Shirley Riffle was celebrating her daughter's marriage in Reading when the walls of the reception hall started to flutter. The crystal lighting fixtures began to tinkle.

People looked up from their dinner, Riffle recalled. "We thought 'Oh my God.' "

An earthquake had been the remotest thing from their minds. But Riffle had good cause for alarm.

When she returned to her split-level in Spring Township, Berks County, she saw that half the chimney had toppled and a first-floor window had blown out. Some of her family's possessions, including a valuable Canterbury pottery collection, had been destroyed.

She's still paying off the roughly $30,000 in damage, she said. "We have cracks all over the walls to this day."

Riffle was singularly unlucky that bitterly cold weekend back in 1994. The epicenter of the 4.7 magnitude temblor that also buckled roads and pried open sinkholes happened to be under her house.

Last month's magnitude 7 quake in the Caribbean stirred painful memories for Riffle, who said her trauma was nothing compared to the massive death and destruction suffered by Haitians.

"I'm very lucky," said the mother of four. "I'm over it. I'm past it."

But 16 years is a geologic eyeblink.

Could a damaging earthquake again strike close to home? How secure is this place, anyway?

Even more of a mystery
Pretty secure, scientists say. But not entirely so.

There's simply too little known about the nether regions of the earth to rule out the big one.

Charles Scharnberger, a geophysicist and Millersville University professor emeritus, notes that the Spring Township quake was the strongest ever recorded in the immediate area.

He figures the odds of a local magnitude 6 quake happening in a given year to be one in 1,000.

By comparison, places like Haiti or California, which are perched on the slowly grinding junctions of tectonic plates, are at much greater risk of periodic, calamitous quakes.

The eastern United States sits squarely on the North American plate, which stretches about 2,000 miles seaward to the Mid-Atlantic Ridge.

Plate interiors are relatively placid geological zones, said Rob Sternberg, a Franklin & Marshall College geosciences professor who posts quake information on shakingearth.blogspot.com.

On the other hand, many midplate quakes occur worldwide every year. Most are small. But some, even in the eastern U.S., have been severe.

Foremost in the lore is a trio of 1811-12 quakes in the Mississippi Valley, the strongest and noisiest of which was said to have shot sulfurous fumes into the air, awakened sleepers in Washington, D.C., and opened up fissures that swallowed seven Indians.

Reported one newspaper of the day: "The earth was so convulsed, as to tender it difficult for one to keep their perpendicular position."

An 1886 temblor estimated to have equaled 7.5 on the then-nonexistent Richter Magnitude Scale heavily damaged Charleston, S.C., and clanged bells in Boston, according to Scharnberger.

Other quake-prone regions in the East include the St. Lawrence River Valley and coastal New England.

Powerful shocks rocked eastern Massachusetts in 1658, 1727, 1755 and 1925, according to Scharnberger.

Should similar events unfold in those regions in the future, Scharnberger said, they'd likely be too distant to have much effect on Lancaster.

On the other hand, Pennsylvania occasionally uncorks quakes of its own.

The strongest of these, magnitude 5.2, rocked the Erie area Sept. 25, 1998.

And the hottest quake zone in the state is right here in southeastern Pennsylvania.

Movements within the Lancaster seismic zone, angling southwest-northeast from the Susquehanna River to Reading, roughed up Riffle's house and spawned Lancaster's infamous Easter Sunday quake of 1984.

Sternberg said he experienced the holiday disruption as a rumbling sound in his house on Pine Street.

"My wife thought it was a boiler explosion or perhaps an accident involving a truck," the professor recalled in an e-mail.

"I was proud of myself for instantly recognizing it as an earthquake, but only because a smaller one had occurred in Lancaster the previous week."

Since 2008, a so-called "swarm" of mini-earthquakes has shivered through the Dillsburg, York County, landscape.

The phenomenon is "even more of a mystery" than the more isolated quakes, Scharnberger said, because swarms are often associated with magma and volcanic activity.

"We don't expect any volcanoes to erupt in Dillsburg," Scharnberger added reassuringly.

Nobody has ever been able to put a finger on exactly why seismic stuff takes place here.

The 200-million-year-old fault lines corresponding to the breakup of the supercontinent, Pangaea, appear to be inactive, according to Won-Young Kim, who tracks Eastern earthquakes at the Lamont-Doherty Earth Observatory at Columbia University.

However, scientists note, tectonic plates continue to shuffle around over the globe. The spreading out of the Mid-Atlantic Ridge exerts east-west stress even in this relatively quiet geologic sector.

Local quakes seem to be linked to areas where hard volcanic rock meets weaker sedimentary layers, Kim said.

The temblors also originate near the ground's surface, thus heaving it more violently than would a deep-earth tremor.

On the plus side in Pennsylvania, according to Kim, "you are sitting on rock," which absorbs seismic waves better than the silty, less stable soils underlying, for example, Long Island and the Chesapeake Bay.

Scientists predict a 100 percent chance of future earthquakes in the county.

"The problem is we cannot give a forecast of when," Kim said.

Geologists continue to scrutinize the subterranean world, a process Kim said intensified in the 1970s when the Susquehanna River corridor was targeted by nuclear power plant builders.

Seismographs at MU and F&M are part of a regional earthquake detection network system based at Lamont-Doherty; Kim said more installations are planned for central Pennsylvania.

Today, he added, many twitches of the needle register manmade activity, such as mining blasts, which are "usually very well timed" to take place just before lunch.

But the next temblor lurks beneath the horizon.

Riffle long ago bought earthquake insurance, just in case.

Scharnberger does not carry it, he said, because the deductibles are high and the chances of a quake causing tens of thousands of dollars of damage to his home are slim.

But those chances are somewhat greater than winning the lottery, he added.

"We don't know."