A hulking research vessel would haul air guns, echo sounders and other instruments along the California coast day and night for several months as part of a proposed $64 million effort to map seismic fault lines near the Diablo Canyon nuclear power plant.
The surveys, proposed by Pacific Gas and Electric Company, could produce a precise three-dimensional map of the fault lines, helping the utility estimate the location and magnitude of potential earthquakes. But the maps would do little to reveal the likelihood of a rupture, experts said, because the proposed surveys will not measure the speeds at which underlying tectonic plates are slipping past each other.
State lawmakers, alarmed that the facility’s reactors could melt down if they prove unable to withstand an earthquake along one or more of the area’s four little-understood faults, passed a measure in 2009 requiring PG&E to perform extensive seismic surveys. The legislation was vetoed by Gov. Arnold Schwarzenegger, but after last year’s earthquake-induced nuclear meltdown at the Fukushima Daiichi plant in Japan, the worst since Chernobyl, PG&E, under public pressure, announced that it was committed to completing the studies.
Information on the speed of tectonic movement could be gathered by placing an expensive array of global-positioning devices on the ocean floor, revealing how often the faults rupture; faster slip rates lead to more frequent earthquakes. But no such devices will be used, although utility officials said they could be in the future.
“To get an idea of how likely an earthquake is to occur, you need to have an idea of how often earthquakes occur,” said Jeanne Hardeback, a research geophysicist with the United States Geological Survey’s earthquake hazards team who has been tracking PG&E’s plans, which require the approval of multiple government agencies. “These studies do pretty much nothing to address that aspect of it.”
L. Jearl Strickland, director of nuclear projects at PG&E, said it “would be nice” to gather data that “gives you a more accurate slip rate.” But he said the utility will rely on calculations about assumed slip rates and that subsequent studies could provide more reliable data.
The studies are planned from September to December using the Marcus C. Langseth, a 235-foot research vessel owned by the National Science Foundation, which will tow four-mile-long streamers equipped with sonar equipment. The studies will be overseen by biologists and timed to minimize any impact on migrating whales. Conservationists and fishermen, however, said they fear that the powerful acoustic blasts will harm porpoises and whales and drive fish away from California’s Central Coast.
Bruce Gibson, a geophysicist who sits on the San Luis Obispo County Board of Supervisors and on a review panel charged with providing guidance on the surveys, questioned why PG&E is not using more advanced equipment, like that used by the oil exploration industry.
“What PG&E is proposing to do is something like a CT scan,” Dr. Gibson said. “Why aren’t you doing an M.R.I.?”