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Wednesday, August 24, 2011

Rare Earthquake Hits Virginia, Rattles U.S. East Coast

Crowds of evacuated workers in Manhattan are seen after an earthquake.
Evacuees crowd Manhattan streets after a magnitude 5.8 earthquake shook the U.S. East Coast.
Photograph by Justin Lane, European Pressphoto Agency
Ker Than
Published August 23, 2011
The magnitude 5.8 earthquake that struck Virginia today was a rare but significant event for the region, according to one quake expert.
"It was quite sizable," said seismologist Hua-wei Zhou of Texas Tech University.
The Virginia earthquake struck at about 1:51 p.m ET near Mineral, Virgina, about 40 miles (64 kilometers) northwest of Richmond (map).
The tremors shook buildings and prompted evacuations as far away as Washington, D.C., and New York City. The quake was followed by a magnitude 2.8 aftershock 45 minutes later.
(Related: "Japan Earthquake Vibrations Nearly Reached Space.")
Earthquakes rarely strike the U.S. East Coast and are generally less severe when they do.
Before this latest quake, for example, the largest earthquake on record in central Virginia was a magnitude 4.8 temblor that occurred in 1875, according to the U.S. Geological Survey (USGS).
Eastern Earthquakes Smaller But Farther Reaching
Earthquakes are rare in the eastern U.S. because the region is farther from a plate boundary—a region where tectonic plates meet and grind together. The closest such boundary is several hundred miles away in the middle of the Atlantic Ocean and in the Caribbean Sea.
By contrast, California has a major fault line, the San Andreas fault, running vertically through most of the state, Zhou explained.
(See "Mexico Earthquake Zone Linked to California Faults.")
Plate boundaries are especially prone to earthquakes because the motion of tectonic plates creates tension that can cause significant shaking when the stress gets released.
Zhou said he suspects the Virginia earthquake was due to the much less frequent release of stress from a small thrust fault in the region.
Still, earthquakes in the central and eastern U.S. are typically felt over a much broader region than on the West Coast.
"A magnitude 5.5 eastern U.S. earthquake usually can be felt as far as 500 kilometers [300 miles] from where it occurred, and sometimes causes damage as far away as 40 kilometers [25 miles]," according to the USGS.
Being far from plate boundaries, the older and denser continental crust is much more like a solid sheet of bedrock than the fault-filled crust on the West Coast, allowing seismic waves to travel farther.
"Most bedrock beneath central Virginia was assembled as continents collided to form a supercontinent about 500 to 300 million years ago, raising the Appalachian Mountains," the USGS says.

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