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Geologists have raised from 10 percent to 14 percent the chance of a major earthquake and tsunami hitting the Pacific Northwest within the next 50 years.
The power unleashed from the powerful quake and resultant tsunami would put coastal areas along the Pacific and possibly inland along the Strait of Juan de Fuca and Hood Canal through a disaster like the one Hurricane Katrina brought to the South in 2005.
Instead of thinking that such Cascadia subduction quakes take place about every 500 years, geologists said that recent research indicates the time span to be from 300 to 350 years — which represents what one official called a near doubling of the chances for a big quake.
The last Cascadia subduction quake was Jan. 26, 1700 — 309 years ago.
It was believed to be around magnitude 9 — a monster compared with the 6.3 temblor that killed nearly 300 people in Italy earlier this month.
How much a monster? About 30 times more powerful.
‘Unbelievable’ damage
“The amount of devastation is going to be unbelievable,” said Rob Witter, coastal geologist with the Oregon Department of Geology and Mineral Industries.
“People aren’t going to be ready for this. Even if they are prepared, they are going to be surprised by the level of devastation.”
In 1700, the Juan de Fuca plate under the Pacific Ocean slipped some 60 feet under the North American plate.
The “megathrust” quake sent tsunamis into the coasts of North America and Japan.
“The geology and numerical models predict tsunamis could reach as high as 80 to 100 feet, which is similar to the tsunami that struck Sumatra [in 2004],” Witter told The Oregonian newspaper in Portland.
“We need to be very cautious and prepare for that event. It may not happen in a person’s lifetime, but if it does, it’s going to be equivalent to a Katrina-like event.”
In about a generation, Witter said, scientists have changed their minds about the dangers posed by earthquakes from the Cascadia subduction zone — the area 50 to 75 miles out to sea where the tectonic plates collide in a distance stretching from the top of Vancouver Island to south of Eureka, Calif.
Change of mind
Twenty-five years ago, they didn’t think it could produce earthquakes.
But the work of Brian Atwater of the U.S. Geological Survey linked a “ghost forest” in a Grays Harbor County tidal marsh — thought to have been killed in a big quake — to a tsunami recorded in a Japanese historical document in January 1700.
Then Chris Goldfinger, director of the Active Tectonics and Seafloor Mapping Lab at Oregon State University, turned up new evidence of big earthquakes.
He studied offshore landslides along the 600-mile fault line.
First he found 20 representing earthquakes of magnitude 9 or greater in the full zone over the past 10,000 years, and then he found 18 more of magnitude 8 to 8.5 in the southern part of the zone.
Witter said those findings put the cycle of giant quakes at 300 to 350 years.
Quake storytelling
Native Americans and other peoples have passed stories of big quakes from generation to generation, said James Roddey, spokesman for the Oregon state geological agency.
“They created a cultural tradition by retelling these events and legends,” Roddey said.
“We see that around the world. When the Sumatra earthquake struck [in the Indian Ocean], the Andaman Islands were right in the middle of the rupture zone.
“There was huge ground shaking, but very few people died from the tsunami because they had also created this culture of awareness.
“They went to high ground. They survived the event.”
Though the Sumatra and Cascadia subduction zones differ, Witter said, a coastal tsunami would be much the same.
