Seismic fault beneath us is ‘fully loaded’ after 311 years

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.