Letters to the Editor

Viewpoint: PG&E must make careful assessment

The lessons of the earthquake-spawned environmental disaster at the Fukushima Dai-Chi nuclear power plant in Japan are still being learned. However, it is clear that the scientific community seriously underestimated the seismic risk to this facility with catastrophic results.

Closer to home, PG&E continues to be unwilling to halt the relicensing process for the Diablo Canyon nuclear power plant while a new 3-D seismic hazards study is still in progress. There is simply no excuse for PG&E’s rushing ahead with the relicensing process when the health and safety of so many communities continues to be at risk while the Diablo Canyon nuclear power plant operates in an active seismic belt.

Furthermore, it is imperative that PG&E’s new study not be narrowly focused on the immediate area of the facility and the newly discovered Shoreline Fault. The scope of the study must be expanded to include the broader geologic setting and the potential for links to surrounding fault zones, and must focus on two major concerns regarding seismic safety at the Diablo Canyon Power Plant:

The risk of undetectable “blind” thrust faults near the Diablo Canyon nuclear power plant. These are faults that have no surface expression and can generate sizeable quakes. The U.S. Geological Survey (USGS) reports that up to one-third of quakes less than magnitude 6.7 occur on “blind” faults that have not been previously detected, and that quakes up to magnitude 7.5 have been reported historically on such blind structures worldwide.

This suggests that, as with the Sendai quake in Japan, much larger events are possible on blind structures if we look beyond the short time range of human records.

Indeed, our very own magnitude-6.5 San Simeon quake of 2005 was a textbook example of a blind rupture, with the epicenter just 30 miles from the Diablo Canyon Power Plant. The 1994 magnitude-6.7 Northridge quake in the L.A. basin was another blind thrust fault.

The big concern is that these blind structures are not detectable by traditional geologic mapping at the surface, and for a variety of technical reasons (lack of offset beds, etc.) they may not even be detected on a 3-D seismic study such as is under way for Diablo Canyon. Thus it is possible that even the current study may not be able to provide the required high level of certainty that such blind faults do not exist in close proximity to, or even directly under, the plant.

If that is the case, and given the horrible environmental consequences of a nuclear accident, the most prudent course of action must be to simply prohibit operation of all nuclear power plants in seismically active belts.

The risk of much larger quakes on the Hosgri Fault than currently planned for by PG&E at Diablo Canyon Power Plant. The Hosgri Fault passes about 2.5 miles offshore from the Diablo Canyon nuclear power plant.

The Hosgri Fault “proper” is currently assessed by the USGS as capable of generating a maximum quake of 7.54. The California Energy Commission reports that Diablo Canyon Power Plant is designed to withstand a magnitude-7.5 quake on the Hosgri Fault. However, the consensus in the geologic literature is that the Hosgri Fault is part of a much longer and potentially much more destructive feature. The Hosgri Fault merges to the north with the San Gregorio Fault zone, which in turn merges smoothly with the San Andreas Fault near Bolinas Bay north of San Francisco.

Should the entire 450 kilometer combined length of the Hosgri/San Gregorio Fault zone rupture at once, the USGS estimates this would produce a magnitude-8.2 event (Wells and Coppersmith scaling method). Such an event would be about 11.2 times more powerful than the current design capacity of the Diablo Canyon Power Plant.

Furthermore, should the rupture extend beyond the Hosgri/San Gregorio complex onto the northern San Andreas Fault, then an even larger, extremely destructive event would be produced.

I urge that the scope of the PG&E seismic study be expanded to include both a careful assessment of the risk of blind thrust faults at the plant and a re-assessment of the potential for large magnitude events along the linked Hosgri/San Gregorio/San Andreas fault system.

Finally, the public must have the opportunity to review the results of this study before any final decision on relicensing is reached.

Erik B. Layman received his bachelor of science and master of science degrees in geology from Stanford University and has been in the geothermal energy business for more than 30 years.

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