Geologist on Big Quake Risk at CA Nuke Plant: 'We've Not Ruled It Out'

Diablo Canyon nuclear power Courtesy of PG&E via

The  scientist who discovered a new earthquake fault just offshore from California's Diablo Canyon nuclear power plant believes it could bring a major earthquake right to the facility's doorstep.

Jeanne Hardebeck, a research geophycicist at the U.S. Geological Survey, discovered the 25 km-long Shoreline Fault in 2008. Diablo Canyon, which went online in 1985, was designed to address the threat of the Hosgri fault, nearly 5 km offshore. According to the plant's operator the new fault lies just 600 meters from the reactors.  

By itself, the Shoreline fault is not considered capable of producing a major quake. "The important issue is whether the two faults can rupture together," Hardebeck tells Rolling Stone. A rupture beginning on the Shoreline Fault that continued on the Hosgri Fault could bring the maximum earth-shaking power of the larger fault directly to the nuclear facility. "We’ve certainly not ruled it out," Hardebeck says.

Diablo Canyon is engineered to withstand a 7.5 earthquake from the more distant Hosgri Fault — a design based on the USGS projections that that fault is likely to max out at a 7.3 magnitude temblor. (For comparison, the cripled Fukushima Daiichi nuclear plant in Japan was built to withstand a 7.9 quake, but got hit by a 9.0.) But Hardebeck cautions that the USGS estimate "is not a very precise number." While she has not calculated the margin of error, Hardebeck says "it’s certainly a few magnitude points of uncertainy — and possibly even more than that.

"A 7.7 on that fault would not be surprising to me," Hardebeck says. 

To recap: A nuclear power plant designed to withstand a 7.5 magnitude earthquake at a distance of 5 km could potentially be subject to a larger quake at a distance of just a few hundred meters. Or closer. The Shoreline Fault is still being studied, and it's an open question whether branches known as "fault strands" could extend onshore. 

For its part the Nuclear Regulatory Commission pooh-poohs the dangers of the Shoreline Fault, based on self-reporting by plant operator PG&E. According to a statement provided to Rolling Stone:

The licensee, Pacific Gas and Electric (PG&E),  has evaluated the ground motion levels at the Diablo Canyon Power Plant (DCPP) from the Shoreline fault and concluded that these motions would be bounded by the ground motion levels previously determined for the larger Hosgri fault.

The statement adds an un-reassuring note: "Although the fault rupture is unlikely to occur directly under the plant, it may cause deformations in the near field.  The potential impact of ground deformation primarily involves the buried components, such as piping and conduits."

I feel safer already.