Shirley Key of York was lying in bed a little after midnight Saturday, watching TV, when she heard the noise. At first she thought it was a train. Then a truck. Then . . . she was out of ideas for a moment.

The low-pitched rumbling steadily increased in volume, seeming to come from everywhere at once. The bedroom shook. The bed shook.

"It scared me," she said.

She woke her husband up. He hadn't heard anything. She called a friend, who hadn't heard anything either.

A couple of hours later, her friend woke Key up with another phone call, just to let her know she wasn't crazy. The news was reporting an earthquake that originated in western Lancaster County and shook things up in York County before subsiding.

Jeri Jones of Jones Geological Services said Saturday morning's earthquake registered a 3.4 on the Richter scale - strong enough to be noticed, but not enough to do any real damage.

"Some cracked plaster might be the extreme," Jones said.

Jones has been looking into a "swarm" of earthquakes around Dillsburg, ongoing since October. He can't say whether there's any connection between those and the one in Lancaster County.

Major quakes unlikely

Jones also can't say for sure, because it's impossible to do so, whether any future earthquakes in the area will pose a danger or cause serious damage. He can say that, based on the East Coast's geologic history, it's very unlikely.

Jones said Lancaster County and Dillsburg are in two distinct geologic regions, based on the age and type of rock that occurs there. In 1997, Jones said, earthquakes occurred in both of those regions within the same year. He can't say whether that's a coincidence or a sign of some correlation.

Charles Scharnberger, a retired Millersville University earth sciences professor, has been collaborating with Jones on the Dillsburg earthquakes.

Scharnberger said this area is on the western edge of a Pennsylvania region prone to such small earthquakes.

Why is that?

Scharnberger and Jones are waiting for results from Columbia University in New York, which placed portable seismographs in the area to monitor the activity, before making a more definitive statement about the likely cause.

Scharnberger does have a theory, though.

Moving plates

Geologists posit that everything from undetectable subterranean rumbles to volcanic explosions result from the movement of tectonic plates - the vast bases of rock that underlie the entire earth's surface and float about freely on the planet's molten core.

The plate that contains North America, which has an eastern edge somewhere in the middle of the Atlantic Ocean, is moving westward by a few centimeters each year.

This area doesn't appear to have any of the major fault lines, which are fractures in the bedrock that cause buildings to topple in other regions of the world. But it does have underlying deposits of diabase, a form of hard igneous rock, pressed up against relatively softer stretches of sedimentary rock.

As the tectonic plate moves, it places pressure on the underground deposits of rock. The softer sedimentary rock would be more likely to simply absorb that force, whereas the harder, and thus more brittle, diabase might simply burst from the pressure.

Scharnberger said he needs to check for deposits of diabase near the epicenter of the Lancaster County earthquake. He suspects that bursting underground rock is what caused it, as well as the earthquakes around Dillsburg.

Whatever the cause, Anna Weirich of Wrightsville is just relieved that it wasn't the nearby County Line Quarry, where her husband works.

That was her first thought when she was walking up her basement stairs and heard the rumbling. It sounded like blasting at the quarry, but it shouldn't be happening after midnight.

Could it be an accident at the quarry? Did a neighbor's gas line burst and blow up a house? She was relieved when she called the York Daily Record newsroom, and found out about the earthquake.

"We were kind of up in arms," she said. "We didn't know what happened."

READ MORE

· U.S. Geological Survey report on the quake
· Talk about it on The Exchange

MAP

The U.S. Geological Survey is collect reports through its Web site from those who felt the quake. Here is a map showing those reports and the intensity at which the quake was felt. To submit your report, go here and click "Did you feel it? Tell us."

FEEL IT?

Jeri Jones of Jones Geological Services said anyone who wants to report what they felt during the earthquake can send him an e-mail at jonesgeo@hughes.net.

FELT IT

Places where people reported feeling the earthquake as of Saturday afternoon, according to the United States Geological Survey:

  • Airville: 2 people reported; 33 miles away

  • Brogue: 11 people reported; 27 miles away

  • Carlisle: 1 person reported; 70 miles away

  • Centreville: 1 person reported; 170 miles away

  • Columbia: 69 people reported; 9 miles away

  • Dallastown: 17 people reported; 33 miles away

  • Delta: 4 people reported; 40 miles away

  • Felton: 10 people reported; 31 miles away

  • Glen Rock: 11 people reported; 47 miles away

  • Hanover: 2 people reported; 61 miles away

  • Peach Bottom: 6 people reported; 43 miles away

  • Philadelphia: 2 people reported; about 100 miles away

  • Red Lion: 31 people reported; 28 miles away

  • Seven Valleys: 5 people reported; 41 miles away

  • Shrewsbury: 5 people reported; 45 miles away

  • Spring Grove: 2 people reported; 49 miles away

  • Windsor: 16 people reported; 23 miles away

  • Wrightsville: 15 people reported; 17 miles away

  • York area: 108 people reported; about 30 miles away

    DON'T CALL 911

    Several York County residents said they called 911 after the earthquake to find out what had happened.

    Brian Morrin, pubic information officer for York County emergency services, said York County 911 got a handful of calls immediately afterward, but they weren't swamped.

    Regardless, Morrin said 911 should be reserved only for emergencies - such as somebody injured or a potentially life-threatening situation.

    If there's a widespread event such as a severe storm, Morrin said, county emergency officials are most likely aware of it.

    A tree blown down on power lines, for example, would merit a call to 911. But a tree blown down in a yard with nobody injured wouldn't constitute an emergency.

    PAST QUAKES

    Some of the earthquakes that have been felt by York County, according to past newspaper reports:

    Dec. 27, 2008: A magnitude-3.4 earthquake hit in Lancaster County, and residents as far away as York city reported feeling it.

    October 2008: Two sizable earthquakes - magnitudes 2 and 2.7 - were felt in Carroll Township. Since then, residents have reported feeling many smaller earthquakes - a "swarm," it is called - and scientists placed portable seismographs around the area.

    2001: A small, a magnitude-1.5 quake rippled across the York Haven area

    1997: A magnitude-2.4 earthquake hit the Dillsburg area.

    April 22, 1984: On Easter Sunday, York County felt an earthquake with a magnitude of 4.1 and an epicenter reported to be in Lancaster County.

    1888: A large, magnitude-5 earthquake hit York County.