Cascadia's Fault - Jerry Thompson [28]
Benioff’s 1949 study revealed the existence of two great faults, previously undiscovered: one off the coast of Tonga nearly 1,550 miles (2,500 km) long, the other off the coast of South America nearly 2,800 miles (4,500 km) long, both roughly 560 miles (900 km) wide and extending approximately 400 miles (650 km) downward into the interior of the earth. When he wrote that the South American sub-sea fault was “larger than any previously known active fault,” it almost sounded like bragging. Eleven years later, when the fault ripped apart and wrecked the coast of Chile in the largest earthquake ever recorded on scientific instruments, his words turned out to have been prophetic.
Benioff explained, “The oceanic deeps associated with these faults are surface expressions of the downwarping of their oceanic blocks. The upwarping of their continental blocks have produced islands in the Tonga-Kermadec region and the Andes Mountains in South America.” When he noted that “the continental mass flowed over the oceanic mass,” it sounded like an endorsement of the still heretical theory of continental drift.
What Benioff observed was that blocks of continental land seemed to be thrusting up and sliding over top of blocks of the sea floor. Another way to see it might be that slabs of the ocean floor were diving underneath the continental coastlines, cutting deep trenches offshore, scraping rock against rock, generating volcanoes, building huge mountain ranges like the crumpled fenders of massive collisions—and causing earthquakes in the process. When these oceanic cracks or faults occur close to the edge of a continent, he explained, the seafloor slab extends downward at a shallow angle of roughly thirty-three degrees. Of specific interest to young George Plafker was Benioff’s calculation that a fault under the Aleutian Island chain off the coast of Alaska dipped at an angle of twenty-eight degrees beneath the mainland.
The picture Benioff saw in a cluster of seismic dots—what geophysicists now refer to as a subduction zone—was a crucial missing piece of the still incomplete great tectonic puzzle. At the time he wrote, in 1954, a significant number of Benioff’s fellow seismologists were unwilling to embrace a concept that ran against conventional wisdom. To most experts of the day, a fault was a nearly vertical crack in the earth. Ten years later though, as Plafker stood in his muddy boots on the wrecked Alaskan shore, Benioff’s idea had the ring of truth.
As he began to write the first draft of his own report, Plafker looked at the plots of seismic data from Prince William Sound and concluded that a “low-angle fault” had caused the catastrophic earthquake of 1964. He described a crack in the crust that was almost horizontal instead of vertical—sideways compared to the San Andreas. He described a colossal continental collision in matter-of-fact terms, with what seemed to Plafker a logical conclusion about what had caused the rupture and why there was no visible fault at the surface. But a politically significant part of the science community did not agree.
“No, no, no. Not much of this was accepted,” Plafker laughed. “Hell, people gave me big arguments about Alaska. And the trouble was that one of them happened to be a world-class scientist who later became the president’s science advisor and the head of the National Academy of Sciences.”
It came down to a question of geometry. Frank Press, the prominent seismologist at Caltech who later became science