Cascadia's Fault Page 7
When I phoned him in 2009 to talk about the turmoil of the times, it was hard for Plafker to remember after so many years exactly what he knew when he flew north from Seattle that day in 1964, but one name did stand out. Hugo Benioff, who in the 1930s had designed and built the most sensitive earthquake detection equipment in use, was one of the three wise men who pioneered the young science of seismology at the California Institute of Technology (Caltech) in Pasadena. Benioff had written a classic series of papers between 1949 and 1954 that drew the first hazy picture of big slabs of the ocean floor thrusting underneath the margins of the Pacific Rim.
Benioff borrowed the voluminous and detailed charts of worldwide seismic data compiled by his famous Caltech colleagues, Charles Richter and Beno Gutenberg, to compile the first truly quantitative description of an earthquake mechanism. When he plotted on a map where most of the Pacific Rim ruptures had happened, Benioff noticed that they were not randomly distributed but instead concentrated in distinct zones of intense seismic activity parallel to most of the major island arcs, “curvilinear mountain ranges,” and deep ocean trenches along the coastlines of the Pacific Ocean basin. He had charted in precise numeric detail the infamous Ring of Fire, as we know it today.
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 advisor to U.S. president Jimmy Carter, thought Plafker was wrong. He examined the seismograms from Good Friday, calculated what is known as a “fault plane solution,” and concluded that “a near-vertical fault plane is uniquely indicated.” How could Press and Plafker look at the same raw data and see such radically different pictures? Unfortunately, a fault plane solution does not provide a single, unambiguous answer.
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. In January 1912, Berlin-born meteorologist Alfred Wegener dared to express for the first time his idea of “continental displacement.” Like others before him, Wegener had noticed that the coastlines of Africa and South America seemed to fit together as if they were pieces of a jigsaw puzzle and that parts of North America and Europe also seemed to match up. He speculated that all these far-flung land masses might once have been part of a single supercontinent (he named it Pangaea) approximately 200 million years ago.
His idea was that Pangaea had broken apart and that huge masses of land had moved sideways across the surface of the earth, jostling and crashing into each other over millions of years. While others had speculated about the significance of matching coastlines, Wegener had the audacity to put it in writing, making himself an instant lightning rod. Published in 1915, his book The Origin of Continents and Oceans triggered a controversy that was still raging in 1964. If Plafker and Benioff turned out to be right about how that fault in Alaska worked, then the essence of Wegener’s big idea might be right as well.
Wegener’s notion that “tidal friction” and differences in gravity caused by the earth’s imperfect, oblate shape had caused the continental breakup and that huge slabs of the earth’s crust somehow plowed across the ocean floors like ships through pack ice was considered by physicists to be impossible. What on earth—what mechanism—could possibly generate a force strong enough to fracture and move entire continents horizontally? To many it sounded like utter nonsense. The evidence that big land masses had once been joined together, however, was harder to dismiss.
The dominant view among scientists at the time was that the continents had been locked rigidly in place from the very beginning of time as the earth solidified from a molten state. With the interior of the planet gradually cooling, the outer crust began to shrink and slump, to crack and wrinkle like a drying apple’s skin—creating mountains along the way. Others thought parts of the crust rose and fell periodically as if they were floating on a semi-fluid interior.
Most who doubted Wegener were aware of the work of other scientists pointing to fossil match-ups, the remarkable similarities between rock layers on different continents, and the evolution of nearly identical plants and animals on opposite sides of the oceans. They realized that sooner or later there would have to be some way to account for all this. Wegener offered a theory to explain how the various bits and pieces might have fit together, even if he couldn’t say for sure why they had come apart.
He argued that if continents could move downward as the planet cooled and contracted, or even upward—rising slowly as ice ages ended and the enormous weight of glaciers melted away—then they could probably move horizontally as well. Figuring out how and why this happened would be the crucial next step. Even before Wegener published his theory there were reasons to question the orthodox view that the earth was cooling and shrinking. In fact, some researchers already thought the opposite might be true.
With the discovery of nuclear radiation at the turn of the century came the understanding that some elements generated energy all by themselves, that rocks containing these elements deep underground might be pumping out an enormous amount of heat that accumulates faster than it can dissipate into space. If true, then perhaps the earth was heating up rather than cooling. The surface of the planet might actually be expanding rather than shrinking. It might also explain how continents could slide or drift sideways.
Scientists began to speculate that heat generated in the earth’s interior might get trapped beneath the continents. Radioactive elements could be generating enormous “convection currents” of melted rock that would rise from the planet’s white-hot mantle toward the surface, like bubbles in a pot of soup. In fact the soup analogy seemed to make so much sense it was still being taught in Geology 101 courses when I entered university in 1970.
I can still recall the lecture. The professor, whose name is lost to me now, asked us to imagine the earth as a large cauldron of thick soup that has been brought to a slow boil. Bubbles of heat rise up from the bottom of the cauldron. At the surface the soup cools and forms a crust that floats atop the hotter liquid material below. When new heat bubbles rise to the surface, they push the older crust aside.
Propelled against the outer walls of the pot, the older crust is pulled down into the interior of the cauldron, where it gets reheated and eventually bubbles back to the surface to form crust again. This continuous, circular motion of a heated liquid is now known as a convection cell. And that, concluded the professor, is how we might explain the way continents get dragged or pushed across the surface of the earth.
Just think of continents as great rafts of floating soup crust. In the 1920s, however, this was still just a wild idea with no solid evidence to back it up. Wegener and his supporters might have been cheered by these new developments, as they seemed to provide an explanation—the mysterious force that could move continents around like pieces of a jigsaw puzzle—and make sense of continental drift. Sadly that’s not how the story ended for Wegener himself. A meteorologist by profession, he got lost in a blizzard during a research trip to Greenland in 1930 and did not live to see the discoveries that would rehabilitate his theory more than three decades later.
If continents were indeed moving around like rafts of soup crust, there had to be some way to prove it once and for all. The next several breakthroughs in earth science came as a result of military research begun during World War II. The U.S. Navy needed a new technology to detect German U-boats and better maps of the ocean floor to keep track of where enemy (and their own) submarines might be able to hide. In those days the bottom of the sea was as uncharted as outer space and a generation of young scientists was eager, willing, and able to explore the planet’s last frontier.
As warships sailed from one battle to the next, echo sounders pinged day and night, creating detailed, never-before-seen profiles of the ocean floor. In the Pacific, they charted undersea volcanoes and steep canyons like the Marianas Trench which, according to the new measurements, was seven miles (11 km) deep. What process had created such a steep canyon in a mostly flat ocean floor? Could this be where the soup crust buckled under and got recycled into the earth’s interior cauldron?
The threat of enemy subs seemed just as real during the Cold War, so the Office of Naval Research continued sending exploration teams to sea during the 1950s and early ’60s. Along the way scientists discovered amazing new details about the Mid-Atlantic Ridge, a chain of undersea mountains halfway between Europe and North America. The so-called ridge turned out to be a set of parallel mountain ranges with volcanic vents oozing hot magma onto the ocean floor.
As the magma spewed out and cooled in seawater, it expanded and hardened to a rocky crust, forming a new piece of ocean floor. Over millions and millions of years, the lava had piled up into those volcanic ridges while a seemingly constant spew of new magma kept pushing the ridge flanks farther and farther apart. Here, at last, was direct physical evidence of the convection currents that might be causing continental drift.
At first it was thought this mid-ocean ridge existed only in the North Atlantic. Further mapping confirmed that it wandered down between Africa and South America and then snaked around the entire globe like the seam on a baseball, a fifty-thousand mile (80,000 km) chain of volcanic ridges. When they put all the new charts together, these mid-ocean ridges turned out to be the longest continuous mountain range in the world. In terms of scientific significance, the undersea ridges had morphed into the most prominent geologic structure on the planet. Research done on these volcanic slag heaps would make or break the theory of continental drift.
In the early 1960s,
with hot convection cells to power the system, the next question to answer was what happens when two moving portions of crust collide. Like two cars crashing head on, the obvious result of two continents slamming into each other would seem to be crumpled fenders—mountain ranges like the Himalayas and the Alps. When a segment of ocean floor crashes against a continent, the sea floor, being made of heavier, denser, volcanic rock, apparently buckles under the lighter continental crust, creating a deep ocean trench like the Marianas.
As the heavier ocean floor continues to move, it gets forced downward, grinding against the underside of the continent as it goes. Coastal mountain ranges get shoved upward in the process. As the seafloor slab goes deeper, it gets so hot it begins to melt, spewing a volcano up through the overlying continental rock. If two masses get stuck together by friction instead of sliding past each other smoothly, enormous pressure builds up and is finally released in megathrust earthquakes. So the old soup cauldron story had endured, and the conversion of many skeptics into tentative believers was underway. Finally there was a logic to continental drift and a way to put the planet’s jigsaw puzzle together.
By the mid-1960s J. Tuzo Wilson at the University of Toronto would weave the bits and pieces of new discovery together in a comprehensive theory that filled in most of the blanks in Wegener’s original concept. Wilson also changed the terminology. He described the earth’s surface as being divided into “several large rigid plates” rather than continents. By Wilson’s definition, a plate was considerably larger than a continent and could include segments of ocean floor that had been jammed against and welded to the edge of a continent. The North America plate, for example, included everything from the Mid-Atlantic Ridge to the California coast—meaning all that “new ocean floor” being generated underneath the Atlantic had become part of the older continental mass. It was all of a piece, a westward-moving tectonic plate.