Cascadia's Fault (30 page)

 

In all the mucking about along the coast, scientists had become increasingly aware that people had in fact been living on the edge of Cascadia's rupture zone when it tore itself apart the last time. Beneath a thick layer of tidal mud in the Nehalem and Salmon River estuaries of northern Oregon, Rick Minor of Heritage Research Associates in Eugene and Wendy Grant of the USGS found fire pits full of charcoal and woven cedar mats. Near Willapa Bay in Washington State, fishing weirs and cobblestones modified by fire were found buried in submerged shorelines. Trying to figure out when the campfires had been doused or when
coastal villages and their Aboriginal residents had been shaken from their sleep or perhaps drowned by killer waves turned out to be exceedingly difficult.

Anthropologists had been collecting flood and disaster stories from tribal elders since the 1850s but it was only in the 1980s that they began to compare notes with geologists, who found the oral histories intriguing and frustrating at the same time. Judge James Swan, who had lived among the Makah people of Neah Bay, Washington, for a time in the late 1860s published the family history of Billy Balch, a Makah leader, who told him about a catastrophic flood that had turned Cape Flattery into an island. The Balch story began “a long time ago” although “not at a very remote period,” when the water of the Pacific flowed like a swollen river through the swamp and prairie between his village and Neah Bay.

This, of course, was the same tide marsh where Brian Atwater found his first evidence of land subsidence caused by large seismic ruptures. Balch's story does not mention the ground shaking, so perhaps the flooding he talks about was caused by a distant tsunami from across the Pacific. In any case, Balch told Swan that after the initial flooding, the ocean began to recede and left Neah Bay dry for four days. It then rose again “without any swell or waves” and submerged the whole of Cape Flattery.

“As the water rose, those who had canoes put their effects into them and floated off with the current which set strong to the north,” Swan paraphrased Balch. “Some drifted one way and some another; and when the waters again resumed their accustomed level, a portion of the tribe found themselves beyond Nootka [on Vancouver Island] where their descendants now reside . . . Many canoes came down in the trees and were destroyed, and numerous lives were lost.”

“Could this be an account of a great tsunami?” asked Tom Heaton of the USGS in a paper he wrote with Parke Snavely in October 1985. “Great tsunamis may have periods of tens of minutes to hours [between
waves] but 4 days is without precedent. If the event is real, then it is apparent that the effects must have been substantial. However, it seems incredulous that any tsunami could have overtopped the entire Cape Flattery region since the highest elevation of the cape exceeds 400m.”

Careful not to dismiss the story as myth, Heaton and Snavely tried to imagine what sort of geological event might explain it. The tsunami from the 1964 Alaska quake, which did so much damage to Port Alberni and Crescent City, delivered to Neah Bay a wave that was only 4.3 feet (1.3 m) above the tide, so a tsunami that could completely submerge the entire cape seemed a remote possibility. Then they wondered—what if the land behind Neah Bay had been hoisted up during a
seismic
event, draining sea water from the bay and leaving the land temporarily dry? Heaton cautiously wrote that “crustal deformation associated with a nearby subduction earthquake could explain the uplift. However, this and any other conjectures about the significance of this legend are purely speculative.”

If there was a quake, why didn't Balch mention the shaking when he told the story to Swan? Living as close as they did to the subduction zone, the Makah people must have experienced the worst imaginable ground motion. This doesn't sound like the kind of detail a person could easily forget. Heaton and Snavely found enough inconsistencies to consider that the story might be “entirely fictional.” In the end, however, they decided it was “noteworthy that such a report exists for a region where there is growing concern that large subduction earthquakes and subsequent tsunamis may be a real possibility.”

Farther down the coast, along the beaches of southern Oregon and northern California, Deborah Carver gathered stories told nearly a hundred years ago by Wiyot, Yurok, Tolowa, and Chetco people. From six different villages along at least two hundred miles (320 km) of coastline came similar cultural memories of a violent rupture that struck at night followed by many aftershocks and tsunami waves that rolled in for hours. Many people were killed as villages were inundated or swept
away. By the light of day the survivors found places where the ground had liquefied and slumped. In a Yurok village near what is now Redwood National Park, south of Crescent City, Carver learned that one purpose for the ceremonial “jumping dance” was to repair or relevel the earth after an earthquake.

As geologists continued their investigations, more and more stories like this came to light, including one that I heard in Pachena Bay on the west side of Vancouver Island. Seismologist Garry Rogers of the Pacific Geoscience Centre had told me about a reference he'd found in the provincial archives to an earthquake that rocked villages along the west coast of the island in the middle of a winter's night long ago. It didn't take long to find an elder who knew the details, a descendant of the people who once lived in Pachena Bay. The year was 1998 and I was filming another documentary for the CBC called
Quake Hunters.

Crossing the rocky spine of the island westbound, I thought about living on the edge of the known world two or three centuries ago when the earth tore itself apart.
Wild, remote,
and
spectacular
were words that came to mind. Not for the faint of heart. Since getting from the sheltered eastern shore at Nanaimo to the wild west coast presents a challenge even today, imagine what it was like back then.

After a fast and curvy two-lane tour along the postcard edge of Cameron Lake and through the ancient cedar tunnel of Cathedral Grove, the slow grind up and down the switchbacks of Mount Arrowsmith and the deeply rutted trail beyond Port Alberni became more work than fun. Potholed and scattered with fallen rock, the unpaved washboard logging road that runs parallel to the fjord out to the fishing village of Bamfield and Pachena Bay was a challenge. But once the camera started rolling, we knew the ride had been worth the effort. We were closing in on a story about Cascadia's last big quake.

Around a crackling fire in a big stone hearth in a house just above the beach sat half a dozen young children, mesmerized by one of their elders. Outside, pounding surf and a fine mist of rain washed against
the dark green shore. Sitting on a spattered windowsill, his back to the churning sea, Robert Dennis, chief councilor of the Huu-ay-aht Nation, waited until all the young eyes were trained on his weather-beaten face and then began his story in a low, quiet voice that forced the children to sit forward and listen carefully.

“The story that I'm about to tell happened a long, long time ago and was told to us by Chief Louis Nookmis,” said Dennis, grandson of Nookmis, the man who originally told this story to an anthropologist back in 1964. “Some of you are directly related to that man,” the chief added, nodding and calling the names of several children at his feet.

Like the ancient cedars of the ghost forest in Willapa Bay, the Huu-ay-aht people felt a violent shudder in the earth one dark and stormy winter night many generations ago. “They had gone to bed,” said Chief Dennis, “just like any other normal night. Had gone to bed, gone to sleep. And they were awoken during the night when the earth began to shake.” He paused, letting the words sink in. “The earth shook. Startled the people. They woke up.” He shrugged very slightly. “Thinking everything was over, they just relaxed. A little while later, the water came in real fast. Swept their homes away. Swept everything away.” The children were transfixed. “The water just came too fast. They didn't have time to go to their canoes. So all the people that were living there were drowned. They were all wiped out.”

Chief Dennis let the thought sink in and then explained how it was that the story itself survived the death of an entire village. The modern fishing village of Bamfield is next door to the rebuilt Native community where Chief Dennis and his people now live. On slightly higher ground, however, in the trees behind the beach, was another settlement of people who watched what happened that horrible night long ago and who did live to tell the tale.

“They who lived at ‘House-Up-Against-Hill' the wave did not reach because they were on high ground . . . Because of that they came out alive. They did not drift out to sea with the others.” This according to
the 1964 transcript of the story told by Chief Nookmis. For scientists eager to follow the story back in time, the original translation was a bit garbled, making it impossible to figure out when this disaster might have happened. Years later Robert Dennis would help organize a more thorough translation that narrowed the timeframe somewhat. The newer, more detailed version placed the event some time between 1640 and 1740, the same period in which geologists had pegged the last megathrust earthquake on Cascadia's fault.

 

Brian Atwater is fond of saying that “the earthquakes had written their own history” by dropping segments of the coastline a meter or two, by forcing sandy seawater across the freshly sunken land, and by causing the edge of the earth to crack. The problem was that by the mid-1990s researchers had reached an impasse in their attempts to define how large the quakes had been. The answer to this question was important because emergency planners and civil engineers needed to know what they were up against when it came to designing new buildings and reinforcing old ones. “How great an earthquake should a school or hospital be designed to withstand? How large a tsunami should govern evacuation plans on the coast?” asked Atwater in a book he cowrote several years later.

Atwater's colleague David Yamaguchi had come closer than anyone else to nailing a more specific date. He went back to the ghost forest at Copalis River several times in search of wood cores that might contain enough growth rings to reveal what year the trees had died. He put together a database, a pattern of rings collected from twenty-three ancient cedars that had been alive at the time of Cascadia's last violent outburst. Like the Native people who lived above Pachena Bay on Vancouver Island, the trees of Washington State stood on slightly higher ground and had witnessed the disaster unfolding below.

When they compared the rings of “witness trees” with rings from the ghost forest, they knew the forest floor had sunk beneath the tides
some time in the late seventeenth century. “The closest I could get was 1691 as a limiting date for the earthquake,” Yamaguchi told me. “The earthquake had happened some time shortly after then, but I didn't know how many years afterward.” Insects, rot, and roughly three hundred North Pacific winters had pitted and peeled away too much of the outer bark of these standing dead cedars. The final few years of growth rings—the crucial last clues—were missing.

A dating technology was being tested at the University of Washington, however, and it might pick up where the tree rings left off. Atwater was keen to try it. “Brian realized that he could use the preliminary 1691 date as a tool to help him do something called high-precision radiocarbon dating,” Yamaguchi continued. Wood samples were locked for weeks inside shielded atom counters and the resulting timeline was accurate to “the 95 percent confidence interval.”

Like the weather-beaten tree trunks, though, radiocarbon technology fell slightly short of the mark. It moved the goalposts closer together without truly scoring a victory. The window had shrunk a little more, putting Cascadia's most recent event some time between 1690 and 1720, but there was still no certainty about whether a single, giant rupture or a “swift series of merely great” earthquakes had done the job. Nevertheless, when news of this tighter timeline became public, it created a buzz. And a scientist from Japan had one of those eureka moments that researchers everywhere dream of.

CHAPTER 17

The Orphan Tsunami: Final Proof of Cascadia's Last Rupture

At a science conference in Marshall, California, in September 1994, Kenji Satake was having lunch with Alan Nelson of the USGS when Nelson mentioned the frustration he and others were having with radiocarbon dating. Nelson and eleven colleagues were putting the final touches on a paper that was meant to collect and focus all the physical evidence for past quakes on Cascadia's fault in a single volume. Satake's specialty was subduction zones and tsunamis. He had studied the effects of the Okushiri disaster in Japan the previous year and was very interested to hear the latest news about Cascadia. Especially Atwater and Yamaguchi's work with tree rings.

“I did not know much about the calibration technique of radiocarbon dating using tree-ring records,” Satake told me. “I met Alan. He explained to me the basics of the technique and I was very much impressed by the small uncertainty—only a few decades.” Something about those dates—a time span from 1690 to 1720 for Cascadia's last rupture—caused Satake to pay special attention to this latest study.

He was working at the University of Michigan as an assistant professor. Before that he'd completed a two-year term as postdoctoral
researcher at Caltech, where he had studied Cascadia's quake and tsunami potential with Tom Heaton. Heaton, Hiroo Kanamori, and Stephen Hartzell were then banging the drum about similarities between Cascadia and Chile and other big subduction zones. But in 1994 there was still not enough
unequivocal
evidence of past ruptures in the Pacific Northwest for the case to be ironclad.

Later that same year Satake made a point of meeting Brian Atwater on a field trip to the coast as part of the Geological Society of America's fall meeting. Satake would soon depart the United States for a job with the Geological Survey of Japan and was keen to see the Cascadia evidence first-hand before he left. Curiosity piqued by Atwater, Satake wanted to see more.