The quake that hit the East Coast on Tuesday afternoon was notable, but not unprecedented, for the eastern half of the country, geoscientists say.
Additionally, the shaking was felt over such a large area — as far south as Atlanta and as far north as Ontario, according to eyewitness reports — largely because the eastern part of the North American continent is different than the West Coast, where quakes are more common. [Album: The Great San Francisco Earthquake]
"The crust is different in the east than in the west," United States Geological Survey (USGS) earthquake geologist David Schwartz told LiveScience. "It's older and colder and denser, and as a result, seismic waves travel much farther in the east than in the west."
Additionally, said Andy Frassetto of the Incorporated Research Institutions for Seismology, the sediments along the east coast can make quakes feel stronger.
"The sediments of the coastal plain along the eastern seaboard can trap waves as they propagate and produce a minor amplification of the shaking," Frassetto told LiveScience.
A much more extreme version of this effect occurred during the earthquake that hit Christchurch, New Zealand, this year, Frassetto said. Faults that rupture east of the Rockies usually create quakes felt over more than 10 times the area than those west of the mountains, according to the USGS. A magnitude-5.5 quake in the Eastern U.S. can usually be felt as far as 300 miles (500 km) away.
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According to the USGS, the 5.8-magnitude quake struck at 1:51 p.m. Eastern Time. The epicenter was 5 miles (8 kilometers) from Mineral, Va., and 84 miles (135 kilometers) away from Washington, D.C. Despite the distance, the Pentagon, the Capitol and other buildings were evacuated. [Read: Large Earthquake Could Strike New York City]
The quake was only about 3.7 miles (6 km) deep, according to the USGS. That's typical for the eastern U.S., Frassetto said. In the east, he said, quakes usually originate in the upper part of the crust.
In contrast, subduction zones such as the Pacific Ring of Fire, where one plate is being pushed under another, produce very deep quakes — sometimes 435 miles (700 km) down, Frassetto said. These super-deep quakes may not even be felt on the surface.
