Earthquakes far and near

Many local news outlets probably have had stories like the one in today's Lancaster newspaper, which compared the earthquake threat to Haiti (always significant) to that in Lancaster (small, but not zero).  I was asked via email about my own experience with earthquakes. As the article says, the only one I ever personally experienced was in Lancaster in 1984.  Charlie Scharnberger also comments.

Part of the full article is repeated below:

How safe are we?  Though not immune to earthquakes, Lancaster County is relatively secure compared to others areas of the United States.

Sunday News   Feb 07, 2010 00:12 ES

By JON RUTTER, Staff Writer

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 [that's me, above] 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.