County Seeks Review of Quake Faults Near Diablo Nuclear Plant

Saying the destructive Central Coast earthquake Dec. 22 has increased concerns about the safety of Diablo Canyon nuclear power plant, the San Luis Obispo County Board of Supervisors is asking federal geologists to survey the area’s subterranean fault system.

“We’re behind in mapping our earthquake faults up here,” Supervisor Shirley Bianchi said this week. Bianchi said she is aware that the San Simeon/Hosgri fault that runs closest to the plant is not the one on which the quake occurred. “But we want to know how those faults may be connected and what impact the quake may have had on the San Simeon/Hosgri,” she said.

Led by Bianchi and fellow board member Peg Pinard, the supervisors voted unanimously this week to ask the U.S. Geological Survey for a thorough assessment of the county’s fault system. That move came a day after U.S. Sen. Barbara Boxer (D-Calif.) sent a letter asking the Geological Survey whether the county had been thoroughly surveyed. “Better information can ultimately lead to better planning,” Boxer wrote. “If there is a lack of information about the extent and threat of faults, I believe it is important to complete the necessary mapping.”

Although the requests have yet to be received and studied by the Geological Survey’s regional earthquake hazards office in Menlo Park, scientists there believe that much of what the supervisors and Boxer are asking for already exists. “There already are detailed maps of the fault systems there,” said Stephanie Hanna, a spokeswoman for the office. She said there also are natural hazard maps showing the areas where there is the greatest potential for damage.

The Geological Survey, however, said the relative paucity of modern recording instruments in the Central Coast area shows a need to develop a more comprehensive seismic network there. Scientists say that because of the lack of such equipment, they have yet to pinpoint the epicenter of last month’s quake, other than to place it somewhere near San Simeon.

The 6.5-magnitude quake caused damage estimated at more than $250 million, killed two people and injured more than 50. Most of the damage was in Paso Robles and Atascadero. Instruments showed that the nuclear plant experienced shaking densities of only 15% to 20% of gravity, not nearly enough to cause any damage.

The concrete walls of Diablo’s reactors are 3 1/2 feet thick, reinforced with six layers of 2 1/2-inch steel rebar. The plant, located about five miles northwest of Avila Beach, is designed to withstand a temblor 10 times the power of last month’s quake, according to Pacific Gas & Electric Co., which operates Diablo.

Bianchi said she knows geologists have said that because all known faults in the area are relatively small, it is unlikely that the county’s coastal region will experience the sort of catastrophic quakes possible along the San Andreas fault. She added, “They may not be worried about it, but they don’t live here.”