Even before the Fukushima disaster, state law mandated that utilities conduct extensive seismic studies of nuclear facilities, but did not specify the type of research.
Perched on an 85-foot bluff along the scenic Central Coast, Diablo Canyon sits within three miles of two underwater earthquake faults, including one that was discovered in 2008.
PG&E came up with a four-pronged approach that includes the use of high-energy seismic imaging technology. Under the ratepayer-funded study, a research boat would tow 18 air guns that would emit sonic blasts into the ocean every 10 to 20 seconds for several days. The utility had hoped to conduct the seismic survey between November and December to avoid peak breeding and migration seasons.
In August, a State Lands Commission environmental impact study determined there would be unavoidable consequences to marine life during the testing. But the panel ultimately decided the project's benefits outweighed the environmental risks.
PG&E faces a big test before the coastal commission, which received scores of letters opposing the offshore study. Conservation groups contend the company has not done enough to explore other less damaging options.
"If you're going to harm the coast, you've got to make sure that there's no alternative," said Michael Jasny, an attorney with Natural Resources Defense Council.
To minimize impact to sea life, PG&E proposed starting off with one air cannon at a low decibel before ramping up to full power. It also planned to have spotters on the vessel and in an aircraft to alert operators of marine mammals in the region. Air guns will be silenced and work will cease if an animal strays too close.
The twin-reactor Diablo Canyon generates enough electricity to power more than 3 million homes in Central and Northern California. After the Japanese nuclear crisis, the utility asked federal nuclear regulators to delay issuing extended operating permits until thorough seismic studies are completed. The permits expire in 2024 and 2025.