Cascadia's Fault (33 page)

Because only one of the four existing stations had displayed this maverick behavior, Dragert convinced himself the data had to be wrong. “So I said, ‘Okay, most likely our monument is unstable.' For some reason, even though this is in concrete—we have rebar drilled
into the bedrock, so there's a very good coupling to the bedrock, and it 's very competent bedrock, it 's not fractured or weathered—so, strange as it may seem, maybe we just didn't see something. Maybe there's a fracture zone somewhere that we were unaware of and it's tilted our monument.”

To find out what had gone wrong, Dragert and his colleagues drove back out to Albert Head, set up a laser transit, and resurveyed the antenna. “We did exactly the same survey as we did in'92 [when the system was first installed]. And according to both surveys—‘Hey, the monument hasn't moved at all!' Less than 0.3 millimeters was the difference between the '92 survey and the '96 survey.” The GPS tower itself was locked solidly in place, so it had to be the
ground
that was moving back and forth. In essence, Vancouver Island was being shoved to the east most of the time, but every fourteen months or so it would slip backward as if the underground stress had somehow been temporarily released or reversed.

How could that happen? Dragert laughed heartily. “We couldn't explain it. We simply said, ‘That's life,' and we went on.”

But Dragert and his colleagues kept watching the incoming data, determined to solve the mystery. Then one day—there it was again. The same apparent glitch, the same displacement. But now they had fourteen continuously monitored GPS stations in the Pacific Northwest, giving them much more precise data.

So Dragert started calling around to find out if any of the other research teams had seen anything this weird. Sure enough, when asked to take a closer look at their data plots, several of them saw a similar kind of reversed movement. Six other GPS antennas in both Canada and the United States had “jumped backward.” In all a cluster of seven adjacent sites strung out across southern Vancouver Island from Ucluelet to Nanaimo and to Victoria and down Puget Sound as far south as Seattle had suddenly slipped backward in what looked like a slow, silent earthquake that took anywhere from six to fifteen days to happen.

And it was definitely a geographic cluster rather than a random scatter. The antenna up at Holberg, on the north end of the island, didn't move at all. Neither did the towers at Williams Lake or Chilliwack, British Columbia, or Linden, Washington. Only the stations in the middle moved. The stations at the extreme north and south ends of the GPS array didn't flinch.

Not only that but the slippage had started a few days earlier down in Washington State and then moved gradually toward Vancouver Island—almost like the slow-motion unzipping of a fault. Dragert beamed. “We were saying, ‘Holy crap, this is great! This is absolutely great!' This not only told us that the signal was real, it told us the signal was constrained to a given area. And it took time to travel from the south to the north.” Basically the signal looked like a ripple moving through rock.

“It was like something migrating underneath our feet,” he said. There was still no logical explanation, however, for the odd, backward-jumping movement of a handful of GPS antennas, so a professional skeptic's first response was to say it still was probably some kind of mistake. If the GPS monuments are locked in solid rock, then something else must be wrong. Before publishing their data, Dragert and his colleague Kelin Wang had to rule out every conceivable analytical glitch and recalculate all the GPS orbits, just to be absolutely sure what they were seeing was real. Eventually they arrived at the conclusion that the silent backward slip was not a fantasy.

On his computer screen Dragert again traced the upward-slanting line of data with his finger. His hunch was that somehow, way down in the lower part of the subduction zone, a small measure of tectonic stress was being temporarily relieved. The deepest part of the zone had—in his words—come unsprung. It had slipped.

When he expanded the timeline to display several years of continuous movement, the zigzag pattern became even more obvious. The reversals put spikes in a straight-line graph that made it look like a saw
blade with evenly spaced, sharp teeth. The thing that struck me about it was the regularity. How could anything in nature be that punctual? Why would it keep coming back every fourteen months?

“Interesting question,” Dragert replied. “We have no idea.”

In a paper published in
Science
on May 25, 2001, Dragert and Wang released their mysterious findings to the world. “In the summer of 1999, a cluster of seven sites briefly reversed their direction of motion,” they wrote. “No seismicity was associated with this event.” Meaning there were none of the normal seismic shockwaves one would expect from something that looked in every other respect like an earthquake.

Could two plates really slip
without
producing detectable seismic waves? Dragert and Wang noted that all the GPS tracking stations that moved backward were some distance to landward of the locked part of the zone. So whatever was causing the backward movement had to be happening way down deep, where the rocks were hotter, softer, and less likely to stick together for long periods. They calculated that if an area 30 by 190 miles (50 by 300 km) were to move backward less than an inch (2 cm), the fault would still be generating the energy equivalent of a magnitude 6.7 earthquake. But where was that energy going?

They concluded that these slip events were probably transferring stress “uphill” to the shallower part of the locked zone in “discrete pulses.” So even though nobody could feel them at the surface, each time one of these bizarre reversals happened, it was probably pushing the fault one notch closer to failure—a giant earthquake.

 

The mystery of “silent slip” took another unexpected turn when Herb Dragert traveled to a science conference in New Zealand, where he learned from Kazushige Obara that something very similar was happening in Japan. “He's the one that discovered tremors,” said Dragert, “but he didn't know what they were. He had no idea there was crustal displacement—crustal motion—involved with these. He just kind of said, ‘Hey, these are weird signals that aren't earthquakes, but they're
not volcanic.' So he called them deep, non-volcanic seismic tremors.”

After quizzing Obara about the details, Dragert returned to the Pacific Geoscience Centre and started hunting for a connection between
tremor
and
slip.
Garry Rogers, by now one of Canada's top seismologists, had an office just upstairs and down the hall. Dragert provided Rogers with dates when the zigzag patterns had showed up on the GPS. Rogers then dug out boxes of seismograph records from the PGC archives and both were stunned to find a near-perfect match-up.

“I opened the box,” said Rogers, recalling the search, “and
there
was the tremor!” He showed me the seismogram and pointed to a squiggle of tremor noise that coincided with one of the backward jumps on the GPS. “Herb gave me the next date—I opened the next box—and
there
was the tremor event. And boy, the hairs just stood up on the backs of our necks.”

“Oh yeah,” Dragert said, beaming. “Yeah, that was exciting. Every time a slip event occurred, there was a huge increase in this background noise, a huge increase. And so it was at that point the eureka came through. We said, ‘Hey, these things are intimately related.' We knew we had something.”

Condensing it all into a neat little sequence, Rogers explained that the deepest part of the fault—way down where it's hot and gooey—fails, or slips loose, every fourteen months. When it slips—for a period of about ten days—the GPS antennas on the surface record a backward jump. The land actually recoils as the fault slips, and the seismographs record a silent tremor. Fourteen months'worth of deep tectonic stress is transferred upward into the colder, harder rocks of the part of the fault that has remained stuck. Then, with the stress transferred, the lower part of the fault locks up again and the cycle repeats itself. “It's a very unique phenomenon,” Rogers said. “We called it episodic tremor and slip, or ETS for short.”

The most fascinating revelation in all of this was that the upper part of the fault—the part that's been locked in place and building stress
ever since the last great Cascadia earthquake more than three hundred years ago—gets another increment of stress added to its load on an incredibly and mysteriously regular basis. Almost like clockwork. In other words, the stress doesn't just add up gradually until the fault ruptures; it comes in discrete little jolts every fourteen months.

“It's like adding straws to a camel's back,” Rogers suggested. “Probably when one of these straws are added it will break the camel's back.” And we'll have the megathrust earthquake we've all been waiting for. But maybe—just maybe—with this bizarrely regular timing of the ETS events, there might be a way to anticipate that quake.

CHAPTER 19

Turbidite Timeline: Cascadia's Long and Violent History

“Initially, my colleague Hans Nelson and I didn't believe it,” said Chris Goldfinger with a smile. “We thought it was probably wrong. It was way too simple. It can't be right. So we wrote a proposal to the NSF [National Science Foundation] to go prove John Adams wrong.”

This was Goldfinger holding court in the ship's lounge off the coast of Thailand en route to Sumatra to collect piston cores of ocean mud from landslides triggered by the catastrophic quake and tsunami of Boxing Day 2004. He was setting the scene for those who had not yet experienced the intrigue and frustration of trying to study deadly subduction zones that lie hidden beneath the sea. For Goldfinger and his research partner at Oregon State University, Hans Nelson, the story had begun back in 1985 when John Adams, at the Geological Survey of Canada, wrote a controversial paper based on some old OSU core samples.

The original work in question had been done in 1968 by graduate student Gary Griggs and his thesis advisor, LaVerne Kulm, who (along with Bob Yeats) later became Goldfinger's thesis advisor as well. The Griggs and Kulm piston cores revealed a series of undersea landslides
that had traveled hundreds of miles downhill from the edge of the continental shelf into deeper ocean water along a network of seafloor canyons and channels off the coasts of Washington, Oregon, and northern California. The question in 1970, when the original data were published, was what had caused the landslides.

The core samples had been gouged with steel tubes and plastic pipes from a web of deep-sea channels many miles apart, each showing evidence of thirteen or more landslides. Griggs and Kulm over beers after work one night came up with several possibilities. Either the sediment flows “self-triggered” once the seafloor mud had piled up deep enough to collapse under its own weight. Or big storms with very deep waves might have done it. Or perhaps it was big earthquakes.

Griggs and Kulm knew from looking at the core samples and measuring how thick the layers were and how far they were from the Mazama ash layer that each of the thirteen slides had happened within minutes of each other—all up and down the coast. How could so many turbid flows happen in so many canyons so far apart all at once? If it were mere coincidence, the same coincidence had happened thirteen times, once every six hundred years. In one of a series of papers on the subject, Chris Goldfinger would later refer to this as “coincidence beyond credibility.”

The self-triggering idea seemed the least plausible because it was unlikely that sediment would accumulate at exactly the same rate along hundreds of miles of the continental shelf. Rivers of different size and volume dump differing amounts of debris in different places along the coast at different rates—so how would piles of sand so far apart all collapse into turbidity currents at exactly the same moment? Thirteen times?

Storms with waves big enough to trigger deep-sea landslides also seemed a tad improbable, especially on such a wide geographic scale. They were pretty sure that turbulence from waves generated by winter's worst storms generally did not reach that far below the surface, so how could they disturb heaps of sand at the heads of steep canyons (where
most of these landslides began) that were anywhere from 500 to 1,300 feet (150–400 m) deep? If storm-triggered landslides had happened, the odds were higher that they would have occurred at different times in different places—not all at once, as the core samples showed.

Seismic shockwaves also seemed unlikely culprits because in 1970 there was no historical record of megathrust ruptures in the Pacific Northwest. When one of the grad students casually suggested the quake hypothesis that night over beers at OSU, Professor Kulm legendarily replied, “Nobody would believe it.” Thus the earthquake story entered a state of limbo, with no obvious way to prove or disprove it.

Goldfinger can still recall the doubt and disbelief that flashed through the corridors of the marine geology department when the first John Adams paper appeared in the scientific literature saying, in essence, that Griggs and Kulm should have ignored the potential skeptics. Without going to sea, without collecting a single new sample himself, Adams, the outsider, this transplanted New Zealander who had moved to Canada via Cornell University, wrote a paper based on the Griggs and Kulm cores and concluded that seismic jolts were the best—indeed the only logical—explanation for the thirteen turbidites.

It must have seemed as though a foreign spy had raided the OSU lab and stolen the thunder of the home team's original discovery. As it turned out, not a bit of skulduggery was involved. Adams simply wrote to OSU officials asking permission to look at some of the core logs that had been sitting in storage since 1968. Picking up the storyline more than three decades later, Chris Goldfinger told his colleagues aboard the
Roger Revelle
off the coast of Sumatra how an amazing feat of deductive reasoning had come about.