Posted at 03:47 PM ET, 08/23/2011

Earthquake aftershock felt in Virginia, more aftershocks likely to come


People crowd Pennsylvania Avenue in Washington, Tuesday, as they evacuate buildings after an earthquake his the in Washington area. (Charles Dharapak - AP)
The U.S. Geological Survey has confirmed that an earthquake struck central Virginia just before 2 p.m. EST Tuesday near Mineral, Va., 87 miles southwest of Washington D.C.

The quake measured a 5.8 on the Richter scale. And the tremors may not be over.

Just 45 minutes after the initial quake, a 2.8 magnitude aftershock was reported five miles south-southwest of Mineral, according to USGS. At 3:20 p.m., a second aftershock occurred 8 miles south of Louisa, Va., with a magnitude of 2.2.

Mike Blanpied, USGS associate coordinator for the earthquake hazards program, says there could be more aftershocks any magnitude up to 5. “Aftershocks could go on for days, weeks, or even months. They’re most likely to be felt under the next three or four days,” Blanpied said.


The quake was felt from New York City to Charlotte, N.C., and as far west as Cleveland, according to reports on the USGS Web site.

While Californians are ridiculing the East Coast for their overreaction to a 5.8 quake, Dr. Graham Kent, Director of the Nevada Seismological Laboratory, said, “it's a lot bigger deal than a 5.9 would be in California or Nevada.  You might see damage further away from the epicenter than you might expect.”

Blanpied explains why: “The rocks are old and cold and they carry the seismic energy very far. Even a magnitude 6 or less earthquake can be felt over a considerably large area, unlike California where the shaking is more concentrated.”

Earthquakes are relatively rare events on the East Coast.

An aftershock is a smaller earthquake that occurs after a previous earthquake, in the same area. Aftershocks form when the crust around the displaced fault plane adjust to the effects of the initial shock.

By definition, aftershocks cannot be larger than an earthquake. If this earthquake were to be followed by a more powerful tremor, then the initial quake would be redefined as a foreshock.

Slate explains that there are two principles that describe the typical behavior of aftershocks. “The first, called Omori's Law, predicts that most shocks will occur immediately following the earthquake and become less and less frequent over time.”

The two aftershocks this afternoon are indicators of Omori’s Law.

The other law, known as Båth's Law, states that “the largest aftershock is, on average, about 1.2 magnitudes smaller than the main quake,” Slate writes. Because the Richter scale is logarithmic, an eartuquake measuring 6.0 is 10 times larger than a quake measuring 5.0.

See video of the damage already done in Tyson's Corner, VA.

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By  |  03:47 PM ET, 08/23/2011

 
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