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Coastal Commission eyes implications of Japanese quake on California

Despite 1,100 miles of coastline and a history of powerful earthquakes, most of California is not susceptible to the kind of temblor and tsunami that devastated Japan, according to a report by the California Coastal Commission.

But there is a cautionary note: The area known as the Cascadia Subduction Zone, which runs from about 25 miles off Eureka to north of Vancouver, B.C. That zone, where a jumble of tectonic plates meet deep below the earth’s continental crust, could produce a quake – and tsunami – on the scale of Japan’s Tohoku Quake.

The 21-page study by staff geologist Mark Johnsson, released March 24 and presented to the commission members, noted that the majority of faults in California, including the San Andreas fault, could not produce a magnitude 9.0 earthquake and that most of the state “is not susceptible to an event on the scale of the Tohoku Earthquake” that struck Japan on March 11.

To produce a magnitude 9 quake, faults must be deep and wide, the study noted, and California’s seismic faults are shallow.

“A magnitude 9 earthquake requires rupturing a fault surface thousands of square miles in area. The shallow faults making up most of California’s fault systems, including the San Andreas, simply do not have sufficient area to generate such an earthquake.”

“Nevertheless, “ the report noted, “it is important not to become complacent; large earthquakes are inevitable throughout coastal California, and could be devastating in their own right. There is a large population and much infrastructure at risk in central and southern coastal California.”

But while most faults are shallow, the crucial exception is along the 800-mile-long Cascadia Subduction Zone, where a number of plates are moving and being thrust under the North America plate under the continental crust. There are two sets of fracture zones in the Cascadia Subduction Zone that are zones of weakness. “Most seismologists agree that a megathrust earthquake involving any of these plates would be in the magnitude 9 range, similar to the Tohoku quake,” the report said.

The Japanese quake and tsunami killed about 13,000 people, a figure that includes a dozen people killed last week in a 7.4 magnitude aftershock. Much of the loss of life and property damage occurred when the quake-spawned tsunami averaging about 30-foot-high struck the northern Japanese coast and pushed inland about six miles.

The quake also damaged nuclear power plants at the Japan’s Fukushima Daichi Nuclear Power Station, leading to explosions and radioactive leakage.

In California, that nuclear scenario appeared “extremely unlikely,” according to the report.

“The combination of strong ground motion and massive tsunami that occurred in Japan cannot be generated by faults near the San Onofre Nuclear Generating Station and the Diablo Canyon Power Plant,” the study said. “Nevertheless, the geologic conditions near those plants are very likely different than previously believed and ongoing study is warranted. This has been understood for at least the past three years, and some of these studies, and the environmental planning

process for other such studies, are under way.”


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