Plants Tell a Tale : Northwest: Is a Monster Quake Due?

Standing knee deep in slime, Brian Atwater plunged his bare hands deep into the muck that has formed a biological time machine along the banks of this western Washington river and pulled out a clump of weeds. The weeds had been buried for about 300 years, perfectly preserved in the oxygen-depleted mud.

“That’s beautiful,” he yelled with as much enthusiasm as someone who had just won the California Lottery. “Have you ever seen a better Triglochin rhizome in your life?” he shouted at about 60 scientists and engineers and insurance executives who had just landed their canoes along the river.

At another site a few miles away, Atwater has discovered a “ghost forest” of giant cedar trees, as well as various types of vegetation that thrived and then perished because of changing conditions.

May Hold a Story

Many scientists now believe that the weeds and the trees found by Atwater and his colleagues tell a story of cataclysmic events in the distant past when the marshes in which they were growing suddenly plunged below sea level in an earthquake of extraordinary energy. The land on which they were growing apparently dropped suddenly, was flooded with salt water, and then gradually rose again to allow new vegetation to flourish. That, in turn, suggests repeated and massive earthquakes of the type that have devastated similar geological regions around the world.

Planners and geologists have long assumed that such a thing couldn’t happen here, but evidence now being collected suggests they should prepare for temblors far more powerful than even the most potent earthquake believed possible on California’s mighty San Andreas Fault. What that means, according to a growing number of experts, is that few of the buildings that now stretch along the coast from southern Oregon544501536the kind of earthquake that probably has struck here in the past, and surely will strike again.

10 Times More Powerful

Such an earthquake could be at least 10 times more powerful than the jolt that destroyed much of San Francisco in 1906, and hundreds of times more powerful than the large earthquakes that have hit Southern California in recent years.

“We don’t have the proof” that such a massive quake should be expected here, Atwater said. “But we have very strong evidence.”

Atwater’s pioneering research has uncovered the first in what will have to be a mountain of evidence before the complex picture is clear enough for multibillion-dollar corrective measures to be taken. Other scientists have moved into wide areas of the Pacific Northwest to see if they can also find evidence. Among the features they seek are massive landslides triggered by violent earthquakes, or evidence of liquefaction, a form of ground failure in which the land assumes the characteristics of a liquid because of intense shaking.

However, since much of that evidence could have been washed away by the constant rains, or buried beneath dense vegetation, the process is likely to be long, difficult and quite possibly inconclusive.

“We may need at least one quake to remind people that it is possible,” geologist Derek Booth, a county planner in Seattle, told a recent symposium in Olympia. Booth is one of the people who will have to decide at some point whether the evidence is compelling enough to require people to build stronger facilities, a requirement that could raise building costs in some areas by as much as 20%.

Booth noted that in places like California, earthquake awareness is at its greatest intensity immediately after an earthquake.

“Learning experiences,” he noted dryly, “only come around about once every generation.”

The people of the Pacific Northwest have had their own learning experiences, including large earthquakes in Olympia in 1949 and Seattle in 1965 that totaled $200 million in damage and 15 deaths. But those quakes, measuring up to 7.1 on the Richter scale, were mild compared to what Atwater believes may be possible from an area called the Cascadia subduction zone.

In that zone the Juan De Fuca Plate is being subducted, or pushed, under the North American Plate in the continuous process of plate tectonics. Giant tectonic plates, which support the Earth’s oceans and continents, are pushed around the planet by spreading centers on the ocean floor, where new material from the Earth’s molten mantle slowly works its way to the surface, creating new crust and pushing the plates apart.

In some cases, plates rub against each other in a lateral motion, creating such geological features as the San Andreas Fault. But in the most violent areas, one plate is pushed under the other in a dramatic clash of forces that produces such things as the volcanoes that dot the coastline of the Pacific Northwest, part of the so-called “Rim of Fire” that rings the Pacific Ocean.

In nearly all subduction zones, the process also produces giant earthquakes because of the powerful forces involved when one plate suddenly drops as it is pushed under the other. Experts had long thought, however, that in this area the Juan De Fuca Plate was sliding smoothly under the North American Plate without accumulating strain in the system, and thus giant earthquakes were unlikely.

But now there is a growing conviction among geologists that this area is accumulating strain, and a temblor of up to 9.5 magnitude cannot be ruled out.

Such a quake would rumble along the coastline from Northern California to Canada, a cataclysmic event so great that it could claim thousands of lives. The U.S. Geological Survey has started a detailed study of the area in an effort to determine just what the impact would be, but no one seriously doubts that it would be staggering.

Even Atwater admits, however, that the evidence at this point does not prove that such a monstrous quake is inevitable.

The current search for evidence was triggered several years ago when a young geologist began studying the area because of plans to build nuclear power plants near here. Tom Heaton, now the scientist in charge of the U.S. Geological Survey’s office in Pasadena, compared the Cascadia subduction zone to other areas around the world and found the results rather unsettling.

The Cascadia, for example, turned out to be very similar to the subduction zone in southern Chile. “And that,” he said, “has produced the strongest quake of this century,” a 1960 monster measuring 9.5.

The zones were similar, for example, in that in each case the plate being subducted was relatively young--only a few million years old--and buoyant. In such cases, Heaton told the Olympia symposium, the subducted plates are resistant to sinking into the Earth’s molten mantle on which they float, and their buoyancy pushes them up against the bottom of the overriding plate.

That, in turn, locks the plates together and strain accumulates at the interface of the two plates. When that strain is released, the bottom plate suddenly plunges downward in an earthquake of immense magnitude.

“The biggest earthquakes anywhere in the world are at the interface between two plates in a subduction zone,” said geophysicist Robert S. Crosson of the University of Washington.

Heaton added that such interfaces produce “very exceptional earthquakes, very large, and with a long duration of several minutes.” The long duration is particularly troubling to planners because many structures that could withstand 30 seconds of shaking might collapse if the shaking lasts for even one or two minutes.

Heaton found the similarities between the Cascadia zone and Chile, as well as Japan, so troubling that he published a series of scientific papers questioning the widely held assumption that such an earthquake was not likely in the Pacific Northwest.

That assumption was based on the fact that no such earthquake has occurred in the last couple of hundred years, when the event would have been recorded by local residents, and that there was no obvious geological evidence--like the giant valleys in California created by the San Andreas Fault--of major activity.

Atwater, meanwhile, had completed his doctoral studies in geology and moved to the Pacific Northwest, where he began working for the Geological Survey.

Shortly after arriving, he ran across speculation from other scientists that the coastal area of Washington had been pushed up about 30 feet as the Juan De Fuca Plate was forced under the North American Plate. That suggests that one plate was sliding smoothly under the other and gradually pushing the upper plate higher, minimizing the danger of a great earthquake.

“I figured if that had happened, I should be able to find the evidence,” Atwater said in an interview. “You know how self-confident you are when you are just starting.”

So he set out to explore the marshes along the coastline, and there he found his first real evidence.

“I just blundered into it,” he said.

But instead of evidence that the North American Plate had been pushed up, he found evidence that the Juan De Fuca Plate, which runs right along the beach, had suddenly dropped. That evidence, Atwater believes, can best be explained by a violent earthquake.

Much of the evidence consists of freshwater grasses and plants buried a couple of feet below the surface along the banks of the many rivers and estuaries throughout this region. The plants were perfectly preserved, indicating that their oxygen supply was suddenly cut off by a layer of mud.

In many cases he found saltwater plants in soils above the freshwater plants, indicating that for some time the area remained below sea level, allowing saltwater plants to grow in the mud that had covered the earlier marshes. Then, the evidence suggests, the marshes slowly rose out of the sea again, possibly because the Juan De Fuca Plate was stuck on the bottom of the North American Plate. As the system accumulated strain, the lower plate could have warped slightly, pushing the marshes back up to above sea level.

The youngest fossils are about 300 years old, but Atwater and his colleagues soon found others at various depths, suggesting that the process has been repeated over and over at uncertain intervals.

In addition, the researchers found giant cedar trees that had been dead for about 300 years, still standing among the spruce that have since grown up along the river banks. In some areas, the cedars number in the dozens, creating a “ghost forest” bearing a troublesome message from the past.

“There’s no cedar living here at all today,” Atwater said as he motioned toward a dead tree towering above the green forest. “There’s something out of place about that tree.”

Atwater and others are now convinced that the cedars died because the land on which they were growing was “jerked downward suddenly” and flooded with seawater.

“They got just a tad too much salt,” he said.

If the Cascadia subduction zone did, in fact, rupture in a giant earthquake about 300 years ago, as Atwater’s evidence suggests, then similar evidence should exist along the entire Oregon-Washington coastline.

Other scientists, including Curt D. Peterson of Oregon State University, have moved the search into other areas and they are reporting some success.

“We have found evidence of six and possibly seven subsidence events in northern Oregon” over the last 3,000 years, Peterson told the Olympia symposium, indicating that the Oregon coastline, too, dropped suddenly.

“Some layers show organisms that live only in fresh water, overlain by saltwater organisms,” Peterson said. So the area repeatedly has dropped below sea level, then it has been been pushed back up slowly, only to drop again, Peterson’s research indicates.

Similar evidence has been found in western Canada, suggesting that the phenomenon has been widespread.

However, at this point it is impossible to determine if all those areas dropped at the same time, as would have been the case in a giant earthquake. The Cascadia subduction zone could have broken on a piecemeal basis in a series of smaller quakes, as some geologists believe.

“I know of no place globally where the entire subduction zone has ruptured,” said William Spence of the U.S. Geological Survey. Spence believes that subduction zone earthquakes in the Cascadia are possible, but he sees no “hard evidence” of a giant earthquake produced by a rupture along the entire zone.

Spence said the Juan De Fuca Plate is the slowest moving tectonic plate in the world, and he believes “it’s questionable to take other events and say it should happen here.”

“It’s a dying subduction zone rather than a normal subduction zone,” Spence added.

Scientists like Heaton, however, insist that it would be a serious mistake to conclude that the Cascadia is anything but a sleeping giant. The Chilean earthquake, he said, rumbled along most of that subduction zone with devastating results.

The evidence uncovered by Atwater and Peterson, he noted, is “quite compelling.”

“I’m not aware of any feasible explanation” other than a massive earthquake, Heaton said.

In the years ahead, geologists will explore wide areas of the Pacific Northwest, looking for such things as giant landslides that could have been triggered by a catastrophic earthquake. But even if they find the evidence, it will be difficult to tie it all together into a single event.

“We are now trying to identify major prehistoric landslides,” said Robert Schuster of the Geological Survey. “But even if we find them, we can’t always prove that they were earthquake-induced.”

Atwater, a painfully shy man whose discoveries have transformed him into somewhat of a folk hero in the Pacific Northwest, admits that the matter remains very fuzzy.

“Reading earthquake history from the kind of materials we are slogging through is not a precise matter,” he said.

That concession is scant comfort to the insurance executives who flocked to the Olympia symposium to see what they may be faced with in the future, and the planners who showed up hoping for a few answers, and the scientists who admitted they are better with the questions than the answers.

“We are leaving ourselves naked when we expose ourselves without having all the answers,” lamented Tom Urban of the Geological Survey. “It’s going to be a while.”

Structural engineer Roger McGarrigle of Portland commented:

“What (earthquake magnitude) in the world are we supposed to be designing to? All I have is questions. I sure don’t have the answers.”