwestern corner of the state, near Erie, in 1998, registering 5.2 in magnitude. It caused some damage such as fallen chimneys and was felt as far away as Illinois and New Jersey.

At 3.3, the Lancaster quake was large enough to be felt 48 miles west, in Carlisle, and in parts of Chester County to the east. While most residents of the surrounding area would have likely felt it had it occurred during the day, the fact that it hit just after midnight meant many slept right through it.

But not Lancaster resident Amy Gaston, who was awake at home, seven miles from the epicenter, when the quake struck.

Gaston was in bed reading when she heard a rumble, felt a vibration and looked up to see her windows shaking.

"I thought it was a truck coming down the road or an airplane flying low," said Gaston, an administrative assistant for the Historic Preservation Trust of Lancaster. "I made sure to look at the clock - it was 12:06. It was there and gone."

She learned later that day it was an earthquake. "I thought, 'I'm not crazy, I just experienced my first earthquake,' " Gaston said.

At the Three Mile Island nuclear plant, 20 miles west of Salunga, crews reported feeling nothing during the quake, but did conduct a precautionary walk-through and found nothing amiss, said spokesman Ralph DeSantis, adding that the plant was built to withstand natural disasters such as earthquakes and tornadoes.

Scientists are unsure why a sudden swarm of quakes is occurring in York County or why a temblor occurred about the same time in Lancaster - that county's first quake since 2000.

While the major earthquake activity occurs where tectonic plates meet - as they do on the West Coast - the intraplate activity in the East is much harder to explain, scientists say.

"People want to know where the fault line is," said Charles Scharnberger, a professor emeritus of earth sciences at Millersville University who set up a seismic center in Lancaster County in 1973, a year after a 3-magnitude earthquake struck near Lititz. "But there's no surface evidence, no rupture."

Scientists say little faults, some as small as the room in a house, run below Southeastern Pennsylvania.

"The so-called East Coast earthquakes are problematic because they don't fit into the plate-tectonics view," said Furlong. "The nearest plate boundary is in the middle of the Atlantic Ocean."

He added, "There are various theories, such as the plate under North America is under stress, squeezing existing fractures."

Earthquakes have been experienced by Philadelphia residents over the last century - a New Jersey-centered quake in 1968 shook the tollbooths at the Ben Franklin and Walt Whitman Bridges - but the epicenters are difficult to pinpoint.

Scharnberger, who is retired but still operates the seismic center at Millersville, is hoping the recent flurry of geological activity in Southeastern Pennsylvania will draw professorial candidates - and perhaps research dollars - to the geology department at the university.

"You could count on two hands the number of seismologists in the eastern United States," said Scharnberger, adding that most research money is directed to areas where big quakes occur. "I'd like to see someone come in at Millersville who is interested in seismic study, who'd like to take over for me, and maybe there would be a more active research program."