Years ago, West Coast had its own giant wave

Could a tsunami like yesterday's hit the Washington Coast?

It can and, in fact, it already has.

On Jan. 26, 1700, a magnitude 9.0 earthquake buckled the ocean floor from Vancouver Island to Northern California — the Cascade subduction zone — setting off a tsunami that swamped the West Coast of North America and washed away houses in Japan.

Today, geologists and emergency agencies use the evidence left behind from that quake to design tsunami-response programs for the Pacific Coast of North America and Puget Sound.

Yesterday's earthquake off Sumatra bears an eerie similarity to the one that shook our coastlines nearly 305 years ago. The two quakes are estimated to have been about the same magnitude, both occurred on north-south faults along the western edge of an ocean and both had "rupture lengths" — the length of fault that slipped in the quake — of about 600 miles.

"The parallels are incredible," said Brian Atwater, a U.S. Geological Survey geologist based at the University of Washington.

Over the past several thousand years, the Cascade subduction zone has generated a severe earthquake about once every 500 years. Geologists put the likelihood of another major quake in the next 30 years at 5 to 10 percent.

The Sumatra quake "is a reminder for people in our region that the earth can do this here, too," Atwater said.

The tsunami from the 1700 quake left a layer of sand in many of the flooded areas.

Using that evidence in conjunction with simulation models, the National Tsunami Hazard Mitigation Program, a partnership of several state and federal agencies, has developed inundation maps and evacuation plans for at-risk coastal areas in the Pacific states.

In the Puget Sound area, Whidbey Island, Port Angeles, Port Townsend and Bellingham are at particular risk from a Pacific tsunami.

In addition, the West Coast and Alaska Tsunami Warning Center in Palmer, Alaska, combines seismic and deep-sea pressure data with tsunami-prediction models to produce constant updates on potential threats.

Vasily Titov, a research oceanographer at the National Oceanic and Atmospheric Administration's Pacific Marine Environmental Laboratory in Seattle, develops models of tsunami propagation. He said international monitoring has focused on the Pacific Ocean, because that's where 90 percent of tsunamis occur.

"In the Indian Ocean, there is tide-gauge information, but there isn't a 24-7 warning system," Titov said.

The international effort was started after a powerful 1946 earthquake in the Aleutian Islands. That quake touched off a tsunami that raced across the North Pacific, hitting Hawaii with 55-foot waves five hours later and killing 173 people.

Tucked into Puget Sound, Seattle is not at significant risk from a tsunami from the open Pacific, Atwater said. But a strong Seattle Fault temblor could touch off damaging waves on Puget Sound and even Lake Washington.

Tim Walsh, program manager for geologic hazards with the Washington Department of Natural Resources, helps the state's coastal towns prepare for a tsunami on short notice.

He said he was notified about the Sumatra quake before the waves hit the shores. The monitoring system for the Pacific correctly reported that the quake wouldn't be a threat to the Washington coastline.

"We knew about that here but didn't realize how bad it would be over there," Walsh said.

He said education and agency coordination are as important as monitoring systems in dealing with a tsunami.

"It's a very tricky problem when you have only 30 to 40 minutes to evacuate," he said, — particularly when buildings and roads may be damaged from the initial impact of a strong quake. "What I've told people is that keeping a mountain bike handy would be a good idea," Walsh said.

Jim Downing: 206-515-5627 or jdowning@seattletimes.com