Cascadia's Fault (9 page)

Somewhere between 66,000 and 77,000 square miles (170,000–200,000 km
²
) of the ocean floor had been hoisted up, while vast areas of dry ground inland (
behind
the beach zones) had sunk. The sea floor southwest of Montague Island appeared to have been lifted more than fifty feet (15 m). The sudden upthrust of the ocean bottom was clearly what had displaced so much seawater and created the deadly tsunamis that hit Port Alberni and the West Coast.

Wearing gumboots and hauling a surveyor's level across the slippery
rocks, Plafker spent most of that summer in Alaska making more than eight hundred separate measurements of uplift or subsidence (relative to sea level) along thousands of miles of shoreline between Bering Glacier and the Kodiak Islands. In some places he didn't need equipment to see what had happened. He could tell how far a beach had been raised simply by examining the whitish band of dead barnacles, algae, and mussels that had been killed when seafloor rocks were lifted above the reach of tides. Without their daily slosh from the ocean, all the sea creatures clinging to those heaved-up rocks had died and been bleached by the sun. Their reeking bodies painted a marker line on the rocks that measured how much the earth had moved.

In other places, where the ground had
subsided,
he found bands of dead brush that had been killed by seawater. As the incoming tides extended their reach over newly sunken beaches and marshes, the salt was slowly poisoning huge shoreline trees that once had lived above the tides altogether. It would take a while for the big trees along the beach to die and wither, but their mossy hulks standing in knee-deep, newly created saltwater lagoons would become vital clues in a later investigation.

Plafker found hoisted sea cliffs, drained lagoons, new reefs and islands—all indications of violent and widespread upheaval. A short time later another crucial piece of the puzzle came from the USGS survey crew, who rechecked a network of triangulation points and discovered that the earth's surface had also been
stretched
horizontally as much as sixty-four feet (20 m) between Anchorage and the outer island of Prince William Sound. This extraordinary piece of geographic distortion would eventually help prove what kind of rupture this really was and why nobody could see the fault from the surface.

Plotting his elevation numbers on a map, Plafker also noticed an invisible line of “zero change” in the level of land, approximately parallel to the south coast mountain ranges and to the deep Aleutian Trench offshore. On the seaward side of this invisible line, the land had been raised; on the landward side it had dropped down. “What I was doing was just trying to
get some feeling for whether these areas of uplifted and subsided ground might be pointing to a fault in between them,” he told me.

If there was a hidden crack in the earth, it seemed odd that heaving up and dropping down—especially on a scale as grand as this—could have happened
without
breaking the surface. How could so much land be jacked up or slumped with no visible fracture line? And yet “we never could see the fault,” he said, and that made the Alaska mystery all the more fascinating.

In numerous places he saw the “squeezing up of the rocks,” which he likened to a crumpled fender. It all looked very different from the kinds of surface damage he'd seen when plates slid past each other along a fault like the San Andreas. In California the earth was fractured vertically—and it was plain to see—but in Alaska the rocks were being folded up and shortened. Or stretched horizontally like taffy.

The essential unknown of the Good Friday rupture—the true nature of the fault—needed an explanation, so Plafker and his colleagues spent months living on a converted river tugboat, prowling the shore in small skiffs, measuring rocks and crunching numbers trying to make sense of what they'd found. Their data logs were so chock-full of bewildering new information it would take until June the following year to get it organized and published.

To make the job more challenging, Plafker, a relatively young scientist who had not yet earned his PhD, was preparing a report about earthquakes—not his chosen specialty. He was a geologist who'd spent most of his career up to that point mapping rock formations, searching for oil and other natural resources. He was not trained as a seismologist, yet here he was writing about an unseen fault that had behaved contrary to what most experts in the field were familiar with. This invisible crack along the Alaska coast appeared so unlike the San Andreas that the facts and figures Plafker came back with beggared belief. And got him into a bit of hot water.

The new science that would eventually explain what happened in Alaska—the revolution in geology now known as plate tectonics—was in mid-evolution in 1964. The controversial theories had not been refined, tested, or accepted. “They were still just barely getting to it at the time of the earthquake,” recalled Plafker. Strange as it may seem today, there was no broad consensus then on how mountains and volcanoes were formed or what kinds of forces generated earth tremors. Geophysicists didn't even know for sure whether faults caused earthquakes or, the other way around, earthquakes caused faults. Was the earth's surface cooling and shrinking and cracking? Or was it expanding and cracking because of radioactive heat from the deep interior of the planet? All these big ideas were still very much in play.

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.