Wednesday, February 25, 2004

The last major earthquake to strike the East Coast came in 1886 near Charleston, S.C. The reverberations could be felt over 2 million square miles.

D.C. area residents might think they can breathe a little easier knowing that, but they shouldn’t take those facts at face value, says James R. Martin, professor of civil and environmental engineering at Virginia Tech.

The risk for significant earthquake-related damage to the Eastern United States “is less than the West, but comparable,” says Mr. Martin, the co-founder of the Earthquake Engineering Center for the Southeastern United States.

Recent studies show this region is vulnerable to such an event, he says, in part because there haven’t been preparations for the worst case, as on the West Coast. Regional building codes are starting to address earthquake concerns, he says, but they only apply to new construction.

“Our infrastructure is so weak,” he says. “We’re much more susceptible to damage and collapse of structures under much lower levels of ground shaking.

“In California … you have small events which show where the weak structures are,” he says.

An earthquake is a vibration that travels through the earth’s crust. An 18-wheeler rumbling down the highway can make the earth tremble temporarily, but the source of most earthquakes of consequence is movement of the earth’s many plates along fault lines.

According to “Living on an Active Earth” by the National Research Council of the National Academies, earthquakes caused an average of 10,000 deaths per year worldwide in the past century.

Last year proved to be the deadliest 12 months of earthquake activity since 1990, according to the U.S. Geological Survey. As of Jan. 7, 32,819 earthquake-related deaths were confirmed by the United Nations Office for Coordination of Humanitarian Affairs, though the U.S. Geological Survey cautions that many more deaths have been reported in December’s earthquake that shook Iran.

The average year features 18 major earthquakes — magnitude 7.0 to 7.9 — around the globe and one great earthquake of more than 8.0 on the Moment magnitude scale, a measuring practice similar to, but more precise than the commonly referenced Richter scale.

Earthquakes typically are measured using either the Richter scale or the Mercalli scale. The former measures the magnitude of the earthquake, or the amount of energy released. The latter measurement, which is much more subjective, concerns the amount of damage an earthquake causes in Roman numerals. A minor earthquake might be rated a II. A major catastrophe might earn a XII, the highest rating.

An earthquake occurs every 11 seconds, but the overwhelming majority of them are so weak they aren’t noticed. A quake registering about 3.5 on the Moment magnitude scale could be felt if a person was standing near the source; it might feel as if a big truck had just driven by, but it might not be felt by those a few miles away.

One quake Virginia residents did notice was a 4.5 magnitude temblor that hit the central part of the state Dec. 9, 2003, the shock of which was felt throughout a chunk of the state and spread as far as Marietta, Ohio. The quake made its presence felt in the Washington area in slight but perceptible ways.

What makes predicting potential earthquakes around this area difficult is the nature of earthquakes themselves. Earthquakes are caused when two tectonic plates, or sections of the earth’s crust, move together and stress both plates. Where the plates meet is called a fault, and a few of them, including the San Andreas fault, are visible. Many faults, particularly those that may be deep underground, are not, Mr. Martin says.

“The faults in the Eastern U.S. aren’t easily explained,” he says. “We don’t see them. They’re not exposed to the ground surface.”

The District rests on the middle of the North American plate, which makes it hard to predict how the earth will react when stressed, he says.

It’s like breaking a pane of glass, he explains. “You wouldn’t be able to predict where the fracture occurs.”

Gerald Baum, director and program chief of environmental, geology and mineral resources with the Maryland Geological Survey, says the nearest known major system is the New Madrid fault in Missouri.

That may seem far enough away from the area, but given the structural nature of the East Coast, it doesn’t mean this region is free of the fault’s reach.

“If a very large one occurs there, it’ll be felt for quite some distance,” Mr. Baum says. “The faults dissipate the energy out West. With the East Coast, the energy isn’t broken up. The energy travels for longer distances.”

John Taber, education outreach program manager for the District-based Incorporated Research Institutions for Seismology (IRIS), says people trigger some earthquakes.

“If you pump fluid into the ground, say, when a large dam is built, it lubricates the faults,” says Mr. Taber, who recalled a case in Colorado in which pumping water triggered earth shakes that stopped when the water was shut down.

Without man’s intervention, earthquakes occur all around, he says.

“In California, there might be 50 a day located by the [seismograph] machines,” he says. “In this part of the world, it’s less.”

One area that’s fairly well understood is why aftershocks are felt after a major earthquake. If California experiences a 6.5 earthquake, then a subsequent aftershock could register in the neighborhood of 5.5.

“It’s the redistribution of stress,” Mr. Taber says. “If a fault slips, there’s still stress left over in different places.”

Dazhi Jiang, an assistant professor with the University of Maryland’s department of geology, says 90 percent of the world’s earthquakes can be attributed to activity along plate boundaries. That leaves 10 percent that occur in the interior of the earth, where the causes are less certain.

“There’s a lot to be learned,” Mr. Jiang says.

Mr. Martin knows how high the stakes are regarding the damage even a midsize earthquake can create. He just isn’t sure we’ll ever be able to do more than prepare for the worst.

“We’re not able to predict earthquakes reasonably yet, and I’m not sure we ever will,” he says.

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