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

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?

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