Seismic Safety at Diablo Canyon

All U.S. nuclear power plants, like Diablo Canyon Power Plant, are designed to withstand the most severe ground motions, or shaking that experts believe could be generated from earthquake faults in their specific locations.

PG&E remains focused on ensuring that Diablo Canyon continues, and improves upon, its strong record of safe operations. PG&E employs a seismic department staffed with experts who continually study earthquake faults in the region and global seismic events as part of the plant's comprehensive safety program, known as the Long Term Seismic Program.

In November 2008, the U.S. Geological Survey (USGS), working in partnership with PG&E's Geosciences department, identified the Shoreline Fault zone and PG&E evaluated whether that new feature presented a safety risk to the plant. PG&E submitted its evaluation to the Nuclear Regulatory Commission (NRC) under the commitment of its current operating licenses. In 2012, the agency confirmed Diablo Canyon has adequate safety margin to withstand the maximum ground motions from faults in the region, including the Shoreline Fault.

The advanced seismic studies will help provide a more detailed picture of the region’s complex geology. The research was called for by the State of California, and will help further define the level of seismic activity and ground motions that earthquake faults in the region, including the Shoreline Fault, are capable of producing.

PG&E completed the 2D/3D onshore and 3D low-energy offshore studies in 2012, and installed ocean-bottom seismometers in 2013 to detect and record seismic activity.

An additional 3D high-energy offshore study did not receive approval by the California Coastal Commission in 2012.

PG&E is currently focused on evaluating the data from the recently completed onshore and offshore studies, as well as other available geophysical data that have been used to define the characteristics of offshore faults, to determine whether or how to proceed with additional data collection efforts under the advanced seismic study program.
PG&E will use the data to support its ongoing work to continually assess and validate the seismic design of the plant. PG&E will also share information collected with local public and government agencies so they can incorporate it into their respective emergency preparedness plans and ensure the safety of critical infrastructure.

The data collected will also support a new, federally mandated seismic risk evaluation. Under this process, existing and new seismic information is reviewed and publically evaluated by independent experts that make up the Senior Seismic Hazard Analysis Committee (SSHAC). The Committee’s conclusions will be used to update the model that characterizes the seismic hazard near Diablo Canyon.

All nuclear power plants are required by the Nuclear Regulatory Commission to conduct such an evaluation after the Fukushima Daiichi power plant tragedy in Japan.
Using state-of-the-art scientific methods, the data derived from PG&E's advanced seismic studies will help further define the level of seismic activity and ground motions that onshore and offshore earthquake faults in the region are capable of producing.

2D/3D Onshore Research: From 2010-2012, vibrosis and accelerated weight drop trucks produced sound waves that penetrated the Earth's crust several kilometers deep. More than 2,000 strategically-placed recording devices captured the reflected sound waves, producing data that experts are currently interpreting to help further define the seismic characteristics of nearby onshore faults.

PG&E Demonstrates Seismic Test Equipment for Public

3D Low-Energy Offshore Research: In 2012, PG&E completed low-energy seismic research work off portions of California's Central Coast. PG&E began the first phase of this low-energy offshore study in 2010, and completed the second portion in 2011. The third phase, finished in 2012, studied offshore faults near San Luis Bay, Estero Bay and Point Sal.

To perform the work, equipment onboard a research vessel generated low-energy sound waves that were directed toward the ocean bottom. An array of sensors, or hydrophones, towed behind the vessel captured the sound waves as they reflected back to the surface. The data were recorded and are being used to create a detailed three-dimensional map of offshore seismic faults. Like the onshore work, this data will be used to help further define the seismic characteristics of nearby faults.

Ocean Bottom Seismometer Project: In 2013, PG&E deployed four long-term and two temporary seismometers on the sea floor as part of the Ocean Bottom Seismometer Project. The seismometers are passive listening devices that will detect and record seismic activity in the region of Diablo Canyon Power Plant. The devices are located three nautical miles offshore at an approximate depth of 110 meters. An underwater cable connects the sensors to an onshore recording station, which relays data to researchers. The data derived from the project will be used by experts to provide a more accurate and detailed picture of the region’s complex geology, supporting PG&E’s continual commitment to seismic safety.
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