Viewpoint: Diablo seismic study is crucial to understanding risks

April 18, 2012 

  • Public sessions

    The State Lands Commission is holding two public sessions Thursday to take public testimony on PG&E’s application to conduct high-energy, offshore seismic mapping. The sessions will be at 2 and 6 p.m. at the San Luis Obispo County Government Center, 1055 Monterey St., San Luis Obispo.

For as long as it has existed, the issues surrounding Diablo Canyon nuclear power plant have been complex and the stakes have been high. The production of huge quantities of electrical power, with its enormous economic impact, brings with it difficult discussions on issues such as the handling of high-level nuclear waste and the safety of plant operations in a seismically-active region. We have long wrestled with the costs, benefits and risks of this powerful facility.

In the devastating aftermath of the Fukushima earthquake and tsunami, our concerns about risk have been raised further. The unthinkable did occur in Japan: An earthquake and tsunami much larger than expected caused a nuclear disaster that will not be resolved in the foreseeable future. That event has reasonably caused us to re-examine the risks of nuclear power and its vulnerability to natural disaster.

Assessments of earthquake threats to Diablo Canyon were under way well before the Fukushima event. Pacific Gas and Electric has studied the earthquake potential near Diablo Canyon since the 1980s, having discovered the Hosgri fault 3 miles offshore of the plant.

In 2006, state Sen. Sam Blakeslee sponsored legislation (AB 1632) to mandate review of the seismic hazard posed to both the San Onofre and Diablo Canyon nuclear plants. In 2008, the California Energy Commission issued recommendations for further intensive geophysical surveys in their AB 1632 report. Also in 2008, a fault was discovered to run along the shoreline near the Diablo Canyon plant, and the need for further study of earthquake risks was clear.

Full assessment of the earthquake hazard affecting Diablo Canyon requires a variety of geophysical studies, including a high-resolution, three-dimensional seismic reflection survey, conducted offshore both north and south of the plant. This survey will use reflected sound waves to produce 3-D images of subsurface geologic structures, most importantly, the location, size and orientation of faults capable of generating significant earthquakes.

The main sound sources for this imaging effort are “air guns” towed in the water behind the survey ship. The release of high-energy compressed air from these devices creates waves that reflect from geologic formations below the sea bottom.

It’s well-understood that the noise of these air guns will impact marine life and the activities of our commercial fishing industry. These impacts, and ways to mitigate them, have been studied in a draft Environmental Impact Report, prepared as part of the permitting process required of PG&E. Because the proposed survey covers a large area — more than 500 square miles of ocean surface — some of the impacts are considered significant.

Discussion is now starting on the upcoming decision of the California State Land Commission on granting PG&E a permit to conduct this survey. I believe it is essential that this survey be conducted, with three crucial conditions being met.

A 3-D offshore survey is fundamentally important to precisely locate the faults that could produce earthquakes. More importantly, we need to understand the full vertical and horizontal extent of each fault and how various faults connect with one another. The size, location and connectivity of these faults relate directly to the maximum earthquake they can generate and thus the strength of ground shaking at the power plant. Properly conducted, a 3-D seismic survey is unrivaled at providing a detailed image of these features.

The crucial conditions to be met are, first, that the survey is properly designed. The scale of the survey must be sized to cover only the geologic structures relevant to damaging earthquakes. Comments from the Independent Peer Review Panel, on which I sit as this county’s representative, have addressed the overall footprint of the survey and the panel is further considering the details of the survey design.

Second, impacts to marine life must be absolutely minimized. These impacts can be addressed by timing the survey to avoid migration and breeding seasons and having appropriate observers on board, so that operations are suspended if marine mammals are nearby. The issues of marine life disturbance by survey activities have been extensively studied in recent years and deserve the utmost consideration in the upcoming discussion of the EIR.

Finally, the financial impacts to commercial fishing operators must be properly compensated. The full evaluation of short and medium term loss of access and disturbance to fish stocks is essential. The experience of recent offshore activities, such as the placement of trans-oceanic cables, should serve as a model to determine fair compensation.

No matter what opinion one has of nuclear power generation in general, or the Diablo Canyon plant in particular, the knowledge to be gained from this 3-D seismic survey is crucial to a greater understanding of our region’s complex geology. That, in turn, will inform a more robust understanding of risks to the safety of Diablo Canyon power plant, a critical feature of our county’s environmental and economic landscape.

Bruce Gibson is the 2nd District supervisor for San Luis Obispo County. He holds a Ph.D. in geophysics, having specialized in seismic reflection survey techniques, and serves on the Independent Peer Review Panel convened to comment on PG&E’s seismic survey program.

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