Cascadia's Fault - Jerry Thompson [38]
Plafker told me that after the speech Press cornered him and unloaded. “He came up and he was real mad,” Plafker recalled. “He said, ‘You know, I’ve written a lot papers and I’ve seldom been proven wrong. But you did it to me this time!’ He told me I had caught him in the biggest mistake he made in his career,” said Plafker. “His views on the mechanism of this earthquake had changed and he was man enough to say so.”
But not everyone was convinced. Clarence Allen, another senior scientist who’d heard Plafker’s talk, still needed convincing. And he threw down a challenge that Plafker simply could not resist. How, he asked, could this underthrusting of the ocean floor be happening only in Alaska? Did Plafker think that’s what happened in the Chile earthquake as well? Plafker said yes—even though he didn’t know for sure—and so Allen arranged the funding necessary to send him south.
Plafker spent two months scouring the Chilean coastline by car along the mainland and by chartered boat in the islands of the southern archipelago, measuring areas of heaved-up and down-dropped land. “There again in Chile, in the southern part, vegetation grows right down to the shorelines,” he said. “You could see the effects of subsidence from the drowned and dead trees and brush.” He found a zone of “tectonic warping, including both uplift and subsidence,” that was 125 miles (200 km) wide and roughly 625 miles (1,000 km) long. It affected an area of at least 50,000 square miles (130,000 km2) in southern Chile.
The two-day series of temblors in 1960 had included two main shocks, thirty-three hours apart, along with fifty-six large aftershocks. The sequence of ruptures and the tsunamis they triggered killed 2,000 people in Chile and 230 more in Japan, Hawaii, and the Philippine Islands.
The main finding of Plafker’s paper, however, refuted the previous conclusion that Chile’s wreckage had been caused by a nearly vertical strike-slip fault (like the San Andreas) because it was based on “incorrect” data that were “clearly incompatible” with his newer evidence of tectonic movement. He wrote that the Chilean main shock “resulted from a complex rupture on a major thrust fault or zone of thrusting roughly 1,000 km long that dips at a moderate angle from the continental slope beneath the continental margin.” In other words, the fault was more horizontal than it was vertical, just like in Alaska.
Plafker estimated that to cause such widespread upheaval and deformation of the landscape, there must have been at least 65 feet (20 m) and perhaps as much as 130 feet (40 m) of horizontal slip once the fault broke. This might seem “surprisingly large,” he wrote, but “not excessive” if compared to the horizontal thrust of roughly 65 feet he had seen in the 1964 Alaska earthquake. The bottom line appeared to be that the events had both been caused by the same process: two pieces of the earth’s crust crashing together.
So his gambit in Chile had been a success; he was able to prove plate convergence. “It was very straightforward, once you know what you’re looking for,” he said, putting the apparent mistakes of the first scientists on the scene in Chile into some kind of context, “but you know, it’s just like anything else. If you’ve done it once before, it’s a cinch. And if you haven’t, you don’t know what to do.”
Nevertheless, when Plafker met Gary Carver a few years later and they compared notes on Alaska, Chile, and northern California, the similarities were hard to miss. “It’s pretty clear to me,” said Plafker, “that the southern end of Cascadia is very much like the eastern end of the Aleutian Arc and the area where the ’64 earthquake occurred. We have the same type of continental margin.”
When I asked Gary Carver why he thought it took so long for most geologists to come around to the view that Cascadia was a threat, he could remember clearly one paper—written by Masataka Ando of the U.S. Geological Survey and Emery Balazs of the