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Quake's strength surprises scientists

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August 24, 2011

How noteworthy was Tuesday's quake across Virginia, Maryland, Delaware and much of the American East Coast?

Enough to raise the eyebrows of an earth scientist in central Pennsylvania whose work has taken him onto some of the world's shakiest real estate.

"This was probably the strongest earthquake I've ever been in, and my research has taken me to active volcanoes," said Peter LaFemina, an assistant professor of geosciences in State College, Pa. "We felt it very strongly here, most likely because we're on very stable rock in this region."

It was that same usually steady, deeper rock that helped to send shaking from the quake near Mineral, Va., across Delaware and the rest of the Northeast on Tuesday, researchers said.

Although most of sandy-soiled Delaware is considered to be in a medium hazard area for earthquakes, the entire mid-Atlantic sits atop ancient bedrock crisscrossed by cracks and faults that periodically let go with a bang.

Just that happened Tuesday about 3.75 miles below the surface near Mineral, in an area known as the "Central Virginia seismic zone," a place where few clues are available about the number or location of cracks and weaknesses that can spawn earthquakes.

"Where harder rocks are deep down, those energies can be transmitted up and outward," when something moves, said Peter P. McLaughlin Jr., Delaware's state geologist and interim director of the Delaware Geological Survey. "That's why the shaking was felt as far north as Canada and as far south maybe as Georgia.

The quake itself, McLaughlin said, might have been the release of strains created 150 million or more years ago, during the most recent splitting of the continents and the re-opening of what is now the Atlantic Ocean, at a time when the Appalachian Mountains rose up.

Stefanie Baxter, a geologist at the Delaware Geological Survey, said the effects of Tuesday's tremor were not the most intense in state history. But it might have been the first on record to be felt throughout the state.

Although the magnitude can be measured only at the epicenter, Baxter said the effects reported in Delaware would be comparable to the intensity of a quake of about 2.0 near its epicenter. That might not sound like much, but for the First State, it's quite high, she said. The Geological Survey's tectonic instruments on the University of Delaware campus were designed to measure much smaller geological events. Tuesday's readings sent the seismographs measuring activity at Bellevue and Brandywine Creek state parks spiking off the charts.

Unlike the active earthquake zones of the Pacific, where global plates are grinding alongside, over or under one another, the bedrock sheets of the East are far from their nearest continental neighbor, at the mid-Atlantic Ocean ridge. The land under the eastern United States is nevertheless crisscrossed by deep faults, including the Piedmont area below Mineral, Va., and northern New Jersey.

And from time to time, those faulted areas move and shake.

"There's a fair amount of historic activity in that area, but not this big," McLaughlin said. "This is the biggest that relates to those faults running through there."

At Bell, Book & Candle, a collectible and gift shop in a 150-year-old building on Dover's Loockerman Street, manager Darleen Aragorn needed no convincing.

Aragorn, who previously lived in California, rated Tuesday's event as "a 3 on a scale of 1 to 10," but said it was still alarming.

"I was standing at the counter looking at my customer, and all of a sudden, she was swaying one way, and I was swaying another," Aragorn recalled. "Then all the wind chimes in the store started ringing, all at the same time."

Neither the store nor the building was damaged, Aragorn said, but a phone call quickly showed how far the trouble went.

"I called my daughter in New York, and she said she felt it," Aragorn said. "She said she could see her stove moving."

Thomas Wilson, a professor of geophysics in West Virginia University's Department of Geology and Geography, described the east as a "passive continental margin," mostly stable but with countless potential problems set up by the shifting of the earth's surface hundreds of millions of years ago.

"It's kind of like a bell ringing," Wilson said of Tuesday's wide-ranging vibrations and rattling. "Things that are nice and hard tend to ring a little bit better than things that are really soft, like a lot of the sediment that we have in the near-surface."

U.S. Geological Survey records show that the Mineral quake rivaled a shaker recorded in 1897, in the south-central part of the same state. Loud rumblings were heard, and small quakes were felt for weeks before the big one on May 31, 1897.

Reports from the time show that the shock from that earthquake spread across an area similar to the one reported Tuesday: from Georgia to Pennsylvania and beyond, and from the Atlantic to Kentucky.

The Delaware Geological Survey has reported that 69 quakes occurred under the state since 1871, including an estimated 4.1 shaking in that year that caused serious damage in Wilmington. Some 550 are believed to have occurred within 150 miles of the state.

The largest recorded quake in Delaware, in 1973, was measured at 3.8 and was centered in the Delaware River near Claymont.

Baxter said the East Coast could experience additional tremors in the days ahead, all of them smaller than the initial one. But they will likely be imperceptible to most people in Delaware, she said.

"I would not be surprised at all if we had aftershocks," she said. "In fact, I would be surprised if we didn't have rumbling in the next couple of days or even the next couple of weeks."

Reference: 

http://www.delawareonline.com/article/20110824/NEWS08/108240326/1006
Published in The News Journal: August 24, 2011

For questions and information, contact DGS at
delgeosurvey@udel.edu, 302-831-2833
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