Deadly Analysis: Diablo Canyon Nuclear Power Plant Seismic Hazards

by Lee Andrea Caulfield

In the wake of the Fukushima disaster in Japan scientists have been investigating a potentially dangerous fault system near Diablo Canyon Nuclear Power Plant (DCNPP). As reported by Susanne Rust of California Watch , "while PG&E maintains the Hosgri fault is too small to threaten the aging plant, some government scientists suspect the fault acting with others in the region could produce an earthquake much more powerful than the plant was built to withstand."

The Diablo plant is surrounded by over a dozen seismically active faults, of which the characteristics and complexity are poorly understood. Some of these faults have been evaluated by geologists with the U.S. Geological Survey (USGS). "According to the Nuclear Regulatory Commission and PG&E the nuclear facility is safe. But scientist aren't so sure." (Mark Alpert, Federal News Digest)

The plant was built to withstand a 7.5 magnitude earthquake three miles off shore on the Hosgri Fault. However, Dr Samuel Johnson of the USGS laid out a scenario where the Hosgri Fault extends 250 miles from Point Conception (near Lompoc) to Bolinas north of San Francisco much longer than PG&E's estimate. The longer the fault the more powerful the rupture. Johnson's scenario shows the Hosgri Fault connecting in a system with the San Simeon and San Gregorio faults to the north of the nuclear plant.

According to Johnson,"If the fault were to extend this far than that would be close to a point 8.0."(Susanne Rust, Calif Watch).

Too further add to the uncertainly regarding the seismic hazard at the nuclear plant, Dr. Hardebeck of the USGS, discovered the Shoreline Fault in 2008 which is only 600 yards from the nuclear facility.

Susanne Rust wrote, "Hardebeck points out that  a M. 7.2 quake on the combined Shoreline and Hosgri faults would cause greater ground shaking at the nuclear plant than would a M 7.5 quake three miles off shore on the Hosgri fault acting alone."

Hardebeck indicates "If the Hosgri Fault runs up to Bolinas and it's connected to the Shoreline Fault there is a possibility of a magnitude 7.7 earthquake under the plant."

"The closer the fault, the greater the acceleration will be." according to former Senator Blakesley, who has a PH.D in earthquake studies.  (Sacramento Capitol Public Radio)

In a recent interview Dr. Hardebeck reported she has determined the Shoreline Fault does indeed connect with the Hosgri Fault. She also explained that "PG&E has called part of the Hosgri fault running from Cayucos to Big Sur, the San Simeon fault. Sam Johnson from the USGS shows clear evidence that the Hosgri and San Simeon faults are one continuous fault and could rupture together, potentially producing an earthquake 100 miles long."

Hardebeck says an earthquake can start on one fault and jump to other faults — this is what happened at the Fukushima power plant where scientists thought there were several unconnected faults offshore but when the earthquake occurred it ruptured through all of those fault segments resulting in a 9.0 M earthquake. (Susanne Rust)

Geologist, Eric Layman reported: "Even though existing published information provides ample evidence that a dangerous web of active faults surrounds the Diablo Canyon plant"… "PG&E continues to assure us that the nuclear plant is safe and consistently minimizes earthquake hazards to the plant."

A 2005 paper published by the Geological Society of America clearly links the Hosgri Fault with the San Gregorio Fault which in turn merges smoothly with the San Andreas Fault. This increase in the estimated length of the fault increases the risk of a higher magnitude earthquake than Diablo was built to contain. 

The Alliance for Nuclear Responsibility expressed concern that "the NRC miscalculates in refusing to investigate multi-fault rupture scenarios on the Hosgri and Shoreline Faults" and they are ignoring USGS studies.

A program on Sacramento Capitol Public Radio asks "As PG&E asks the federal government to renew its operating license for Diablo Canyon there are questions about whether the utility has been skewing data to make the plant appear safer from the threat of earthquakes than it really is." (Note licensing procedures have currently been temporarily suspended.)

If the plant continues to operate then as Andrew Christie of the local Sierra Club has written "down the road an earthquake greater than what the plant was built to withstand could render a large part of California unlivable and leave the central coast radioactive for centuries."

BIBLIOGRAPHY/Deadly Analysis

l. Susan Rust, , "Scientists Concerned About Fault System Near Diablo Canyon Plant, July 15, 2011, California Watch in conjunction with PBS and Capitol Public Radio
2. Mark Alpert, , "NRC says Diablo Canyon Safe - Case Closed?"
July 21, 2011, Federal News Digest, Understanding Government
3. Susan Rust, , "PG&E, USGS disagree on Diablo Canyon Fault Danger," July 17, 2011 SF Gate, San Francisco Chronicle
4.Dr Jeanne Hardebeck, interview, December 2012
5. Eric Layman, , "Diablo Canyon Couldn’t Withstand Worst Case Scenario Earthquake",   Sept 2011, SLO Coast Journal,  (the author received his B.Sc. And M.Sc.degrees in geology from Stanford University and has been in the geothermal energy business for over 30 years)
6.Alliance for Nuclear Responsibility, David Weisman, Oct 24, 2012
7. Joe Rubin, , "Double Fault at Diablo Canyon", July 14, 2011, Capitol Public Radio.