The vulnerability of Oregon to tsunami was a reason behind a recent trip to India by Curt Peterson, a professor of geology. Peterson left Portland last Wednesday and will be gone 10 days. He’s not there to supervise relief efforts but instead to collect data for tsunami hazard maps.
Tsunami hazards in East Asia bear substantial similarity to hazard maps of the Oregon Coast. Peterson has spent the past quarter century mapping ancient tsunami deposits on our coast. Offshore topography found in East Asia bear a combination of factors similar to those that exist on our coastline.
This week Ken Cruikshank, associate professor of geology, reviewed the study of tsunamis and the mapping of hazards along Oregon’s coastline.
"The tectonics in Indonesia are a very good match for what we have on the Oregon coast," he said. Tectonics refers to the changing of plates that shift and slide over the Earth’s surface.
"Our hazard maps are based on computer models," Cruikshank said. "One of the things we need to do is find out how well our hazard maps compare with our computer models."
Peterson went to India to study how well existing computer models compare with actual conditions there and where the models may need improvement.
"Which in turn should help produce more accurate hazard maps," Cruikshank said. He maintains a library of hazard maps based on computer models produced by the National Oceanographic and Atmospheric Administration. They are marked with ominous "red zones" that predict the likely areas of extreme hazard, shading off into areas of slightly lesser hazard. Around Seaside, the most vulnerable areas lie near the Siletz River, threatening severe danger to low-lying locations such as Taft and Culver City.
When a tsunami hits the coast at Seaside, it will change direction slightly to follow the shape of the flatter terrain. Red dots on the hazard maps show areas where people have discovered historic tsunami evidence well outside the current red zones. There are tsunami deposits near the Siletz River estimated at about 300 years old, Cruikshank said.
Only a half century ago, in 1964, a tsunami killed people at Seaside – the number variously recalled as two or three. The same tsunami wreaked havoc at Crescent City, Calif. The earthquake that caused it came all the way from Alaska’s Aleutian Islands. PSU geologists warn it could happen again.
Tsunamis have characteristics strange and not entirely predictable. They usually follow earthquakes under the ocean that may occur thousands of miles away but produce the giant waves that travel great distances.
"These waves can sweep across huge expanses of the ocean," Cruikshank said. On the other hand, some earthquakes do not produce tsunamis.
"An earthquake of 8.1 to 8.2 south of New Zealand did not produce a tsunami," he said. "To produce a tsunami we have to have big earthquakes, but not all earthquakes produce tsunamis."
The Pacific Rim, which includes the Oregon coast, has a system in place that can send advance warning of an approaching tsunami. Buoys located on the ocean floor can detect when a giant wave is forming at the surface. The buoys, currently being increased in number, send the data to satellites, which then spread the warning.
All Oregon coast cities in tsunami danger zones display signs warning of tsunami hazard and direct residents to tsunami escape routes.
Cruickshank, along with other geologists, are wary of a worst-case scenario they say is possible for the Oregon coast. In this scenario, there is a close, offshore earthquake of magnitude 8 or 9, followed by a tsunami.
Such an event could potentially wipe out much of the population of Seaside before fleeing inhabitants could reach the safety of surrounding hills.
"You would have 15 or 20 minutes before the tsunami hit," he said, "and the first five minutes the shaking would be so bad you would not be able to stand." The ground would recede suddenly by several feet. In Seaside this would sink the three old bridges that connect the beach with the mainland, making escape by bridge impossible. The highway out of town would become impassable.
Assuming that such an event would give residents approximately 15 minutes to escape, a group of PSU students about five years ago decided to test how far they could run from Seaside beach to mountain safety in 15 minutes. At the end of the 15, they ended nowhere near safe ground.
Cruickshank said people most commonly die in tsunamis when fast-floating debris traps them and they drown. He revealed that tsunamis give warning of their imminent approach but people may not be aware of these danger signs.
"The ocean suddenly goes way out," he said. Unfortunately, some people will go out on the beach to inspect this strange phenomenon and get hit by the following wave. They also don’t understand that the first tsunami is typically followed by a second or third, which may be bigger than the first.
"If I saw the ocean suddenly go out, I’d be running the other way as fast as I could," Cruickshank said. "I’d be getting myself high and safe."
The city of Seaside, cognizant of the virtual impossibility of escape from the beach in such an event, has formulated emergency plans. These would involve building fairly high buildings, like parking structures, with breakaway ground floors to let the wave wash through at ground level. Meanwhile, people could climb up into the higher levels of the structure, where they would be safe from the cascading streams of debris.
What would such a coastal disaster be like for Portland?
"The quake would produce severe shaking in the Portland area," Cruikshank said. "It could have major effects." Chimneys and masonry buildings could topple. The rivers would reflect tidal effects.
"We would probably lose some of the bridges across the Willamette," he said. "I-5 is prone to landslides; Portland would be relatively isolated for a few days. But there would be a relatively small loss of life."
Every few years the state produces an earthquake scenario study that assesses potential damage and loss for the Portland metropolitan area. The largest earthquake on record occurred in 1877 and was estimated at 5.7. A copy of the study about 10 years old assumes a magnitude 6.5 earthquake likely to cause $20 million in damage, injure 39 people and perhaps kill 11 within the 60-block central city area.