Big Question of Yesterday's Quake: Why Virginia?

"Smack dab in the middle of Virginia" doesn't seem like the likeliest earthquake epicenter locale, and yesterday's quake has plenty of East Coasters wondering why it struck there—and if it'll happen again. The Christian Science Monitor tosses out the word "rare": USGS seismic hazard data gives the area a 70% to 75% chance of having a 5.0+ magnitude quake ... once every 1,000 years. The big ones to hit the East Coast are few, but they do happen: a 5.9 temblor near Boston in 1755; a 5.5 shaker in NYC in 1884; a 7.3-magnitude quake in Charleston in 1886.

California straddles the line between two grinding plates, while the East Coast is situated in the center of the North American Plate. So why do quakes happen there at all? Some 200 million to 300 million years ago, the East Coast sat on the boundary of an ancient plate and was connected to Europe and Africa. When the continents began to split, faults were born under the region. Fast forward to modern day: As molten materials push from the mid-Atlantic Ridge, which is located along the floor of the Atlantic Ocean, new crust is formed. That stresses those ancient faults, and when the stress gets to be too much along a fault zone ... well, earthquake. (More earthquake stories.)