Last month’s record earthquake in the eastern United States may have shaken a Virginia nuclear plant twice as hard as it was designed to withstand, a spokesman for the U.S. nuclear safety regulator said on Thursday.

But Dominion Resources told the Nuclear Regulatory Commission that the ground under the plant exceeded its “design basis” only by about 10 to 20 percent, and it plans to prove in the next month that its reactors are safe to restart.

The discrepancy is one of many items the NRC and company must deal with, in the first instance in which an operating U.S. nuclear power plant has experienced a quake beyond its design parameters.

The NRC must sign off on Dominion’s restart plans for the North Anna plant, about 12 miles from the quake’s epicenter — and determine how it will make that decision.