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East Coast earthquake: How does a 5.9 temblor happen in Virginia?

Fault lines in the East are not as apparent or as active as in the West, but certain stresses can lead to a rupture. Tuesday's East Coast Earthquake was the biggest in 100 years.

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Crustal collisions 300 million years ago formed the Appalachian Mountains, which would have grown as tall as today's Himalayas, explains Steven Jaumé, a geophysicist at the College of Charleston in South Carolina. Then, 200 million years ago, the continents began to split, forming the Atlantic Ocean.

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Both the collisions and the splitting generated a network of faults under what is now the East Coast.

Over the intervening millenniums, the Appalachians eroded, providing the soil along the Piedmont that, at Charleston, is some 3,000 feet deep. Meanwhile, the crust cooled and became more rigid, locking the faults in place, but remaining generally inactive.

These faults, however, are still susceptible to stresses on the crust. Some of these stresses can come from the mid-Atlantic Ridge, a structure that looks like a zipper and runs north and south along the Atlantic Ocean floor. It is constantly oozing molten material from deep in the earth, forming fresh crust and pushing North America and Europe farther apart at the rate of about 17 millimeters a year.

As the new crust formed by the ridge pushes westward and eastward, it contributes stress to the ancient faults along the East Coast, explains Thomas Herring, a geophysicist at the Massachusetts Institute of Technology in Cambridge.

When that stress builds along a fault zone with the right orientation, "occasionally it might rupture," adds Chuck Bailey, a geology professor at the College of William and Mary in Williamsburg, Va. But, he adds, the fault zones most susceptible to occasional quakes are not uniformly spread throughout the eastern US. And they don't always manifest themselves in easy to spot features on the surface, the way faults do in the West.

Why it was felt so far away

Some of the quake activity, especially in the Northeast and into Canada, also can be traced to the melting of continental glaciers after the last Ice Age. Hudson Bay is rising about 2 centimeters a year – the crust rising in a process akin to releasing a squeezed ball. That also adds to the stresses on the crust.

Tuesday's quake was felt across a wide swath of the region because the rock formations deep beneath the surface are old and cold, and so transmit seismic waves more efficiently than thick layers of ancient sediment.

For easterners not used to thinking about earthquakes, Tuesday's shaking should serve as a reminder than quakes are more than theoretical occurrences, Dr. Bailey says.

The event also "may produce a focus of energy" on the part of researchers "to think about why we do get earthquakes in these regions that are far away from plate boundaries."


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