It is abundantly clear that the Japanese power industry seriously underestimated the seismic risk to the Fukushima-Daichi nuclear power plant in Japan, with catastrophic results to surrounding communities. Here at home, PG&E continues to assure us that the Diablo Canyon nuclear power plant (DCNPP) is safe and consistently minimizes earthquake hazards to the plant in its public statements.
Yet the seismically active Shoreline Fault, located just 600 yards from the plant, was only discovered by the U.S. Geological Survey in 2010. PG&E is currently undertaking a new 3-D seismic study to evaluate the seismic hazard risk at DCNPP, providing further confirmation that PG&E does not yet have a complete picture of the seismic risks at DCNPP in spite of its definitive statements that the plant is safe.
The 3-year time frame of the new seismic study also conveniently pushes the problem down the road when our memories of the horrors of Fukushima-Daichi will be less fresh. In the meantime, central coast residents are constantly exposed to the risks tied to the continuing operation of DCNPP. Even without results from PG&E’s new study, existing published scientific information provides ample evidence that a dangerous web of active faults surrounds the DCNPP.
Recent studies have determined that these faults in turn link to very large scale fault zones which are capable of very large magnitude earthquakes, significantly larger than the 7.5 magnitude (M) design limit of DCNPP.
My objective here is to briefly summarize for the local community the current state of knowledge regarding active faults near DCNPP and their relation to very dangerous large-scale faults. The two maps of active faults included with this article are intended to provide a clear illustration of this dangerous web of faults for the local community, and raise community awareness of the serious risk of a large-scale earthquake that could cripple DCNPP and potentially release radiation into our central coast environment.
Map of Active Faults and Historical Earthquakes in the San Luis Obispo Region
(Click Here to View PDF File)
This map pulls together information from several recently published scientific papers as listed on the map. The map shows active faults in the San Luis Obispo region which were identified using a variety of methods: 1) mapping of linear belts of epicenters of earthquakes using seismic monitoring networks; 2) mapping of faults on the surface which cut very young geologic deposits, indicating recent activity; 3) mapping of offshore faults which cut very young deposits using seismic reflection methods; and 4) historical records of earthquake activity.
This map shows that DCNPP is sited within a highly active network of faults which has spawned numerous sizeable earthquakes in recent decades. Key active fault zones in close proximity to DCNPP include the major Hosgri fault, located 2.5 miles offshore from DCNPP; the Oceanic – West Huasna fault; the Los Osos fault; and the Shoreline fault, located just 1800 feet offshore from DCNPP.
Studies confirm that the The Shoreline fault links with the Hosgri fault about 5 miles northwest of DCNPP, a linkage which has critical importance to the seismic hazard risk at DCNPP as described below. The most recent sizeable earthquake within this seismic belt was the M 6.3 San Simeon quake of 2003 which was centered just 30 miles north of DCNPP. This quake occurred on a splay of the Oceanic fault zone which had not previously been recognized by the scientific community, and was thus a "blind" fault.
Such blind faults are particularly pernicious, since they are only revealed when they produce a destructive earthquake. Indeed, we cannot eliminate the possibility that an undetected blind fault directly underlies the DCNPP, one which could rupture the plant’s foundations during a quake. The largest historical event in the San Luis Obispo County region was the M 7.1 quake centered west of Lompoc in 1927, which may have been associated with the Hosgri fault zone. This quake produced a small tsunami wave which impacted the central coast region.
Map of Major Faults Associated with the San Andreas Fault Zone
(Click Here to View PDF File)
This map provides a larger-scale view and is derived from a 2005 paper published by the Geological Society of America. The map shows the relation of the larger active faults near the DCNPP to major regional-scale fault zones associated with the San Andreas fault. The San Andreas is a major tectonic plate boundary and comprises one of the largest fault zones on the planet.
The critical feature of this map as it relates to DCNPP is the linkage of the Hosgri fault with the San Gregorio fault to the north, which in turn merges smoothly with the San Andreas fault. This linkage extends the effective length of the Hosgri fault to at least 450 kilometers.
Fault length, and hence the potential length of fault rupture during a single earthquake event, is critical for estimating the potential magnitude of an earthquake event. The California Energy Commission reports that DCNPP is designed to withstand a M 7.5 quake on the Hosgri fault. However, this maximum credible quake estimate for the Hosgri does not take into account its linkage with the San Gregorio / San Andreas fault system, which significantly increases its destructive potential.
Should the entire 450 kilometer length of the Hosgri-San Gregorio fault zone rupture at once, the USGS estimates this would produce a M 8.2 event (Wells & Coppersmith scaling method). Such an event would be about 11.2 times more powerful than the current design capacity of the DCNPP, and would likely lead to plant failure and radiation release. 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.
A final critical point is the linkage between the Hosgri and the newly-discovered Shoreline fault. This linkage extends the reach of the dangerous, 450 kilometer-long Hosgri-San Gregorio fault zone to within 1800 feet of DCNPP. This further highlights the inadequacy of PG&E’s assessment of the seismic safety of DCNPP.
PG&E has not only seriously underestimated the length and destructive potential of the Hosgri, but also has significantly overestimated the separation between DCNPP and this structure, via linkage along the Shoreline fault. The 1800 feet separation is just 15% of the 2.5 mile separation distance used by PG&E in its assessment. The greater proximity of the potential rupture zone to the DCNPP increases shaking intensity and the potential for much greater damage at the plant compared to PG&E’s current assessment.
The U.S. Geological Survey believes that a M 8.2 earthquake caused by a 450 kilometer-long rupture of the very dangerous linked Hosgri-San Gregorio-Shoreline fault zones is a realistic worst case scenario for the DCNPP. DCNPP clearly is not designed to withstand such an event, and PG&E clearly has not addressed this scenario in their assessments of the seismic risk for the plant. Thus PG&E’s continued statements that DCNPP is safe seem hollow indeed.
In light of the above, we can only assume that PG&E sees the safety of central coast residents as a secondary concern, while placing highest priority on insuring continued operation of DCNPP and continued flow of plant revenue to PG&E shareholders.
Erik B. Layman received his B.Sc. and M.Sc. degrees in geology from Stanford University and has been in the geothermal energy business for over 30 years.
