USGS Earthquake Hazard Map
The U.S. Geological Survey’s 2008 National Seismic Hazard Map shows that the mid-Atlantic states have one of the lowest likelihoods of earthquakes in the country. U.S. Geological Survey

The Virginia earthquake Aug. 23 was unusual for the region, but not unprecedented.

Earthquakes, though fairly commonplace along the Pacific Coast, are few and far between on the Atlantic Coast because of its different geological structure. Western states like California are prone to earthquakes because they stand near the meeting point of two tectonic plates. But while there are many seismic faults along the Eastern seaboard, few of them are thought to be active, according to the Maryland Geological Survey. And because earthquakes occur here so rarely, geologists don't spend much time studying them, so their cause remains unknown.

One of geologists' guesses is that East Coast earthquakes occur because of vertical faults that formed during the opening of the present Atlantic Ocean during the Triassic Period about 220 million years ago, which would be in the original bedrock of the ocean floor rather than in sediment that accumulated later, according to information from the National Earthquake Hazards Reduction Program cited by the Maryland Geological Survey.

Just last November, a magnitude 3.9 earthquake occurred at the bottom of the Atlantic Ocean, about 100 miles off the coasts of New Jersey and Long Island. In 2006, a magnitude 4.2 quake hit the coast of Maine. There was also a magnitude 4.1 earthquake in southeastern Pennsylvania in April 1984, which was felt in eight states and Washington, D.C. However, these quakes were all minor.

The last time an earthquake hit the East Coast hard enough to cause significant damage was more than a century ago. In 1886, a quake with a magnitude between 6.5 and 7 hit South Carolina, creating tremors that could be felt over two million square miles, according to the Maryland Geological Survey. In 1811 and 1812, a cluster of earthquakes with magnitudes up to 8.7 hit Missouri, but were felt along the coast as well. Even farther back, there were major earthquakes in New York City in 1737 and 1884 and one near Boston in 1755.

In the past several decades, there have been dozens of very minor earthquakes (mostly magnitudes 2.0 or lower), but no major ones.

Serious earthquakes remain unlikely in eastern regions. According to the U.S. Geological Survey's 2008 National Seismic Hazard Map, the northern part of New England -- Maine, New Hampshire, Vermont and a bit of upstate New York -- are moderately prone to earthquakes. However, the mid-Atlantic region, where Tuesday's earthquake occurred, has one of the lowest quake likelihoods in the country.