As the world's attention remains focused on the nuclear calamity unfolding in Japan, American nuclear regulators and industry lobbyists have been offering assurances that plants in the United States are designed to withstand major earthquakes.

But the Diablo Canyon nuclear plant, which sits less than a mile from an offshore fault line, was not required to include earthquakes in its emergency response plan as a condition of being granted its license more than a quarter of a century ago. Though experts warned from the beginning that the plant would be vulnerable to an earthquake, asserting 25 years ago that it required an emergency plan as a condition of its license, the Nuclear Regulatory Commission fought against making such a provision mandatory as it allowed the facility to be built.

Officials at Pacific Gas and Electric Company, the utility that operates Diablo Canyon, did not respond to calls seeking comment before the story was published. After publication, a spokesman for the company said the plant does have an earthquake procedure that had been implemented during a 2003 earthquake near the facility, and that staff are trained to respond. The company did not provide further details upon request.

As Americans absorb the spectacle of a potential nuclear meltdown in Japan -- one of the world's most proficient engineering powers -- the regulatory review that ultimately enabled Diablo Canyon to be built without an earthquake response plan amplifies a gnawing question: Could the tragedy in Japan happen at home?

Experts who recall how the California plant came to be erected offer a disconcerting answer: Yes. And some are calling for more urgent government action to review safety at nuclear plants across the country.

"What they're displaying now is exactly what was wrong in the past with the nuclear establishment, which is that they didn't have their priorities right," said Victor Gilinsky, who served on the Nuclear Regulatory Commission during the Diablo Canyon debate and agreed with the call for greater attention to earthquakes in emergency plans. "They're more concerned about the protection of the plants, and installation of further plants, than they are about public safety. The president should be saying, 'I want every single plant reviewed.'"

Back when the California plant was being finalized in the mid-1980s, local activists and environmental lawyers sued the Nuclear Regulatory Commission in an effort to slow the project, arguing that the clear risks from earthquakes nearby required additional planning.

The case made its way to the U.S. Court of Appeals in Washington, D.C., where a 5-4 majority -- including current Supreme Court Justice Antonin Scalia and former Clinton independent counsel Kenneth Starr -- ruled that earthquakes did not have to be included in the plant's emergency response plans.

The underlying theory was that the plant's design, which came after years of planning and geological studies, could withstand any foreseeable earthquake in the area -- the same assumption that guided thinking in Japan.

"What they're saying is that there could be an earthquake, but in no way could it ever cause a radioactive release at the same time," said Rochelle Becker, who led the San Luis Obispo, Calif., group that first sued the Nuclear Regulatory Commission over earthquake preparedness in the 1980s. "I'm pretty sure we now have evidence that it does."

A spokeswoman for the Nuclear Regulatory Commission confirmed that the Diablo Canyon plant is not required to have an emergency response plan specifically for earthquake-triggered releases because the commission is satisfied that the plant's structure will be able to withstand an earthquake in the area -- calculated as a maximum magnitude of 7.5. But she said that should not imply that the plant lacks an overall emergency response plan.

Officials at Tokyo Electric Co., the operator of Japan's stricken Fukushima Daiichi plant, said over the weekend that the strongest earthquake they had anticipated was much lower than the magnitude-9.0 quake that struck last Friday.

"That's a lesson that we ignore at our own peril, because we could be wrong, too," said Joel Reynolds, the attorney who originally brought the case against the Nuclear Regulatory Commission and who is now a senior attorney with the Natural Resources Defense Council in California. "It is a story as old as science that we're always learning new things. We're always discovering the unexpected."

Critics have raised particular questions about how a standard emergency response to a nuclear disaster could be complicated if it had been caused by an earthquake, where roads and other surrounding infrastructure would also be impaired.

The NRC spokeswoman said that all plants are required to have an overall emergency response plan in place, regardless of what triggers an emergency. At Diablo Canyon, she said there are sensors to alert employees to shut down the plant if tremors are felt, the spokeswoman said. Then depending on the circumstances, the plant would alert local authorities, who would handle questions such as how evacuations would be handled.

"All our plants are designed to withstand significant natural phenomena like earthquakes, tornadoes and tsunamis," the commission's chairman, Gregory B. Jaczko, said earlier this week. "We believe we have a very solid and strong regulatory infrastructure in place now." He added that the commission would "continue to take new information and see if there are changes that we need to make with our program."

Michael Mariotte, the executive director of the Nuclear Information and Resource Service, a group critical of the nuclear industry and the regulatory process, said the pushback on earthquake response planning years ago reflects an environment where the industry is helped along by regulators.

"That's the logic behind a lot of our nuclear regulation, unfortunately, is that it's designed to accommodate the operation of a plant, and not necessarily the protection of the public," Mariotte said. "If they acknowledged that an earthquake occurred that damaged the plant, then they're also acknowledging that an earthquake has damaged the transportation infrastructure, that you can't get people out properly, that the plant doesn't work, and then it can't be approved."

At the time the Diablo Canyon case was being litigated in the mid-1980s, the Nuclear Regulatory Commission and the electric utility looking to build the plant had been dealing with more than a decade's worth of federal and state reviews for the facility. Federal regulators were comfortable with their seismic reviews of the remote coastal area between Los Angeles and San Francisco.

Comments made during closed meetings, later released to the public, showed that some NRC commissioners were concerned that additional public hearings surrounding the emergency response plan and earthquakes would slow the process further.

"One of the things that I think makes me shy away often from hearings is because as soon as we hear the word 'hearing,' you see so much time elapse that it maybe over-influences one," then-NRC Chairman Nunzio J. Palladino, who has since passed away, said at the time. "I do feel that at this late stage, requiring a delay while we wait for a hearing is not in the best national interest."

When the case involving earthquake response was eventually litigated all the way to the federal appeals court in D.C., which ultimately sided with the Nuclear Regulatory Commission, the five-member majority noted that there had already been extensive review of seismic activity around the plant.

"We can think of no potential natural or unnatural hazards, regardless of their improbability, that the Commission would not be required to consider," failed Reagan Supreme Court nominee Robert Bork wrote in an opinion for the appellate court. "That is a prescription for licensing proceedings that never end and plants that never generate electricity."

The four dissenting judges, including current Supreme Court Justice Ruth Bader Ginsburg, noted: "The very purpose of the exercise is to plan for the unthinkable eventuality that the design safeguards will not prevent an accident."

"It defies common sense to exclude evidence about the complicating effects of earthquakes from a proceeding dealing with how to respond to a nuclear accident at a plant located three miles from an active fault, a plant in which seismic concerns dominated the design and construction proceedings for well over a decade," the justices wrote.

In recent years, the utility that operates Diablo Canyon, Pacific Gas and Electric Company, has recently found another fault line less than a mile from the plant after conducting research with the U.S. Geological Survey. The plant's original design had accounted for a fault that was farther offshore -- about three miles from the plant.

The spokeswoman for the Nuclear Regulatory Commission, Lara Uselding, said the utility has not found evidence that the newly discovered fault line would pose a risk to the plant. The commission is currently reviewing the company's geological report.