It wasn’t only the shaking that rattled the East Coast. The surprise earthquake this week created new rumblings about the ability of U.S. nuclear plants to withstand a heavy shaking, an issue with heightened sensitivities after Japan’s meltdown earlier this year.

The North Anna Power Station in Mineral, Va., was less than 20 miles from the epicenter of Tuesday’s quake, and it got fresh scrutiny after both its reactors shut down automatically when outside electricity to the plant was cut off.

The Nuclear Regulatory Commission initially reported that the plant’s emergency safeguards worked just fine as diesel generators automatically kicked in to keep nuclear rods and spent fuel safe in storage facilities and cool water ponds.

But it did not happen without a minor snag.

According to the incident report published hours after the quake, one of North Anna’s four power generators didn’t start properly, as it had been designed to. It was taken off line, and power from another generator off site was routed through to make the system fully operational. Following inspections of the facility and its sensitive parts, both reactors were brought back online.

That scenario raised concerns among nuclear safety and environmental advocates, especially because the NRC already has begun the process of approving a third reactor to be built at the North Anna plant, even as the agency’s own internal research shows the site has far more seismic risk than was thought when it was first chosen and built four decades ago.

Initial research suggested the first two reactors should be suited for the seismic equivalent of a 6.5 earthquake, but the third reactor will be constructed to withstand larger ground movement equivalent to about a 7.5. The disparity is not minor: a 7.5 earthquake is 10 times stronger than a 6.5.

The Blue Ridge Environmental Defense League, which monitors nuclear and environmental safety in the Southeast, has filed a lawsuit to block construction of the third reactor. The entire area is unsafe for nuclear generation, the group says, and the facility, currently run by Dominion Power, previously had sought minor safety exemptions from the NRC for a boiling-water reactor and a pressurized-water reactor. The group also alleges in a report that during the permitting process in the 1970s, plant officials hid from regulators knowledge of a fault line below the area.

“Based on what we know about corruption during the licensing of the first two units in the 1970s, the existing plants should never have been built at North Anna,” says Louis Zeller, BREDL’s director of science. The lawsuit contesting the new construction recently was dismissed by the Atomic Safety and Licensing Board, but is still pending before the NRC.

Scott Burnell, a spokesman for the NRC, downplayed the concerns, saying that the licensing process for all three reactors at North Anna has been proper, and that plant operators are frequently required to address any safety discrepancies. The same is true, he said, at all of the country’s 104 nuclear plants that the agency regulates.

Yet the issue remains of whether old plants, many of them nearing their retirement age, should be retrofitted to address new research in seismic risk, especially in such regions as the eastern U.S. that rarely experience earthquakes, but where ground shakes are still possible, as Tuesday showed.

The answer may depend on the new level of risk. Eric Loewen, president of the American Nuclear Society, a broad professional association of nuclear scientists and engineers, says seismic risk monitoring is a vital part of nuclear power plant construction, and all nuclear activities. “When we look at any technology, it evolves and improves,” says Loewen. “If you take cars, they’re better designed with airbags and other features improving it. We don’t take the old cars out because they’re no good. These reactors were built with the modern construction of the day. But with new facilities it’s easier to get a new design [that’s more advanced].”

Still, the NRC has been scrambling to address new safety concerns brought to light by Japan’s Fukushima crisis, and exacerbated by the Virginia quake. The agency has set benchmarks for new safety measures, including new surveying of all nuclear sites and fortifying fuel storage facilities. The most pressing safety concerns, the NRC has reported, will be addressed by the fall.