Diablo Canyon earthquake vulnerability

Diablo Canyon (Nuclear) Power Plant, located in San Luis Obispo County California, was originally designed to withstand a 6.75 magnitude earthquake from four faults, including the nearby San Andreas and Hosgri faults, but was later upgraded to withstand a 7.5 magnitude quake. It has redundant seismic monitoring and a safety system designed to shut it down promptly in the event of significant ground motion.

In 2008 the Shoreline Fault, which passes less than a mile from the plant, was discovered. The fault has the potential of triggering a 6.5-magnitude earthquake. Because the Richter magnitude scale is logarithmic, Diablo Canyon is designed to withstand an earthquake of shaking amplitude ten times larger than that which the Shoreline fault is capable of triggering, based on an analysis by plant owner Pacific Gas & Electric (PG&E).

Diablo Canyon Power Plant (DCNPP/DCPP) is located proximal to the Los Osos, Hosgri, San Andreas and Shoreline faults. The discovery of these faults required design modifications during construction of the plant.

The Shoreline Fault is described in the November 2008 PG&E report as an "alignment of microseismicity subparallel to the coastline indicating the possible presence of a previously unidentified fault located about 1 km offshore of DCPP." The plant's current operating license expires in 2024 and, in the aftermath of the Tohoku earthquake and tsunami, there is renewed opposition due to public perception that the risk of earthquake or tsunami might make the plant unsafe. Re-licensing is contingent upon consistency with the Coastal Act and thus review by the California Coastal Commission, however seismic issues are more properly within the purview of the Nuclear Regulatory Commission. The NRC in June 2011 announced that it had already completed its Safety Evaluation Report (SER) for the plant. In July, 2016 PG&E announced that it does not plan on relicensing either of its two Units.

Critics contend that the Diablo Canyon (Nuclear) Power Plant was built so close to a set of geological fault lines that it is "for all practical purposes" to be regarded as built "directly over" a fault. They refer to the Hosgri fault, which was discovered while the plant was under construction. No scientific corroboration for these opinions has been presented, and PG&E maintains that "new and extensive scientific re-evaluations performed at the direction of the Nuclear Regulatory Commission (NRC) continue to show that Diablo Canyon can safely withstand earthquakes, tsunamis and flooding that could potentially occur in the region."

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