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Md. quake is a creak in annals of seismic events

A 3.6-magnitude temblor sent residents outside to see what was happening and to their phones to call 911.

"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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