PG&E spokesman Paul Flake said that although the company began work on some new seismic surveys in January, it had not yet sought permits for the most conclusive testing urged by regulators.
"Our license renewal application and our seismic studies are two separate issues," Flake said.
Dan Hirsch, a nuclear policy lecturer at UC Santa Cruz and president of the Committee to Bridge the Gap, an anti-nuclear group, said California's reactors were built when the seismic risks involved were not well understood.
Photos: Before and after earthquake satellite images
In Diablo Canyon's 1967 application to the PUC, PG&E said the site had only "insignificant faults that have shown no movement for at least 100,000 and possibly millions of years." Four years later, researchers discovered the Hosgri fault about three miles offshore, which led to expensive retrofitting of the plant.
In 2008, PG&E argued to the state Assembly that it had thoroughly reviewed its local geography and that no further seismic risks existed.
Yet weeks later, the U.S. Geological Survey revealed that it had found a second fault less than a mile from Diablo Canyon. That fault, called Shoreline, is thought by geologists to be capable of producing a magnitude 6.5 quake, while the Hosgri fault is rated up to 7.3.
Geophysicist Jeanne Hardebeck of the USGS helped discover the Shoreline fault. She said that the network of faults in the area appeared to be connected and that she feared a rupture at one could compound into a larger quake.
"There is a real issue of uncertainty when we put a magnitude on a fault," Hardebeck said, noting that the Japan quake occurred on a fault with a predicted maximum potential quake of magnitude 7.9, but in fact reached 9.
In its 2008 report, the California Energy Commission warned that San Onofre "could experience larger and more frequent earthquakes than had been anticipated when the plant was designed."
NRC spokesman Scott Burnell said that the quake risk at the two plants was acceptable. "All 104 licensed reactors in the country are meeting the agency's requirements to operate safely," he said.
Even so, NRC reports show that Diablo Canyon operated for 18 months with flawed valves that would have prevented cooling water from automatically flowing into the reactor core in an emergency. The problem was discovered in October 2009, and the NRC issued several sanctions against the plant.
The Union of Concerned Scientists, an environmental group, called the event a "near miss," singling it out as one of the most serious incidents at an American reactor in the last several years.
PG&E spokesman Flake contended that valves could still have been opened manually in an emergency. "PG&E has a very strong safety record," he said.
At San Onofre, the NRC cited operators for failed diesel generators in 2007 and again in 2009. In December 2008, inspectors found that a battery used to power emergency systems at the plant had been incorrectly connected and probably had been inoperable for four years.
The NRC noted in January 2008 that San Onofre employees had "willfully" falsified fire safety records for five years. That string of citations led the agency, a year ago, to issue a letter highlighting what it called a "chilling effect" in the plant's safety culture in which employees "have the perception that they are not free to raise safety concerns."
On March 4, the NRC issued its annual review of San Onofre, identifying improvements but noting that in the area of human performance, "corrective actions to date have not resulted in sustained and measurable improvement."
Dietrich, the plant's chief nuclear officer, acknowledged that the plant had safety problems in the past but said they were corrected, and that Edison was "working very diligently to make sure we have an environment where people feel comfortable to discuss these issues."
Dale Bridenbaugh, a nuclear engineer who left his job at General Electric 35 years ago, said that crises at nuclear facilities generally come when small errors add up.
"It's an attitude of not caring about details that in and of itself won't cause an accident, but in certain situations can cause a cascading series of failures," said Bridenbaugh, who worked as a nuclear consultant until he retired in 1996. "Things seem fine and all of a sudden you're in deep yogurt."