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.

"You have the unique opportunity of being at the cutting edge -- and whenever I say that, usually I get a response like, 'It feels like a bleeding edge,'" said Jack Grobe, deputy director of the NRC's Office of Nuclear Reactor Regulation, to officials from Dominion.

"It's not entirely clear to us exactly what we need in this submittal from you. A summary may not do it," Grobe said near the end of an exhaustive three-hour engineering face-off between a dozen top officials from each side.

INSPECTIONS CONTINUE

Neither the company nor the NRC have found signs of serious damage to safety systems at the North Anna nuclear plant, although inspections by both sides continue.

NRC experts from a variety of nuclear engineering disciplines peppered Dominion counterparts with questions about the impact to fuel in the core, the strength of welds in plant steels, and the resilience of underground pipes.

Dominion officials said it appears the North Anna reactors shut when the earthquake caused a problem inside the cores at both units rather than from the loss of power into the plant from the outside grid as was initially believed.

Operators are still working to understand the "root cause" of the plant shutdown, officials said.

Dominion touted a report by Robert Kennedy, a seismic expert consultant from California. The company said he saw no reason why the plant could not be safety restarted.

At Thursday's hearing, the toughest questions were about items that are difficult to see with the naked eye, said Eugene Grecheck, a Dominion vice president.

"You look at the things you know are most susceptible," he told reporters. "If the paint hasn't chipped or cracked, then you know there hasn't been any effect on the steel."

Grecheck would not directly comment on the NRC's comments about the strength of the quake. He stressed that the strongest vibrations lasted only a few seconds.

Dominion's analysis will be considered as the NRC does its own analysis of what it will require the company to do to restart operations, NRC spokesman Scott Burnell said.

The company said one reactor would be "physically" ready to restart by September 22 and the other could be refueled and ready by mid-October. But NRC officials said meetings probably will be needed to explore Dominion's inspection process.

"A fundamental question we have based on the reanalysis is how are you going to look at your design basis for earthquakes going forward?" said Joe Giitter, director of the NRC's licensing division for operating reactors.

SHAKEN, BUT NOT BROKEN

Japan's nuclear disaster at the Fukushima Daiichi plant -- overwhelmed six months ago by an earthquake and tsunami -- put quake risks at the forefront.

The NRC has said it plans to order all U.S. plants later this year to update their earthquake risk analyses, a complex exercise that could take two years for some plants to complete.

The North Anna quake shows the need for the nation's 104 aging reactors to reevaluate earthquake risks using up-to-date geological information, said Majid Manzari, an engineer at George Washington University who studies quake impacts.

A former chairman of the NRC said he expects the broad review ultimately will impact most nuclear plants along the East Coast.

"I think what the East Coast earthquake demonstrated is the design parameters might be changing," said Dale Klein, a mechanical engineer at the University of Texas.

Nuclear power critics have seized on the North Anna quake. "We need a seismic shift in the way in which these plants are protected from earthquakes or other natural disasters." said Representative Edward Markey, a Democrat from Massachussetts.

(Additional reporting by Eileen O'Grady in Houston and Timothy Gardner in Washington; Editing by Jim Marshall, Sofina Mirza-Reid, Bob Burgdorfer and David Gregorio)