As if you didn’t have enough worries, here is one
more to add to that massive list:
“It’s been 300 years,” Bill Steele said Tuesday. “We have a fully
loaded subduction zone.”
Actually, it’s been 311 years since the Great Cascadia Earthquake of
1700.
Steele, a University of Washington
seismologist and spokesman for the Pacific Northwest
Seismic Network, said scientists have determined the monster quake
occurred Jan. 26, 1700 — 311 years ago tonight.
It happened off the Northwest coast, and created huge tsunamis that
devastated shorelines here and in Japan.
What’s amazing is how much is known, considering that in 1700 there
were no Europeans in the Northwest. British Capt. George
Vancouver wouldn’t find his way here until 1792. The Lewis and Clark
Expedition to the West didn’t start until 1804. Historians have no
original account of the 1700 quake written from a Western perspective.
“There’s quite a detective story of how we know all that. It’s
fantastic,” Steele said.
First, a quick explanation of what happened from the online
encyclopedia HistoryLink:
“The earthquake ruptured what is known as the Cascadia subduction zone
— the area of overlap between two of the tectonic plates that make up
the Earth’s surface, the Juan de Fuca plate and the North American
plate.” The 2003 HistoryLink essay by Greg Lange, citing scientists who
studied the quake, said the event dropped the whole Pacific Northwest
coastline 3 to 6 feet, and that the tsunami was as high as 33 feet.
Knowledge of all that comes from both recent science and long-ago
legends and tales of American Indians. Ruth Ludwin, a former University
of Washington geophysics professor, has studied and published evidence
of the quake found in native lore, Steele said.
“There are a large number of Native American legends and tales of
meadows reclaimed by the sea, of great shaking and landslides, and of
whole villages wiped out with canoes found in the trees,” he said. “It
may have killed tens of thousands of people, but there is no written
record of that.”
Scientific evidence of the 1700 earthquake comes from Dr. Brian Atwater
of the U.S. Geological
Survey, Japan’s Kenji Satake and other researchers. Their work
identifying “ghost forests” and an “orphan tsunami” is no less
compelling than the Native legends.
“Brian Atwater uncovered a layer cake of soils,” Steele said. At
Willapa Bay on the southwest Washington coast, he said, there are five
layers where scientists see transitions from marshy tidal estuary to
woody soil, indicating trees, all covered by layers of sand and bay mud.
The layers show places where land levels rapidly changed. And studies
of the roots of trees in what Steele called “ghost forests” showed that
trees died between the growing season of 1699 and before their sap
would have come in 1700.
Steele said Kenji Satake, of the Geological Survey of Japan, found
records of samurai lords, who kept track of rice harvests. Those
detailed records, Steele said, showed that an “orphan tsunami” — a
giant wave without any shaking in the area — hit the coast of Japan on
Jan. 27, 1700. “It did kill a number of people in Japan,” Steele said.
By working with those records and wave speed, he said, scientists
determined that the quake hit the Northwest coast about 9 p.m. Jan. 26,
1700.
“It’s quite a marvelous story, what happened and the impact. No one
wrote about it in the West,” he said.
It’s fascinating, but frightening, too.
Steele said it takes hundreds of years to build up the strain that
causes a subduction zone earthquake. “The toe of North America, the
edge, is being shoved downward. It’s like bending a ruler back,” he
said, adding that the 1700 quake was the last one known to have
occurred on the Cascadia subduction zone.
Remember — it’s “fully loaded.”
“It could produce another one tomorrow, or maybe a century or more
away,” Steele said. “Certainly geologically, in the not too distant
future we’re going to have another one.”
Steele is all for being prepared — whether it’s keeping supplies on
hand at home, making sure homes and public buildings are up to
withstanding big quakes, or assuring that people who live on the coast
have evacuation routes.
It takes money, and recognizing the risks.
“Our purpose here is not to scare people,” Steele said.
Julie Muhlstein: 425-339-3460, muhlstein@heraldnet.com.