Cascadia's Fault - Jerry Thompson [29]
The math will yield two potential answers, only one of which can be correct. One solution will be a geometric plane, a cross-section of the earth, that passes through the focal point of the earthquake. The alternative solution—another slice or cross-section that also passes through the quake’s focal point—is perpendicular to the first. The angle of the fault (and thus the movement of rock slabs during the quake) will be along one of these two planes. But with no obvious rupture at the surface—with no way to examine the crack and “ground truth” the answer—these mathematical plots of shockwave data from deep underground become hypothetical. And in this case, controversial.
When he examined the diffuse pattern of the thousands of aftershocks, Plafker thought the correct solution was obvious: “If you have a big blur of aftershocks, like in ’64, it seemed to me that you could say, ‘Well, it’s the low-angle plane because [the aftershocks are] spread out over a broad region.’” A vertical fault would presumably have produced a vertical or linear pattern of aftershocks. This one did not.
Indeed, the Alaska quake had generated an angular haze of twelve thousand small to moderate tremors that extended 90 to 125 miles (145 to 200 km) from the epicenter—the rupture began underneath the continental mainland—out to sea, reaching the inner wall of the deep Aleutian Trench. Plotted across a wide band of geography, these seismic dots looked like a bad rash on the underbelly of the continent. Connect the dots from the epicenter out to the trench and you might find the missing fault.
In June 1965, when his own paper was published, Plafker openly discussed the conflicting fault plane solutions and acknowledged the elephant in the room. He wrote that everything scientists knew about the angle of this invisible rupture zone had been “deduced from seismological data,” the implication being that nobody could say for sure what kind of fault it was. “Neither the orientation nor the sense of movement on the primary fault is known with certainty,” he wrote. They were all making educated guesses.
Three weeks later, Plafker’s dissenting view made headline news. Walter Sullivan of the New York Times, a regular reader of the latest in Science, saw Plafker’s paper and quickly cranked out a feature article that highlighted the split between Plafker and Press. “Data collected by a number of field parties during the last year has given birth to two alternative accounts of the mighty rupture within the earth,” wrote Sullivan as he zeroed in on Plafker’s main conclusion. “He believes that a mass of material from beneath the ocean floor suddenly thrust inland under the continental rocks.” While most non-scientists probably missed the subtle implication, geophysicists around the world knew that Plafker had essentially said that the emperor had no clothes.
CHAPTER 5
Cauldron and Crust: The Rehabilitation of Continental Drift
A non-geologist might wonder why it really mattered whether the Alaska fault was vertical or horizontal, but the implications for the West Coast were dire. If Plafker and Benioff were right about that slab of ocean floor poking down underneath the continent, then in all likelihood the same would be true for British Columbia, Washington, Oregon, and northern California. An earthquake like the Good Friday disaster would affect millions of people if it happened to the Pacific Northwest. But the debate was by no means resolved by Plafker’s research even though in hindsight the evidence appeared overwhelming and obvious.
The debate about faults and earthquakes and how mountains were built had been raging for five decades.