A Gargantuan Discovery by Geology Gumshoes

The traces, the few that are left in the quiet bays and estuaries off the Oregon and Washington coasts, are subtle: a square depression here, scattered shells there, an ominous stand of towering dead trees.

Yet those simple signs--coupled with years of dogged detective work--are enough to tell geologists and anthropologists exactly what devastated these shores a century before any written history recorded it.

It was, scientists now say with certainty, one of the world’s largest earthquakes. A magnitude 9.0. Large enough to rock the coast from Vancouver to Crescent City, generate killer tsunamis and permanently resculpt the rugged shoreline.

Researchers can even tell exactly when it happened: Jan. 26, 1700. At 9 p.m.

As they commemorate the 300th anniversary of the quake this week, geologists credit this single event with awakening the Pacific Northwest from geologic complacency. As historical details of the quake of 1700 grow ever more vivid, people along the Northwest coast are increasingly preparing for the next big quake and tsunami, which scientists call inevitable.

“If it wasn’t for people studying this event, Oregon would be unprepared,” said Ray Weldon, an associate professor of geology at the University of Oregon. “The actual existence of this event has galvanized people.”

The Pacific Northwest, with new tsunami warning signs, upgraded building codes and school earthquake drills, is the only place in the world, geologists say, where millions of dollars worth of preparation has not been immediately preceded by a disastrous quake.

Native American Tales Assumed to Be Myths

Yet deep into the 1980s and even into the ‘90s, many geologists remained unconvinced that the Northwest was prone to quakes larger than magnitude 7--even though the area sits along the edge of the active Cascadia subduction zone. Such zones, where one geologic plate slides underneath another, cause the world’s most devastating earthquakes.

Local Indian legends told of shaking earth, killing waves, entire villages suddenly flooded and gone. Most assumed that those tales were myths.

Because the area had experienced no major quakes in recorded history, many believed that the region was quiescent. Consultants paid by proponents of nuclear plants proposed for the area encouraged this view, said Weldon, a California-trained geologist who moved to Oregon in the midst of the debate.

Perhaps the zone was more lubricated than other areas, many geologists argued at the time. Or maybe one plate was creeping along at an exceptionally slow rate, relieving tension.

But there were no tiny quakes--”1s and 2s” in the parlance of seismologists--to indicate any creeping, Weldon said. After examining all the existing evidence in the mid-1980s, Caltech geologists Hiroo Kanimori and Thomas Heaton declared the zone active. Earthquakes were coming, they said. And they had come in the past as well.

So Brian Atwater went looking for evidence. A U.S. Geological Survey geologist in Seattle, he reasoned that such massive devastation would have left some record.

Working in waist-high muck, slicing into peat with chain saws, he found the telltale footprints of a tsunami: a dusting of misplaced sand over dirt that had once been a coastal marsh. There were also miles of shoreline that had fallen several feet and mysterious stands of dead cedar and spruce--ghost forests that Atwater suspected were poisoned by salty water.

To test his ideas, Atwater teamed with David Yamaguchi, a tree ring expert working amid the devastated forests of Mt. St. Helens. The rings that Yamaguchi analyzed under a microscope were records--a bar code of narrow and wide bands--stretching back a thousand years. They showed how well trees fared during certain years and when they may have died.

Yamaguchi started examining dead Sitka spruce in the spring of 1987. It was frustrating work. Because the lightweight wood was so prone to rot, few of the trees he needed to study remained standing.

Months later, he turned to cedar trees less likely to rot and spent a summer canoeing through shallow waters to tree stands, sawing out telephone book-sized chunks of wood. “You really felt like Lewis and Clark out there,” Yamaguchi recalled.

Back at his lab, he was frustrated again. The oldest outer layers, those that could divulge when the trees had died, had been erased by erosion.

Atwater, used to digging trenches to study the geological record, noticed that submerged tree roots were well preserved. But because rings are so distorted in gnarled roots, Yamaguchi could give only an estimate: the disaster had occurred between 1680 and 1720.

Then, Japanese tsunami expert Kenji Satake grew interested. The time frame was narrow enough for Satake to search Japanese historical records for tsunami reports in 18th century government records.

Working with historian Kazue Ueda, he struck pay dirt. Several villages on the main island of Honshu reported that their markets and warehouses were destroyed by a 6-foot tsunami that hit Japan at midnight Jan. 27, 1700.

Estimating the speed of a tsunami to be equal to that of a jet and working backward, Satake calculated that a magnitude 9 quake occurred off the Pacific Northwest at 9 p.m. on the 26th.

The newest work on the prehistoric quake has come from anthropologists, who are starting to understand the human toll of the disaster by excavating previously discovered villages and fishing sites. Some are accessible now only at low tide; clearly they were flooded. Some are strewn with debris.

“In most archeology of the American West, you can’t pin things down to a single day,” said Robert Losey, a University of Oregon graduate student who is sorting through mounds of clam shells and fish bones left by Northwestern Indians to determine how the quake affected hunting and gathering. “It’s so compelling.”

A Chilling Picture of Death and Destruction

The work of Losey, fellow graduate student Scott Byram and others is piecing together a chilling picture of what may have happened that cold January night. At that time of year, coastal Indians were most likely gathered in densely packed villages near winter stores of dried salmon, clustering around fires and gathering by the hundreds for religious ceremonies. It was likely, the anthropologists said, that the quake’s death toll was high.

But Byram’s analysis of fishing weirs left by Indians at tidal channels shows that the tribes did rebound and quickly resettle in new areas.

“It’s poignant to think they survived such a massive disaster and then they were torn down by disease” and killed by white settlers, he said.

Byram and others are working to preserve what oral histories remain among the ever-thinning Indian population. Those old legends of disaster that came from the sea, it appears, were eerily accurate.

“There’s an explicit tale of a mythological character called Earthquake traveling along the coast sinking Prairie into Ocean,” Losey said. “That’s exactly what happened geologically.”

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Clash of Two Plates

Over millions of years, massive quakes in the Pacific Northwest have been triggered by the pressure of the Juan de Fuca Plate pushing against the North American Plate. The last such quake, a magnitude 9, was 300 years ago. Geologists say another is inevitable. Using faint clues and traces, scientists are piecing together the geological history of the Pacific Northwest.

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Subduction Zone

Washington state is near the northern end of the Cascade Range subduction zone, where the Juan de Fuca Plate meets the North American Plate. The volcanoes in the Cascade Range have been uplifted where the offshore plate is underthrusting the continent.

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Source: U.S. Geological Survey