Officials tried to reassure Californians on Monday that the kind of nuclear crisis facing Japan was highly unlikely at the state's two nuclear power plants.

Southern California Edison officials acknowledged that the San Onofre nuclear power plant was built to withstand a magnitude 7.0 quake — not the 9.0 temblor that hit Japan. But quake experts said the chance of a similar-sized quake —and a tsunami — occurring in the southern half of California were highly unlikely.

"There's no offshore fault in any of Southern California that's exactly like the one that broke in Japan," said Thomas H. Jordan, director of the Southern California Earthquake Center at USC.

Photos: Scenes of earthquake destruction

Steven Day, a seismology expert at San Diego State, said the highest magnitudes believed to be possible at the nearest significant fault lines to the two Central and Southern California plants — the Hosgri fault in the case of Diablo Canyon and the Newport-Inglewood/Rose Canyon fault in the case of San Onofre — would be in the low 7s.

Experts said there was essentially no risk of an earthquake that would generate a tsunami like the one that struck Japan at either site because there is no subduction zone — where one plate slides under another — off the shore.

An earthquake could cause undersea landslides that would potentially generate a tsunami, but it would be of a much smaller scale.

Northern California and the Pacific Northwest are at a greater risk of a major tsunami because of their proximity to the Cascadia subduction zone.

Because it's on the beach, San Onofre's twin domed facility south of San Clemente is protected by a 30-foot-high, reinforced concrete "tsunami wall," said Pete Dietrich, chief nuclear officer for San Onofre's operator, Southern California Edison. The twin units, which have been in operation since the mid-80s, have multiple safety barriers that are newer and better designed than the Japanese reactors built a decade earlier, Dietrich said.

"Since Three Mile Island in 1978, we have gone through several rounds of evaluation and analysis on how we can make things safer at all U.S. nuclear facilities," Dietrich said. "The severe accident analysis has not been done in Japan as it has here. That's just a fact."

San Onofre's units were built in layered shells, like Russian nesting dolls. The outer shell is made of reinforced concrete that is 4-feet thick and is designed to capture any unexpected release of radiation. An inner casing housing the reactor is made of 8-inch thick steel.

Inside the reactor, fuel rods and control rods are surrounded by pressurized water. San Onofre's cooling system uses a 3,000-foot pipe to draw water from the Pacific Ocean. Water is dispersed from 1,500-foot pipes through hundreds of openings, helping maintain a temperature that varies by only a couple of degrees from that of the ocean.

In addition to diesel generators, the plant has a battery system. Even if all power is lost, pressurized tanks of water can be directly driven into the reactor cooling chambers, Dietrich said. Another Edison spokesman, Gil Alexander, said the company will analyze the Japanese event and further tighten its system, if necessary.

"We will comb through the details of their emergency very carefully and whatever lessons can and should be applied here will be noted,'' he said. "That process will unfold very vigorously."

While the risk factors in Southern California may be lower than in Japan, some contend that the risks do not justify placing nuclear power plants in earthquake territory.

"Earthquakes happen — they happen a lot in California, they happen often on faults we don't even know are there, and no one can predict if there's a small or large chance," said Dan Hirsch, president of the Committee to Bridge the Gap, a nonprofit nuclear policy organization.

After construction permits were issued for the Diablo Canyon plant, offshore studies by Shell Oil revealed the previously unmapped Hosgri fault in 1971. Another previously undiscovered major fault line was identified near the plant in 2008. The Nuclear Regulatory Commission concluded that the plant's existing design would be sufficient to withstand an earthquake on the fault. Jordan acknowledged the possibility of an earthquake on a previously undiscovered fault near either of the power plants.

"We are constantly being surprised in this business, so you have to build in the possibility of surprise, and I think that to a significant degree, that has been done," he said.

The Diablo Canyon plant is on a bluff 85 feet above sea level, Pacific Gas & Electric Co. spokesman Kory Raftery said, and a concrete wall surrounds the intake cove where water is drawn in for the plant's reactor-cooling system. It relies on ocean water for its cooling system and has two diesel backup generators. In the event that all power is lost, two reservoirs are located above the plant, ready to supply a cooling system powered by gravity, Raftery said. It was built to withstand a magnitude 7.5 quake.

State health officials kept close watch Monday on potential radioactive releases at Japanese nuclear plants, holding conference calls with local and federal officials every few hours, said Mike Sicilia, a spokesman for the California Department of Public Health.

Sicilia said federal nuclear regulatory agencies have reassured state officials that the Japanese nuclear troubles do not pose an immediate danger to California.

"The Department of Public Health has radioactive monitoring for the water, food and the air," Sicilia said. "We do have a plan of response and constant contact with our partners. From a health standpoint, we're not concerned at this point."

Jordan Scott, a spokesman for the California Emergency Management Agency, said nuclear experts also reassured officials in his office that, "there is no danger at this time."

Photos: Scenes of earthquake destruction

catherine.saillant@latimes.com

abby.sewell@latimes.com

Times staff writer Molly Hennessy-Fiske contributed to this report.