When the Japanese earthquake occurred last week, an area the size of Maryland ruptured on the fault.
To duplicate that kind of a quake in California would require a rupture along a very long length of a fault — and that geology does not exist in the faults closest to Diablo Canyon nuclear power plant, says Tom Brocher, director of the U.S. Geological Survey’s Earthquake Science Center in Menlo Park.
Similarly, seismologists are not aware of any faults farther offshore that could produce a large quake or tsunami. “There are some offshore structures, but they are not the plate-boundary features that you see off of Japan or the Pacific Northwest,” Brocher said Wednesday.
Seismologists are unsure whether a newly discovered fault near Diablo Canyon and a larger fault offshore are connected and could produce a potentially large earthquake if they went off in tandem.
The Japanese fault is shaped like a ramp that dips beneath the country at a shallow angle, with a large area of contact between two tectonic plates. In contrast, California’s faults are thin ribbons that extend vertically into the ground, creating a fault zone about 10 miles deep.
“We just don’t think there is any way we can get a magnitude-9 quake in this part of the state,” Brocher said. “The faults are just not long enough.”
The closest subduction fault is the Cascadia that runs down the coast of the Pacific Northwest into Northern California.
The longest fault in the state is the San Andreas, which runs about 580 miles, including through the northeast part of San Luis Obispo County.
In order for the San Andreas to generate a massive earthquake, it would have to rupture along most of its length. This is considered unlikely because a section in the middle around Parkfield regularly has many small earthquakes, and that relieves some of the pressure.
By comparison, the Hosgri Fault offshore of Diablo Canyon nuclear power plant is about 120 miles long. The newly discovered Shoreline Fault near the plant is 15 miles long.
“The Hosgri Fault is just not long enough to give you one of the mega-quakes,” Brocher said. Similarly, earthquakes along faults such as those in California do not displace the huge amounts of ocean water needed to create a powerful tsunami.
“The Hosgri is a very vertical fault,” Brocher said.
Seismologists have learned a lot about the faults around Diablo Canyon since the 1980s, when Diablo Canyon was built, Brocher said.
They were initially unsure of the Hosgri Fault’s exact characteristics because it was newly discovered, he said.
Original estimates of the Hosgri showed a quake potential of 7.0 to 7.5 magnitude.
At a recent meeting of the U.S. Nuclear Regulatory Commission in San Luis Obispo, PG&E seismologists announced that they are reducing the quake potential to the 6 to 6.5 magnitude.
NRC officials at the meeting said they are studying the PG&E re-analysis and hope to announce whether they can verify it by the end of the year.
Brocher said his agency has not been asked to review the PG&E report submitted to the NRC. However, the USGS does update its seismic mapping on a regular basis and includes new information that has been scientifically vetted, he said.