Cascadia's Fault (18 page)

Dragert and Schmidt and their team used the laser Rangemaster to redraw the original triangles between the peaks and see whether
they
had changed. With the Rangemaster's new measurements they knew exactly how long each leg of the triangle was and could then calculate the precise angles between the peaks. Then they compared the new triangles to the old ones from 1937. When they did—bingo!—they saw that the angles had changed, which meant that at least some of the peaks had moved since 1937. “We proved that the margin was deforming,” said Dragert. “The mountains were indeed being squeezed landward,” ever closer to the continental mainland.

He pointed to a specific example on the old map, a triangle of dark lines drawn by the original surveyors. The new laser triangle clearly did not match the old one. It was bent out of shape because the mountain closest to the west coast—Mount Grey, a 4,570-foot (1,390 m) peak about halfway up the Alberni Inlet—had been shoved eastward nearly eight inches (20 cm) in less than forty years. Not a huge amount, by the sound of it, but imagine the entire island coastline, hundreds of miles'
worth of mountain rock, being pushed horizontally like that. When I thought of another few inches of horizontal movement each year—for 310 years since the last Cascadia earthquake—the total amount of accumulated strain built up along the fault was mind-boggling.

“And that was totally consistent with our expectation,” said Dragert, “that if the subduction zone is
locked,
we
have
to see deformation.” He poked his finger emphatically at the map again. “And indeed we
saw
the deformation which was the final nail saying, ‘Look—this is
not
slipping smoothly. This subduction zone is
locked!
'” One might think that should have been the end of the aseismic subduction idea. But it was not.

 

For those who knew what the data were saying, the late 1970s and early'80s must have been an exciting time. To Herb Dragert's great satisfaction he had proven his mentor, Tuzo Wilson, right after all. The general public, however, knew almost nothing about it because the latest evidence and the debate it spawned were confined primarily to academic journals, some so specialized that only a handful knew where the cutting edge of this new science could be found.

Even a lot of scientists were unaware of the variety, the volume, and the geographical extent of the data that were piling up. At the annual meeting of the American Geophysical Union in December 1981, more than a few heads turned when John Adams spoke about mountains of the Coast Range tilting to the east. Like Herb Dragert and Jim Savage, Adams was convinced this could only be happening if the two plates were locked together. His presentation at the AGU had an impact on several other scientists.

“I guess something of it caught the eye of the people in San Francisco who were doing the WPPSS project,” Adams told me. When he uttered the unfortunate acronym, it sounded like
whoops,
which is the expression almost everyone would come to see as appropriate for the infamous project eventually. The Washington Public Power Supply System was a megaproject involving construction of five nuclear power
plants, two of which were to be built in the small town of Satsop, west of Olympia along the mountain highway leading out to Grays Harbor on Washington's west coast.

Even though the nuclear plant at Humboldt Bay in California was already known to be in trouble because of crustal fractures caused by Cascadia's tectonic motion, another pair of reactors were going to be installed pretty much on top of the same subduction zone in Washington State. As luck would have it, several of the geo-engineers from Woodward-Clyde Consultants, the group doing the seismic risk analysis for WPPSS (the same company that had done the assessment of Humboldt Bay), happened to be in the AGU convention hall that December.

As Adams remembered it, “One of them basically said, ‘We're interested in what you're doing. Would you like to come down and talk to us?'” The Woodward-Clyde geologist who extended the invitation was David Schwartz. He had met Adams a few years earlier and considered him “one of the more interesting guys in paleoseismology,” with a new take on active faults.

The presentation was “for our enlightenment,” Schwartz explained. He wanted to stimulate discussion within the consulting team about “what was going on up there” in Washington State in terms of the seismic risk factors that might affect the WPPSS project. Adams arrived at the San Francisco offices of Woodward-Clyde a few weeks later and quickly glanced at a preamble document prepared for the meeting. “I guess you could say it was open-minded,” said Adams. “It certainly wasn't coming down very strongly one way or the other.” Meaning Schwartz and his colleagues appeared to be scientifically neutral when it came to the question of Cascadia's fault.

The day's agenda included two presentations. Masataka Ando went first and laid out the argument that most geologists at the time believed to be true. As David Schwartz distilled it, “You knew the plate was going down but the question was—
how
was it going down? Was it
going down aseismically—in which case you can make the argument you aren't going to have large earthquakes? Or was it locked? That was sort of the crux of the issue.” Ando, of course, thought it was aseismic.

John Adams delivered the second presentation, his summary of the Coast Range tilting data. He saved the kicker for the end, wrapping up his talk with an overhead slide of the Griggs and Kulm turbidite landslide data from the Oregon coast. Huge offshore mudslides in deep-sea channels from river systems hundreds of miles apart could have been caused only by very large earthquakes, in his view.

He told the team from Woodward-Clyde, “This is symptomatic of an active subduction zone.” He then tried to encourage follow-up research by suggesting, “The earthquakes don't happen more often than every four hundred years or so, but here's the sort of evidence you could use to tie it down.” Meaning those mud cores.

David Schwartz felt himself being swayed by the data. “From my perspective even back then, it was hard to get all of the secondary deformation if things were just sliding aseismically,” he said. The folding and fracturing of rocks along the shore, the compression in Puget Sound, the tilting of mountains—how could all that deformation happen on the surface if the oceanic plate were sliding smoothly underneath?

Schwartz explained that his company had been hired to perform the FSAR—the Final Safety Analysis Report—on the Satsop power plants, which would soon be presented to the Nuclear Regulatory Commission. Their preliminary draft back in 1974 had been written in a completely different atmosphere, when “the Cascadia Subduction Zone was never a consideration.” At that time the official U.S. seismic hazard maps “did not identify it as an earthquake source,” explained Schwartz.

The science was changing quickly, however, and so was the political environment surrounding nuclear power plants. In the next draft of the FSAR, circulated internally to the consulting board at WPPSS, a new, more cautionary tone had replaced the earlier optimism. “In that document we were definitely opening the possibility that the subduction
zone should be considered as a source of strong ground motion,” he told me. This was a very different view of the situation and, not surprisingly, it didn't go over well with the WPPSS officials. They were “not delighted by this turn of events,” Schwartz remarked dryly, and things slid downhill from there.

A different firm of consulting engineers had been hired to design the reactor, and they clearly disagreed with this alarmist talk from Woodward-Clyde of potential seismic shocks—Alaska-size jumbo quakes, backed up by a handful of mostly theoretical papers with equivocal conclusions and very few hard facts—which threatened to wreck their chances of building a pair of billion-dollar atomic power stations. They insisted on a face-to-face meeting to put things back into some kind of perspective.

Woodward-Clyde, meantime, had gone ahead, drawn its own conclusions and submitted its final draft of the safety analysis to the Nuclear Regulatory Commission, suggesting that Cascadia might indeed be a problem. Thus the tone of that next meeting was fairly poisonous, according to Schwartz's memory. Two rival groups of engineers faced off against each other with their client—representatives from the Washington Public Power Supply System—clinging to a ringside seat.

“Looking back at this, I have to appreciate the chutzpah they had in coming to our tank and trashing us in front of the client,” said Schwartz. “They simply said we were wrong in every assumption we made that the subduction zone could produce large earthquakes, that they could demonstrate that this wasn't the case, that mention of even the possibility of seismic subduction would kill the project with the NRC.”

The implication seemed to be that some other company should take over the entire project (removing Woodward-Clyde from the equation) and “obviously this didn't go down well with us,” Schwartz continued. “It turned into a year of hell working with those guys!” In 1982 the NRC decided to call in outside consultants to help resolve the issue. They contacted a young geophysicist named Tom Heaton at Caltech
and asked him to review the earlier preliminary safety report, the one concluding that Cascadia was
not
likely to generate large quakes.

Heaton told me that after reading the first, more optimistic WPPSS document, he concluded that “the reasoning was weak” and decided to consult a more senior Caltech professor to help him write a response. Dr. Hiroo Kanamori had already done extensive studies of other subduction zones around the world. Kanamori was, in effect, Heaton's mentor and together they had just completed a draft of the paper that would eventually be credited with turning the tide against the aseismic subduction hypothesis.

In essence Heaton and Kanamori compared Cascadia (back then it was still being referred to as the Juan de Fuca Subduction Zone) to all the other active quake-prone subduction zones along the coasts of Chile and Alaska and to the Nankai Trough off the coast of Japan. They found more similarities than differences. Bottom line: if giant ruptures could happen there—in Chile, Alaska, or Japan—the same would probably happen here, in the Pacific Northwest. Their paper, published in the
Bulletin of the Seismological Society of America
in June 1984, argued that the absence of quakes in recent history didn't mean they wouldn't happen in the future.

Like the other seismic danger zones around the Pacific Rim, Juan de Fuca's spreading ridge was so close to the edge of the continent that the new slab of ocean floor pumped out from the bowels of the earth was still relatively warm, thin, and buoyant. Put another way, the seafloor plate had not had much time to cool before it got jammed underneath the landmass of North America. Because it was warm and buoyant, the plate was in all likelihood scraping under the continent at a very shallow, almost horizontal angle. And because it was relatively smooth it was probably sticking to the upper plate the same way wide, flat-surfaced tires known as racing slicks build friction and stick to pavement.

Heaton and Kanamori found that the biggest megathrust events were directly related to young, buoyant plates being strongly coupled
to the overlying landmass at shallow angles—which fit the description of the Juan de Fuca Subduction Zone perfectly. All the other extreme danger zones had shallow trenches full of thick sediments that had been piled up against the outer coast and compressed, causing folds, faults, and uplift of the overriding plate. Same with Juan de Fuca.

They found that weakly coupled subduction zones like the Marianas Trench had much steeper angles of dip and much deeper trenches, which caused the down-going oceanic plate to melt sooner and therefore not get stuck to the upper plate. Without being “strongly coupled” to the upper plate, the subduction process does occur smoothly, creeping along aseismically, with no big jolts. But the Juan de Fuca Subduction Zone was nothing like the Marianas Trench. Therefore, Juan de Fuca was probably
not
aseismic.

At least that's how Heaton and Kanamori saw it. Compared to all the other places around the Ring of Fire, the Juan de Fuca zone looked just as capable of deadly temblors as the worst of the lot. “This 500-kilometre gap in seismic activity is one of the most remarkable to be found anywhere in the circum-Pacific seismic belt,” they wrote. Their concluding paragraph must have sounded like a call to arms for other geologists: “The Juan de Fuca and North American plates appear to be converging at a rate of between 3 and 4 cm/yr. The Juan de Fuca subduction zone shares many features with other subduction zones that are strongly coupled and capable of producing very large earthquakes. Although the shallow part of this subduction zone shows little present-day seismicity and no significant historical activity, we feel that there is sufficient evidence to warrant further study of the possibility of a great subduction zone earthquake in the Pacific Northwest.”

 

The problem, as Tom Heaton explained it to me, was that he did not have direct physical evidence of earthquakes. All the comparison studies in the world could not prove unequivocally that Cascadia's fault had ruptured in the past. The compression evidence from Jim Savage's
survey markers near Seattle, like those squeezed-together mountaintops Herb Dragert was studying on Vancouver Island, were
symptoms
of stress, not proof that big quakes had already happened here. And that's what Heaton was urging others to find—the proof.

CHAPTER 11

Quake Hunters: Finding Cascadia's Ghost Forest

Fog drifted over the hissing surf and followed us as we paddled in tandem, two canoes across a swollen winter tide up the Copalis River toward a grove of trees that had no business being where they were. In the distance, beyond the glassy green water, stood a thick fringe of marsh grass the color of wheat. Behind that, on higher ground, the dark outlines of heavily timbered hills.