Things could have been more dramatic during the August 23 East Coast earthquake. Not Fukushima level bad, but bad. 25 of 27 spent uranium casks were shaken up to 4 inches off their place at North Anna nuclear power plant.
Apparently, the quake "potentially exceeded" the plant's design, which is located in Louisa County, Virginia. The fact is that they don't really know yet. Why? According to a spokesman: "It's complicated."
A spokesman for Dominion Virginia Power—the company that runs the plant—said that they didn't report it to the public because "it was not considered damage", even while the casks were pushed off their position by 1 to 4 inches. They don't really know if the quake exceeded the plant design yet, even while the plant is supposed to withstand 5.9 to 6.2 earthquakes. I con't understand how a nuclear power station is designed to easily withstand a 5.9 to 6.2 earthquake, but suffers considerable damage with a 5.8 alone.
The plant did report an alert status right after the earthquake.
The Nuclear Regulatory Commission believes that there are no issues at this point, even while North Anna's two nuclear reactors—built in 1971—are still shut down after the earthquake hit. According to the NRC, "it does not appear to be a safety issue at this point."
According an August 2010 story by the Nuclear Regulatory Commission, the risk of an intense earthquake causing core damage to the one reactors is 1 in 22,727 yearly. Doesn't seem too remote to me. [Wikipedia via USA Today and Washington Post]