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Japan-style earthquake and tsunami unlikely to hit Southern California, experts say

March 14, 2011 |  1:20 pm

Fault map of Southern California.

Although Southern California is riddled with geological fault lines, and some are relatively close to both the San Onofre and Diablo Canyon nuclear sites, experts said the region was at little risk of experiencing a catastrophic earthquake and tsunami like the ones that hit Japan.

“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 the University of Southern California.

Thomas K. Rockwell, an earthquake expert at San Diego State University, estimated that the offshore faults near the San Onofre nuclear power plant could generate earthquakes of magnitude up to 7 to 7.5, but he said a major earthquake in the area might occur only once every couple of thousand years.

Diablo Canyon lies on the far western end of the Transverse Ranges, where the high-end magnitude possible is slightly higher, he said.

However, Rockwell 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 of Southern California.

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

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

While the risk factors in Southern California may be lower than in Japan, some contend that the danger still does 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 Committee to Bridge the Gap, a nonprofit nuclear policy organization. “The world is filled with lots of risk, and placing big bottles filled with massive amounts of radioactivity near earthquake faults doesn’t make much sense.”

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.

San Onofre is built to withstand a magnitude 7.0 earthquake, said Gil Alexander, a spokesman for the generation station's operator, Southern California Edison. That is greater than the 6.5 shaker that scientists predicted could strike the plant before it was built 42 years ago, he said. But it's less than the 8.9 quake that hit Japan last week.


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Fault map of Southern California. Credit: UC Santa Barbara