New studies, using state-of-the-art seismic mapping technology, show that fault lines threatening the Diablo Canyon nuclear power plant are more complex and interconnected than Diablo's designers could have known. This complexity negates the seismic predictions used to justify the plant's location. Unfortunately, we now know that the network of earthquake faults nearby and underneath PG&E's nuclear facility could be activated by a mega-thrust earthquake far to the north, at the Cascadia Subduction Zone. Subduction-zone earthquakes are the most powerful quakes in the world and can exceed magnitude 9.0.
The Cascadia, which begins near Vancouver Island, is a 620-mile-long fault line that intersects the San Andreas Fault just off Cape Mendocino in Northern California. This region of powerful and unpredictable earthquakes connects directly to the Diablo Canyon site. After evaluating the U.S. nuclear power facilities in the wake of the Fukushima disaster, the Nuclear Regulatory Commission has recently ranked the aging Diablo Canyon nuclear plant as uniquely vulnerable to unanticipated seismic activity, a "Group One … hazard."
In the early ’60s, Pacific Gas & Electric first chose Bodega Bay as the site for its proposed nuclear plant. The company began excavating the foundation, but then a fault line was discovered on-site, and that nixed the plan for Bodega Bay. PG&E then proposed a spot fairly close to where the Diablo Canyon plant sits today. This second location also got crossed off the list because of faults. When, finally, construction of the Diablo Canyon nuclear facility began near Avila Beach, PG&E maintained that there were no active faults within 30 miles of the facility. The plant was originally designed to withstand a magnitude 6.75 earthquake but was later upgraded to weather a magnitude 7.5 shaker. Unknown at the time, the plants’ two reactors were situated near undiscovered faults.