In 2003, a strong 6.5 quake on the San Simeon fault shook the town of Paso Robles, and it had no effect on the Diablo Canyon nuclear power plant. However, it's earthquake faults closer to the plant that have raised concerns, and now President Barack Obama and Sen. Barbara Boxer, D-Calif., are both calling on the Nuclear Regulatory Commission to conduct a review.
The Diablo Canyon nuclear power plant was initially designed to withstand a major quake on the San Andreas fault. That was the fault that scientists knew about in 1968, and it was roughly 70 miles away. In 1971, the discovery of the offshore Hosgri fault forced a redesign of Diablo, and then just two years ago scientists discovered another fault just a half-mile from the plant.
"It runs very close to the shoreline. That's why it's called the shoreline fault," says Thomas Brocher who led the Menlo Park United States Geological Survey team that discovered the fault. "It ends near or at the Hosgri fault. We haven't really determined that precisely at this time."
Original estimates of the Hosgri fault put the quake potential at a maximum 7.5. Pacific Gas & Electric seismologists recently petitioned the NRC to lower that number.
"In the 6.0 to 6.5 region for maximum earthquakes," says PG&E spokesman Kory Raftery.
The director of the Earthquake Science Center in Menlo Park doesn't agree with the company numbers.
"We would say it would still be capable of a magnitude 7.5 earthquake," says Brocher.
The NRC says it's seismologists will study PG&E's projections. In the meantime, State Sen. Sam Blakeslee, R-San Luis Obispo, is calling on PG&E to withdraw its application to extend the life of the plant for another 20 years.
PG&E says it's moving ahead with its license renewal. At Thursday's White House briefing, the chairman of the NRC, Gregory Jaczko, was asked if Diablo Canyon sits on an earthquake fault. His answer was indirect.
"Well, certainly with the plants in California, they are designed to a very robust seismic standard and for the ones that are on the coast they are also designed to deal with a very significant tsunami," said Jaczko.
A question we're hearing a lot is: Could the seismologists be underestimating the earthquake potential? After all, the engineers in Japan probably thought they were building a nuclear plant that would hold up. The difference is our earthquake faults don't have the same seismic structure. California has strike-slip faults -- one side slips against the other in a horizontal motion. In Japan we're seeing the results of subduction faults where plate is moving under the other. A plate the size of Maryland moved 20 feet. So you get much more potential for powerful earthquakes and tsunamis.