Nuclear plants from North Carolina to Michigan are under increased scrutiny after a 5.8 magnitude earthquake rocked the US East Coast.
Twelve nuclear plants declared an "unusual event", the lowest level of emergency, after Tuesday's earthquake, the Nuclear Regulatory Commission said.
Virginia's North Anna Power Station, about 20km from the epicentre, issued an alert, the next highest emergency level.
The quake was centred 65km northwest of Richmond. Two nuclear reactors at North Anna were automatically taken off line by safety systems.
No damage was reported at the plant, which is being powered by emergency diesel generators.
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The quake was also felt at the Surry nuclear plant near Newport News, Virginia.
Besides Surry, the other plants declaring an unusual event were the Calvert Cliffs nuclear plant in Maryland; Peach Bottom, Three Mile Island, Susquehanna and Limerick plants in Pennsylvania; Salem, Hope Creek and Oyster Creek in New Jersey, Shearon Harris in North Carolina; and D.C. Cook and Palisades in Michigan.
All were placed under increased scrutiny but continued to operate. Steve Kerekes, a spokesman for the Nuclear Energy Institute, an industry group, said US nuclear plants responded as designed.
The NRC requires that plant designs take into account earthquakes, tornadoes and other natural disasters.
"US nuclear energy facilities have been tested repeatedly by Mother Nature this summer, with tornadoes in the Southeast and record flooding in Nebraska.
They have successfully met these challenges," Kerekes said.
