Nov 22

1973 Undersea fault near Diablo Canyon

11-24-73-hosgri-fault-hayes.jpgNovember 24, 1973

Time gets measured in different scales. Geologic time runs in cycles with spans of hundreds, thousands or millions of years, hard to put in a human context because the Earth’s pace is not a human pace.
A few signposts of 1973: Richard Nixon was president, and the jaws of the Watergate scandal were closing on him. Telegram-Tribune cost 15 cents and did not have a Sunday edition. Graphics were drawn by hand then photographed, not created on a computer. Thrifty was the name of a drug store. The nuclear power industry answered to the Atomic Energy Commission.
Almost exactly 35 years ago the discovery a fault a mile off the coast was reported in the then Telegram-Tribune.
The United States Geological Survey and the AEC had commissioned a study of the seabed off of Diablo Canyon after scientist Gary Greene had discovered an active fault offshore from Davenport in Santa Cruz. PG&E dropped plans to build a nuclear power there in the wake of that report.
Staff writer Jim Hayes quoted Greene:
“Of course, PG&E had other problems there. There was the fault at Ano Nuevo and then they had a landslide.”
Later in the article the scientist spoke about the newly discovered fault,
“Length, becomes a critical factor, generally the longer the fault the more recent and active we think it is.”
A PG&E spokesman Frederick R. Draeger downplayed the discovery saying Diablo Canyon had been designed “to handle the greatest earthquake that could occur.”
In 1973 both units of the plant were expected to open within two years and the price tag stood at $650 million dollars. Both numbers would balloon as the plant was retrofitted in the wake of the discovery.

Today David Sneed has an article on a fault discovered about a mile offshore from the plant.

NOTE the link expired, here is the story:


Published: Saturday, November 22, 2008
Section: A-Section
Edition: Tribune
Page: A1
Lines: 106

By David Sneed

Anew earthquake fault that could run as close as 1,800 feet offshore of Diablo Canyon nuclear power plant has been discovered by Pacific Gas and Electric Co.

The fault was found using new computer programming that allows geologists to better map the epicenters of the many small magnitude earthquakes in the area, said Lloyd Cluff, head of PG&E’s earthquake risk management program.

Based on this data, the new fault appears to run from offshore of Point Buchon to within a mile of Diablo Canyon, extending to just off Point San Luis, near Avila Beach. The fault could generate a 6.5-magnitude quake. Diablo Canyon is built to withstand a 7.5-magnitude quake, so the new fault does not pose a threat to the operational safety of the plant, Cluff said.

“We don’t see anything that exceeds the plant’s design basis,” Cluff said.

Assemblyman Sam Blakelsee, R-San Luis Obispo, who has a doctorate in geophysics, said  that much more research needs to be done. There is too much uncertainty about the size of the fault and the magnitude of earthquakes it could produce to say that the fault does not pose a safety threat to the plant, Blakeslee said. That is why he wants Pacific Gas and Electric Co. to do additional studies of the fault.

“This is not a cause for panic,” he said. “It’s a cause for concern.”

Specifically, research needs to determine whether the fault extends further south than currently thought, Blakeslee said. Generally, the longer a fault is, the more powerful the earthquake it can produce.

Research needs to also examine what happens during an earthquake to structures, such as the power plant, that are located close to a fault. It is common for buildings next to a fault to be unscathed after a quake while structures miles away are flattened, Blakeslee said.

County Supervisor Bruce Gibson said he had been briefed about the existence of the new fault.

“I think this is an important discovery they’ve made,” said Gibson, who represents the county’s North Coast from Los Osos to San Simeon. “The fault definitely needs further study, and they are on their way to doing that.”

Smaller than Hosgri fault

The new fault, which is unnamed, is thought to be smaller than the other fault off the plant’s coastline, the Hosgri fault, but it is closer to shore. The new fault is less than a mile offshore, while the Hosgri fault is about three miles offshore.

“Currently, the available geophysical data is not sufficient to definitely classify the offshore feature as a major fault, although a minor fault could exist,” Cluff wrote in a notice sent to regulators earlier this week.

The new fault is described as a vertical strike-slip fault as indicated by the earthquake activity along it. The two sides of such faults move horizontally.

Recent studies have indicated that the Hosgri fault is also a strike-slip fault, meaning it is less likely to cause a tsunami than other types.

It is estimated to be from nearly two miles to more than eight miles below the Earth’s surface and from nine to more than 15 miles long and could intersect the Hosgri fault, which is 68 miles long.

The utility is planning on doing additional geophysical surveys in the spring and summer of 2009, which will provide two-dimensional information about the new fault. Information from that work is likely to be available a year and a half from now, Cluff said.

Calls for new studies

In response to the discovery of the new fault, the state Energy Commission is recommending that PG&E and the operators of the state’s other nuclear power plant at San Onofre in San Diego County do seismic mapping and other advanced techniques regardless of whether it is warranted by a cost-benefit analysis. The agency will evaluate whether these studies should be required before PG&E can apply to renew Diablo Canyon’s licenses in 2025.

Although the federal regulators govern the operational safety of the plant, state regulators have a responsibility to ensure that Californians have reliable sources of electricity, Blakeslee said.

The Assemblyman said he wants PG&E to take the seismic mapping to the next level and produce three-dimensional maps, which are commonly used when siting offshore oil platforms. Such research is expensive and may require new legislation to provide the funding. Blakeslee promised to author such legislation, if needed.

Public outcry

The discovery of the new fault may renew a public debate over the future of Diablo Canyon in such a seismically active area. The Hosgri fault was discovered before Diablo Canyon was licensed, and caused the utility to spend billions of additional dollars to seismically upgrade the plant.

A large earthquake a year and a half ago in Japan caused the world’s biggest nuclear plant to shut down. That plant has not restarted, Cluff said.

“How many earthquake faults does it take to close down a nuclear power plant?” asked Rochelle Becker of the Alliance for Nuclear Responsibility, an anti-nuclear organization based in San Luis Obispo. “If there is an earthquake on the Hosgri fault, would that trigger a quake on this new fault and would there be a cumulative affect? I don’t know.”

Reach David Sneed at 781- 7930.

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