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AN EARTH-SHAKING DAY

In region, small temblors common, larger ones rare

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By Carolyn Y. Johnson
Globe Staff / August 24, 2011

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Even though yesterday’s 5.8-magnitude earthquake was centered 470 miles away in Virginia, the ground in Boston probably moved a “pretty good fraction of an inch,’’ according to the initial rough estimate of John Ebel, director of the Weston Observatory at Boston College.

The extent of shaking people experienced varied, however, depending on factors such as the type of building they were in and the composition of the earth beneath their feet.

“For me, the earthquake lasted zero seconds; for someone down the road, it might have lasted 15, 20, 30 seconds,’’ said Ebel, who did not feel the quake. “If you’re in a building that responds a bit more to earthquake shaking, you’ll feel the shake a little bit longer. . . . Two people not far from each other can give you very different reports about how strong the earthquake was.’’

It was the East Coast’s most powerful earthquake in 67 years. Because of the solidity of the earth’s crust on the eastern seaboard - unlike California, where there are networks of active faults - earthquakes tend to be felt over longer distances here, according to Thomas Herring, a geophysics professor at MIT.

Geophysicists said the earthquake was unusually strong for the area, but not unprecedented. In 1897, an earthquake estimated at 5.9 magnitude shook Giles County, Va., according to the US Geological Survey. Eight years ago, a 4.5-magnitude earthquake was recorded in that state.

“Although it’s surprising to see one this large, it’s not completely unheard of to have a large earthquake in a relatively quiet area,’’ said John Bellini, a geophysicist with the National Earthquake Information Center, run by the Geological Survey in Golden, Colo.

Ebel said that many places in the eastern United States that are not typically thought of as earthquake zones have the potential to experience rare, larger quakes. The last major temblor in the Boston area occurred in 1755, when a 6.2-magnitude quake struck off Cape Ann. But smaller quakes occur regularly, and it is far from the first time people in the Bay State have felt the earth shake. For example, in June 2010, a 5.0-magnitude quake on the Ontario-Quebec border was felt in the Greater Boston area.

Even more powerful quakes may have occurred in the Northeast. In a paper published this summer, Ebel argued that a 1663 earthquake in the Charlevoix region of Quebec that caused chimneys to tumble in Roxbury was a 7.5-magnitude quake. In 1925, a 6.2-magnitude quake in the same region was also felt in the Boston area, Ebel said.

“That would have caused somewhat stronger ground shaking’’ in Boston than yesterday, Ebel said. “The reactions of people were very comparable to reactions of people today.’’

The amount of shaking felt in a place depends not only on the magnitude of the earthquake, but also on the distance from the epicenter.

“A place twice as far away gets four times less shaking; that’s a rough rule,’’ Herring said.

The Virginia earthquake occurred in the middle of the North American plate, a section of the earth’s crust, and was probably caused by pressures building up along the edges of the plate where it meets other plates, Ebel said.

“It’s more like putting a brick in a vise and cranking the vise,’’ he said. “The brick will crack in the center.’’

Bellini said that the quake was also probably caused by the buildup of ancient stresses from past geological activity, such as mountain formation, and glaciers in the region.

The shaking occurs as seismic waves move away from the epicenter, something Bellini compared to dropping a rock in a pond and watching ripples spread outward.

Ebel said that it would have taken about a minute for the waves to arrive in Boston from yesterday’s 1:51 p.m. quake. The primary wave would have arrived first, followed by a stronger secondary wave.

“I think it’s a pretty typical quake for the East Coast of the US, places that we have fairly regular small earthquake activity seem to, sooner or later, get these rare stronger earthquakes that have the potential for causing damage,’’ Ebel said.

Carolyn Y. Johnson can be reached at cjohnson@globe.com.