In search of our last big Cascadia quake

Written by Nick Grube, The Triplicate April 04, 2009 09:09 am
This image of a simulated tsunami stretching from the West Coast to Japan is used on the cover of “The Orphan Tsunami of 1700 — Japanese Clues to a Parent Tsunami,” to show what would happen during a Cascadia Subduction Zone event. (Courtesy U.S. Geological Survey)
 

More than a century before Meriwether Lewis and William Clark set out on an expedition to reach the Pacific Ocean, the Cascadia Subduction Zone awoke.

When it did, it sent out a series of oceanic surges with devastating force, striking the eastern seaboard of Japan in what is known as the Orphan Tsunami of 1700.

For nearly 300 years, this tsunami’s origin was a mystery.

“We’re still really at the steep part of the learning curve with tsunamis,” Humboldt State University Professor Lori Dengler said. “We’re babes in the woods.”

Seismologists didn’t really pay much attention to the Cascadia Subduction Zone until 1992, when an earthquake near Cape Mendocino sent a small tsunami steaming toward Eureka and Crescent City.

That event, Dengler said, confirmed some scientists’ suspicions that a Cascadia event could be large, on the order of a 9.0 magnitude earthquake, and that it could one day generate a tsunami that took a trans-oceanic journey.

It wasn’t until after the Cape Mendocino quake that researchers, both here and in Japan, finally pieced together unlikely sources of evidence that actually confirmed the true history of our subduction zone.

“It’s circumstantial evidence,” Dengler said. “But it’s very good circumstantial evidence.”

In 1700, the Japanese already had a well-developed writing system that was prolific among the upper class samurai as well as with the merchants and the peasants.

This proved integral in documenting the Orphan Tsunami, which flooded Japan’s coastline on Jan. 27, 1700, without warning. Japan was already familiar with these types of events — the word “tsunami” originated there — but there was no ground-shaking there in 1700 to foretell the waves.

According to the book The Orphan Tsunami of 1700 — Japanese Clues to a Parent Earthquake in North America, by the early 1990s all the collected writings about this event had made it Japan’s “best documented tsunami of unknown origin.”

That’s when the story again crossed the Pacific, according to Redwood National and State Parks Geologist Vicki Ozaki.

“All up and down our coast we have a lot of early tsunami stories,” she said.

Oral accounts of massive earthquakes and flooding from the ocean go back hundreds of years among coastal American Indian tribes, including stories from both the Yurok and Tolowa.

Many of these stories were collected from the early- to mid-1900s, though some of the tribal elders who told these histories passed them along well into their lives.

In the mid-’90s, Ozaki said a husband and wife team, Gary and Debra Carver, compiled these stories and tried to confirm them with geologic evidence that came from core samples taken from areas where the tribal accounts took place.

“She was an ethnographer and he was a geologist,” Ozaki said, and this tandem made all the difference. “They started having the geologic evidence to support that.”

Through this historical and scientific investigation that spanned an ocean, experts have pinpointed what they suspect is the exact date of the last major Cascadia event, Jan. 26, 1700.

“These pieces kind of fell into place,” Ozaki said. “These are the little pieces that are all kind of the picture that we think points to a Cascadia.”