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 |
The key finding of the Benioff-Smith report comes in the last paragraph of the report:
So how did Benioff and Smith come up with the earthquake? It seems that the famous Cal Tech geologist Clarence Allen had given several talks in which he presented a slide of the spatula distribution of aftershocks of a large Chilean earthquake. Since, according to their Cal Tech geologist colleague Dick Jahns, who had studied the Diablo site, there were no active faults there, the magnitude of the earthquake -- or aftershock, would have to be 6.75 or less, otherwise the fault would have broken through to ground surface, and that, according to Jahns, had never happened a the plant site, at least during the last 100,000 years.
Clarence Allen's superposition of Chilean earthquake aftershocks on a map of California. |
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