Md. quake is a creak in annals of seismic events

By David Brown
Washington Post Staff Writer
Saturday, July 17, 2010; A05

Is the big difference between the East Coast and the West Coast that we're weighed down with history and stuck in our ways, while out there everything is newer and more action-oriented?

If the subject is seismology, the answer is definitely yes.

The small earthquake and single aftershock that rumbled through Maryland suburbs northwest of the District on Friday didn't surprise scientists, but they were at a loss to say exactly what happened.

What's certain is the quake occurred in ancient rock that hasn't generated true seismic headlines for about 200 million years. The movement four miles underground was likened to the settling of an old house. As exciting as it was to feel a 3.6-magnitude shock in our own back yard, in geological terms it didn't mean much.

"Nothing to worry about but nice to know the Earth is alive and kicking," said Scott Southworth, a U.S. Geological Survey scientist in Virginia.

Mary Lou Zoback, a seismologist with the California company Risk Management Solutions, concurred.

"Earthquakes up to magnitude 4.5 typically happen about once a year on the East Coast," she said. "This particular one isn't likely to herald anything because the previous ones haven't. It's just part of the ongoing creaking and grinding of the stable part of the continent."

The impression that earthquakes are happening more often in this part of the country is almost certainly wrong, several experts said Friday. Seismographs are detecting ground movement better, and the events are getting more publicity. But they're nothing new.

Three magnitude-5 earthquakes toppled chimneys in New York City in 1737, 1783 and 1884. In Charleston, S.C., a magnitude-7.3 event -- as with the New York ones, that's an estimate -- killed about 60 people in 1886. One off Newfoundland, in the Grand Banks fishing ground, caused a small tsunami in 1929.

"We haven't identified a particular trend in this area. We just know that there are earthquakes along the Eastern Seaboard," said Mark D. Petersen, a seismologist with the USGS, who is in charge of the so-called "national seismic hazard" maps used by engineers, architects and city planners.

Maryland had 61 earthquakes between 1758 and 2002. The two strongest ones -- in Annapolis in 1758, and in Phoenix, in Baltimore County, in 1939 -- are estimated to have been magnitude 3.7, a tad stronger than this week's. Columbia had 19 shakes, most scarcely perceptible, between March and December 1993.

Are there grounds for worry?

"No, I don't think so," said Jeffrey Halka, acting director of the Maryland Geological Survey. "We are on a stable part of the continent. We are not undergoing serious active faulting."

Friday's 3.6-magnitude quake, at 5:04 a.m., and the magnitude-2 aftershock, at 5:16 a.m., occurred in rock about 1 billion years old, some of the continent's oldest. The location, along the Pleasant Grove fault zone, was last active in a big way about 300 million years ago. This part of the country lies deep in the interior of the North American Plate, one of numerous flat pieces of the Earth's crust that float on the planet's molten core.

One boundary of the North American Plate "is in California, and the other is on the mid-Atlantic Ridge about 2,000 miles offshore," said Won-Young Kim, a seismologist at the Lamont-Doherty Earth Observatory of Columbia University. At the eastern boundary, the North American Plate is pulling away from the Eurasian Plate, as it has been for eons. At the western boundary, it is pushing into the Pacific Plate, which is why California is so seismically active.

The ancient rock underlying this part of the country is riven with fissures and faults. Some are filled with once-molten rock that welled up from the mantle, the layer beneath the crust. The age of high-energy collisions and mountain-building is long over. But that doesn't mean there aren't stresses and strains transmitted to the plate's interior that need to be relieved.

Exactly what happened on Friday, however, is unclear. Kim suspects that stress might have built up along one of those faults, where the strength of the rock was not uniform. Something cracked or sheared and moved a fraction of a millimeter.

"The stress is relieved. That portion of the fault is becoming more stable," he speculated.

There's a competing theory, though. It holds that the motion is more vertical than horizontal.

Up-and-down movement is possible if the ancient rock is essentially exhaling after being sat on for millions of years by mile-thick glaciers (which have melted) and heavier mountains (which are slowly eroding away). David A. Vanko, professor of geology and dean of science and mathematics at Maryland's Towson University, subscribes to that view.

"It's unlikely that plate motions generated at the plate boundaries reach this far in," he said. "I think it's more likely that crustal relaxation from the erosion of the ancient Appalachian Mountains is the probable cause."

It is because earthquakes such as Friday's are so rare and so mild that it has proved impossible to study them well enough to resolve this debate with certainty.

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