The Earthquake that Haunts

Long before Fukushima  there was one earthquake that counted the most for the Diablo Canyon nuclear plant, and that earthquake has been kicked around now for half a century. Today I received a copy of that much discussed but rarely seen report on Diablo Canyon seismicity written by Hugo Benioff and Stewart Smith, which I include in full later in this post. True,  the findings date to the mid 1960s when many things were not known. As Smith put it to me in retrospect:

In the Geological Sciences we were still puzzled over the worldwide distribution of seismicity because the plate tectonic model had yet to be discovered.  Benioff had much earlier  recognized dipping zones of earthquakes, which we now call subduction zones, off the coast of South America and elsewhere, but there was not yet an overarching theory to explain how these were related to features like the San Andreas, which we now recognize as  transform fault necessary to account for the plate motion produced by subduction.  As I recall this was an era in which Clarence Allen at Caltech had pronounced that, based on the widespread nature of aftershocks in Chile, a large earthquake could occur anywhere in California.  I don't recall the reference, but I do remember the slide he would show where he overlaid the aftershocks of the Chile earthquake of 1960 on a map of California.  The entire state was filled with earthquakes.

Victor Hugo Benioff

But the Benioff-Smith report conveys remarkably sound judgment in many respects, notably in its conclusion that the Diablo site is subject to a magnitude 6 3/4 earthquake occurring directly beneath the plant. The effects of exactly this type of event have just been presented by PG&E and its SSHAC committee to the NRC, and they are worth examining, contradicting as they do key elements of  the Benioff-Smith conclusion and design recommendation. We can easily and will examine this proposition later by using various online tools to estimate ground shaking at the Diablo Site and in a manner that should be understandable to any bright high school student. 

The key finding of the Benioff-Smith report comes in the last paragraph of the report:

Worried about acceleration?

Ambraseys first law of earthquake
engineering, Imperial College, 1965.
Solid line good, dashed line, bad

In the old days engineers who were old timers (back then, I was a new kid) used to talk about horizontal ground acceleration as the principal source of   disturbance, damage, and failure in structures. Then in the late twentieth century we had some medium sized earthquakes like San Fernando and Northridge down in LA, with these big sharp accelerations, one g or more, twice as much as what experts like George Housner and Nathan Newmark, whom we all respected, had said would be likely in earthquakes; these leaders had led us to believe (we said to ourselves) that we didn't have to worry about more than 0.5 or 0.6g, that's about all the geophysics of the earth could produce and even my prof, Nick Ambraseys, (I've got it her somewhere in my 1965 lecture notes at Imperial College, something about stress drop and velocity and limits.) So everyone, seeing these high accelerations began to waffle around: There must be something wrong about this record. Or: just hold on here, my structure, it doesn't care about high frequency vibration, it worries about longer period motion. 


And then the vision of those old WW2 army barracks being pulled down comes back, that long noisy gradual fall.

Cal Tech engineer Tom Heaton, whose style is much in the solid practical tradition of George Housner or Nick Ambraseys, suggests that designing for peak ground acceleration or for targeted response frequencies, could even be dangerously misleading:

While I understand that many structural fragilities are described in terms of PGA, its use only leads to dangerous mischaracterization of earthquake risk....5% damped response spectral acceleration (sa) is also a common parameterization of shaking intensity. While SA is certainly more useful than PGA, there are serious concerns about using it to predict structural demand. In particular, sa is based on a linear analysis of a structure about its undeformed state. However, there will always be significant ductile yielding prior to catastrophic failure of a structure, and it is more meaningful to use parameters that better characterize a structure that is in its highly deformed state. For example, structures that yield plastically have much lower effective stiffness and much higher effective damping than is typically assumed in current practice. 

1964: Diablo Canyon gives (breech) birth to The Friends of the Earth

Diablo Canyon site in the early 1960s

Feb 27, 1963: Pacific Gas and Electric(PG&E) announces plans to build 5 nuclear reactors at Nipomo Dunes, California.

May 4,1963: The Sierra Club Board meets to discuss PG&E’s plan to build the Nipomo Dunes facility. The club president and Richard Leonard agree to meet with the PG&E president. A few months later the Sierra Club agrees to support the The Diablo Canyon site which becomes the new alternative to Nipomo Dunes and PG&E starts detailed site investigations at Diablo Canyon.

May, 1966: Sierra Board members tour the site and discover it houses the world’s largest Oak trees, a sacred burial ground for the Chumash Indians, the second to last coastal wilderness area in the state and the home to the largest Abalone site in California. The site was also disclosed as being proposed as a state park.

June, 1967: A major campaign to reverse the Sierra Club endorsement is started by several board members and David Brower, leading to the formation of a Sierra Club breakaway group, Friends of the Earth.

Thanks to Enformable for these excerpts from their chronology of Diablo Canyon. Enformable is a rich news digest related to the subject of nuclear power. Enformable.com

Pop quiz: San Simeon or Irish Hills?

Some interesting studies today. 

Above are two hypocenter cross sections (within the circles) crossing the upthrust zones of the area of the San Simeon M6 earthquake of 2003 (close to NS section) and the other crossing the Irish Hills (SW-NE section) at the site of the Diablo Canyon plant, the latter with no recent M6 earthquakes. But maybe one on the way?

The San Simeon cross section is for the period 1988 through 2002, before the 2003 earthquake occurred. If you can identify which of the above is San Simeon do you see portents of a coming earthquake?  All earthquakes in the Hardebeck catalog  from 1988