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http://www.latimes.com/news/science/science/20000127/t000008557.html
Thursday, January 27, 2000

A Gargantuan Discovery by Geology

For years, the Northwest was widely believed to be seismically
quiescent. Then experts found that one of the world's biggest
temblors occurred there in 1700, confirming Indian legend and
prompting large-scale quake preparedness efforts.

By USHA LEE MCFARLING, Times Science Writer
 

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."

                                                                     * * *
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.

                                                                     * * *

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.

                                                                     * * *

Source: U.S. Geological Survey
 

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