Before they could explain the patterns, scientists had to deconstruct the way a subduction zone quake plays out. Fieldwork after the Alaska quake was invaluable, as were early computer simulations.
On a subduction zone that’s locked tight, the researchers discovered, the descending seafloor snags the leading edge of the continental plate and pulls it down like a twenty-pound steelhead tugging on the tip of a fishing pole. Behind the leading edge, the continental plate bulges up as the two masses of rock bear down on each other.
Another way to visualize the process is to hold one edge of a playing card in each hand and bend it into an upside down U. As the edges pull down, the middle of the card bulges up. Let go of one edge, and it whips up while the bulge flattens out.
That’s essentially what happens in a megathrust rupture: The leading edge of the continental plate, which is always offshore, jerks upward, lifting a vast column of water and triggering a tsunami. Behind the leading edge, the ground drops. When scientists figured out the sequence, they saw that whether a particular chunk of land rose or fell depended on its location relative to the plate boundary. The parts of Kodiak Island that sat closest to the leading edge of the continental plate jerked upward. Portage dropped because it was farther away.
Atwater suspected the Northwest coast was riding the bulge of a locked subduction zone like Portage before 1964. That would account for the rising coast and tilting mountains. When the fault ruptured the coast would drop, as it apparently had at Neah Bay.
But he couldn’t leap to conclusions based on a single observation. There were other possible explanations for the buried marsh. Maybe storms flooded the Waatch Valley or sea level spiked for some reason in the past. A small quake might have jiggled the ground hard enough to pack down the soil and allow the tide to wash in.
In Chile and Alaska, expanses of coastline hundreds of miles long sank all at once. A Cascadia megaquake would have rocked the entire region and left its fingerprints in many places. Atwater set out to search for them.
“I went after the Niawiakum because it has a tiny drainage basin,” he said, hopping back into the truck. The bumpy road makes a series of curves as it descends from the stream’s hilly birthplace to a prairie fringed by spruce and fir. Now recognizable as a river, the Niawiakum has carved hairpin turns through marsh grass burnished by the pale sun. Atwater pulled the truck to a stop by a concrete bridge and stepped out to survey the familiar scene. The only sounds were the rush of water and the rustle of grass.
“It’s almost completely natural,” he said. The landscape remains unchanged from his first visit and uniquely suited to capture a history of cataclysm.
A geologist could waste a lot of time in the Northwest searching for the tracks of earthquakes that struck hundreds or thousands of years ago. On the open coast, pounding waves scrub away clues. It’s equally futile to prowl the banks of rivers regularly scoured by winter floods. Forested uplands are so tangled with vegetation that in 1889 it took a party of explorers half a year to claw its way across the Olympic Peninsula. Atwater’s experience in the backwaters of California taught him that some of nature’s best recorders of land-level changes are the tributaries of quiet bays.
Rivers like the Niawiakum meander through marshes barely above sea level. Slam those marshes with an earthquake that lowers the ground a few feet and they’re underwater. Eventually, the tides will wash in enough sediment to raise the ground above sea level again and a new marsh is born. But a record of the old one is preserved like a buried ruin. The Niawiakum’s compact basin doesn’t funnel enough rain to generate floods that could muddle the picture, Atwater explained. He unfolded a map and traced the outline of the watershed with his finger. “It’s wonderfully small.”
Atwater’s destination that November morning was a stretch of river upstream from the Goose Point Oyster plant, where workers stacked wire baskets on the dock for the next day’s harvest. Beyond the plant stretched the watery expanse of Willapa Bay, a “misted, spongy, oozeful kind of place,” in the words of writer Ivan Doig. A log truck roared by on Highway 101 as Atwater suited up for a slog in the mud.
Sitting on the tailgate of the truck, he threaded his legs into the neoprene chest waders that are his cold-weather garment of choice. They’re warm and, unlike rubber boots, resist being sucked off by mud that grabs like wet concrete. He pulled on a lime-green parka encrusted with grime and a fluorescent orange vest, its pockets stuffed with notebooks and tools. A red toque topped off the ensemble. Oystermen changing shifts stared as Atwater hefted his backpack, shovel, and core barrel and crunched through pickleweed to the river’s edge.
A great blue heron glided to a landing on the opposite bank and lowered into a crouch. Like the bird, Atwater is tall and lean with a tendency to hunch. His high-profile profession forced him to overcome shyness, but his preference is to blend into the background. Ask friends to describe him and the words quirky and humble come up again and again.
With a string of seminal discoveries and membership in the elite National Academy of Sciences, Atwater could claim the prerogatives of a celebrity scientist these days. Or, as one friend put it, “If Brian was an asshole, that would be normal.” Instead, he still wedges himself into the back seat so students can sit up front during field trips.
When he first explored the margins of Willapa Bay in 1986, Atwater was an outsider to earthquake science. He didn’t know if his suspicions about giant quakes would fizzle or bear fruit. The Niawiakum changed that, he explained, plopping on his butt and sliding into the riverbed. The water was low enough to expose a four-foot slice of history. Atwater chopped at the surface with his folding shovel and rocked back on his heels. “It really just knocks you out,” he said, beaming at the riverbank as if at an old friend.
Atwater found one buried marsh at Neah Bay. This stretch of riverbank holds four. The topmost layer is peaty and stubbled with bits of plants preserved like flies in amber. As the layers march back in time, they become thinner and less distinct. At another spot along the river, Atwater pulled cores in 1986 that contained nine buried soil layers. It seemed far-fetched at the time to think each one could represent a great earthquake. Then Atwater found something so startling it convinced him he was on the right track.
Leaning closer to the cutbank, he shaved the surface smooth with a blade and pointed out faint white tracings that lay like filigree atop several of the buried peat layers. He plucked a pinch and rubbed it between his fingers. Sand.
The sand came from the bottom of Willapa Bay. And there was only one force Atwater could think of capable of flinging it this far inland. He pulled out a battered field notebook and flipped to his original entry from April 1986. “Sand layers may each represent a tsunami wave,” he read.
He closed the book. “That really made an impression on me.”
This tiny creek seemed to tell of a turbulent history marked not only by repeated megaquakes but also by devastating waves of a type the modern world would not comprehend until the 2004 Indian Ocean disaster swept more than two hundred thousand people from the face of the earth.
Atwater knew he would need a mountain of proof if he was going to rewrite the region’s seismic narrative. In 1986 he set up camp in a KOA near the shores of Willapa Bay, the first of many such outposts over the next several years.
Fieldwork is a side of science the public rarely sees, filled with tedium and, sometimes, terror. The team that discovered magnetic zebra stripes on the bottom of the Pacific sailed on twelve cruises and made thousands of transects, towing their instrument back and forth. At Mount St. Helens geologists dashed through a hail of flying rocks to grab magma samples from the crater. Atwater’s work wasn’t as dangerous, but it was much filthier.
Former USGS scientist Wendy Grant Walter, who worked with Atwater on the Niawiakum, recalled trying to navigate in mud up to her boot tops. “It took all my strength just to yank one leg out,” she said. “Then that would push the other leg in deeper.” Three or four steps and she was
soaked in sweat. Sometimes it took the combined strength of two people to pound the core barrel through marsh grass as fibrous as a doormat.
Atwater’s frugality—a heritage of his Yankee upbringing—ensured that the government got its money’s worth. Base camp was a 1950s trailer that looked like an aluminum bread box. The floor was falling out when Atwater inherited it from a hard-rock geologist nearing retirement. The single axle made the rig so tippy that Atwater would haul it only on back roads. There was no bathroom, but the kitchen had an apartment-size stove. Ten people could squeeze in for dinner on a rainy night.
Atwater would bake bread most evenings and serve it in the morning with hot chocolate, said Boyd Benson, who was a student at UW when he started working with Atwater. “I learned how to cook in that trailer,” Benson added.
Atwater’s work vehicle was a 1978 Dodge pickup, another USGS hand-me-down. Students called it the “beast” for its balky steering and the way it rattled when the speedometer nudged past 50 mph. His personal vehicle was even worse. The 1962 Mercury Comet station wagon cost less than his canoe, an aluminum Grumman he picked up in the late 1970s at a cut rate because of the gouge in its side. More than thirty years later, he still has the canoe. He drove the Comet until the roof was so rusted it wouldn’t hold a patch.
Brian Atwater points out evidence of a megaquake and tsunami in the banks of the Niawiakum River during a 2007 field trip. (image credits 2.2)
Both car and canoe came in handy on the Niawiakum, where Atwater hit on the technique that he would use up and down the coast. When the tide was down and the riverbanks exposed, he launched the Grumman into the upper reaches of the waterway. As he drifted downstream, he stopped to scrape and scrutinize the mud layers. By the time he neared the river’s mouth, the rising tide would provide a free ride back to his starting point. He called it lazy. Colleagues rolled their eyes.
“Brian would be out there digging like crazy, doing the work of three people,” Benson recalled. “So I would try to at least do the work of two.” Catching tides meant early mornings, and Atwater was always the first person up. In the evenings he was the one who didn’t want to stop brainstorming around the kitchen table.
Atwater also drew nonscientists into the search, inviting locals along on his expeditions and picking their brains about promising places to explore. “Some people with doctorates speak a language only known to other scientists,” said John Shulene, a retired schoolteacher who volunteered with Atwater for thirteen years. “Brian is a down-home guy.”
That inclusiveness makes Atwater a rarity: a scientist with a fan club. Members include retired engineers with time on their hands, earthquake junkies, and amateur naturalists. “Brian is willing to sit down with odd ducks,” said his old friend and collaborator David Yamaguchi, a forester who considers himself part of that flock.
After finishing its work on the Niawiakum, Team Atwater shifted north to Grays Harbor, another large bay on the Washington coast. Next they prowled inlets in the estuary of the Columbia River. On every river, in every bay, the scientists found the same buried layers in the same relative order. “I didn’t really catch on to how important it was at first,” Wendy Grant Walter recalled. “But once we started finding the evidence everywhere, things got pretty exciting.”
For Atwater the excitement was tinged with nerves. He was challenging long-held assumptions, and he knew skeptics would pounce on any flaws in his fieldwork or logic. He checked and rechecked his data and interpretations. What if there were another explanation that he was missing? “He knew he had to get it right, to get all the details down,” said his wife, Frances DeMarco. A single-minded focus gripped her husband, as it always does when he prepares his scientific reports. “It takes up all his energy. We just step back and let him be.”
In May 1987, Atwater laid out his findings: Over the past seven thousand years, Washington’s coastline had dropped abruptly at least six times, by as much as six feet in places. He allowed as to how there might be an innocuous explanation, but his money was on the seven-hundred-mile-long subduction zone sleeping like a dragon offshore.
Atwater doesn’t do a lot of fieldwork on the Northwest coast anymore, but he spends a lot of time retracing his steps. Visiting scientists ask to see the buried marshes for themselves, and Atwater obliges. Teachers, city council members, and geology clubs request field trips, and Atwater says sure. In May 2011, he led a group up the Copalis River on the central Washington coast. Among the party were a pair of television journalists, an anthropologist, and the fire chief of Hoquiam, a nearby mill town that was just coming to grips with the damage a tsunami could do to its waterfront.
The Copalis is one of Atwater’s favorite showcases for the Cascadia story. It’s got multiple buried marshes. It’s got layers of tsunami sand. But the exceptional thing about this river is the way it helped him zero in on the two questions emergency planners, builders, and community leaders across the Northwest started asking as soon as Atwater’s report about past megaquakes hit the press: How big? And when’s the next one?
The Grumman was lashed to the roof of a new government SUV. The hybrid vehicle had separate heating controls for the driver’s and passenger’s sides, power windows, and a smooth ride. Atwater hated it. The USGS declared the “beast” obsolete and made him relinquish it. He was still bereft. “I had an emotional attachment to that truck,” he said, untying the canoe and sliding it carefully to the ground so as not to scratch the SUV’s paint.
Pacing the cobbled shore, Atwater dispensed life jackets, then offered a quick paddling lesson. The flotilla straggled upstream and passed under a coastal highway bridge, rebuilt after the tsunami from Alaska’s 1964 megaquake swept logs up the Copalis and knocked out the old span.
The tide was low, and Atwater maneuvered his canoe around sandbars and snags. In 1987, his report kicked up a scientific furor, but he was too busy to pay much attention. “I don’t know what people really thought,” he recalled as he paddled. Bob Yeats, then the geology chairman at Oregon State University, was in the thick of the debate. “There was a lot of surprise and consternation among the ranks,” he said. At a national conference, one of the nation’s top geophysicists cautioned his colleagues not to jump on Atwater’s bandwagon.
Skeptics continued to hold sway at some universities. One geologist was warned by a friend not to mention his interest in Cascadia when he interviewed for an academic job in 1989. “It could be career threatening to go up against the big boys,” he recalled. WPPSS was still hoping to salvage one of the Satsop reactors. The consortium surveyed geologists after Atwater’s report was published, and found them deeply divided over the subduction zone’s menace.
BIRTH OF A GHOST FOREST
Coastal subsidence in a subduction zone quake can drown forests and marshes and leave a lasting record of the earthquake’s occurrence. (image credits 2.3)
But there was no holding back the tide of discovery. Atwater’s report set off a stampede of fieldwork across the Northwest. Geologists and graduate students headed to their closest stretch of coastline to look for similar signs and found them in abundance.
In Oregon, researchers discovered six buried marshes in Netarts Bay. An estuary on the Columbia yielded signs of a tsunami that traveled more than five miles upstream. Scientists digging on the Fraser River delta near downtown Vancouver, British Colombia, uncovered sand that boiled to the surface during fierce ground shaking. By 1995, a summary report listed eighty-six studies blanketing the coast from the tip of Vancouver Island to Cape Mendocino—all pointing toward a long history of quakes.
As he signaled the canoes to follow him to the other side of the Copalis River on the 2011 field trip, Atwater recalled the pressure he felt to put dates on past earthquakes, particularly the most recent one. “There was a lot of interest,” he said. It’s impossible to predict when the next quake will strike, but knowing how frequently a fault has snapped in the past is the next best thing. Solid dates could also address the size question, Atwat
er explained. If marshes up and down the coast were buried at the same time, that meant Cascadia had unleashed a full-rip 9 monster that ruptured the entire fault. If not, the worst case might be a series of 8s, separated by decades or even centuries. “We felt an urgency to figure out how long a stretch of coast was involved,” he said, nosing his canoe up against the muddy bank and stepping into the thigh-high water.
The rest of the party clustered around as Atwater hacked out a section of riverbank with his trenching tool, revealing the familiar layer-cake of mud. He unsheathed a Japanese weeding hoe, called a nejirigama, and plucked out a hunk of several-hundred-year-old vegetation killed during the last megaquake. It was silverweed, a plant that grows on the surrounding marshes. “You can see the bracts on the base of the stem,” Atwater explained, teasing apart the stub with gloved fingers. That the bracts are so well preserved is proof the ground dropped in an instant, he told the group.
Atwater climbed back into his canoe and led the expedition upstream.
When he first discovered fossil plants and tree roots in the buried marshes, Atwater was hopeful radiocarbon dating could reveal the timing of past earthquakes. But the early results proved as blurry as next month’s weather forecast. Atwater could see that the most recent quake hit sometime between the 1600s and the 1800s, but he couldn’t narrow the window. Earlier quakes stretched back in time from roughly one thousand years ago, when Viking raiders terrorized Europe, all the way back to the reign of the Pharaohs in 1400 BC. The intervals between quakes appeared to range from a few centuries to nearly a thousand years. But with uncertainties of two centuries or more, it was hard to say for sure. Atwater chafed at the imprecision.
Then, as happened so many times over the years, he got lucky.
Full-Rip 9.0: The Next Big Earthquake in the Pacific Northwest Page 4
