New seismic analysis around Diablo Canyon nuclear power plant has significantly downgraded the threat posed by the Hosgri Fault, the dominant earthquake feature in the area.
When the plant was designed in the 1980s, the Hosgri was estimated as being able to produce a maximum quake of 7.5 magnitude. A spate of recent studies and more sophisticated geologic modeling have given scientists 10 times more data about the Hosgri Fault, said Norm Abrahamson, a PG&E seismologist.
“Now the risk is about one-half as great as we thought it was in 1988,” he said.
While the Hosgri Fault still remains the dominant seismic feature in the area, its shake potential at the plant is now about the same as three other smaller faults near the plant, the Los Osos, San Luis Bay and Shoreline faults. The quake potential of a fault is determined by its proximity, what type of fault it is and other factors, Abrahamson said.
All of these faults would likely produce quakes ranging from magnitude 6.0 to 6.5 at the plant. The plant is designed to withstand a magnitude 7.5 quake.
The new information was the result of a two-year study that was presented to the Nuclear Regulatory Commission on Wednesday. NRC officials said it will take them about a year to study the bulky new report and come to their own conclusions.
The study was prompted by the discovery in 2008 of a new earthquake feature — the Shoreline Fault — by the U.S. Geological Survey. This fault sits less than a half-mile offshore of the plant.
PG&E officials said the utility will now start doing a new set of three dimensional seismic studies around the plant. These studies will involve techniques that measure much deeper into the ground and will take about three years to complete.
Recent public meetings have shown that earthquakes top the list of public safety concerns about the plant.
Reach David Sneed at 781-7930.