Oregon rethinks tsunami's reach

Story Posted: Wed, Sep 19, 2007

Safety - The old worst-case killer wave scenarios aren't bad enough, scientists say

Wednesday, September 19, 2007
STUART TOMLINSON
The Oregonian Staff

When the next mega-earthquake strikes the Oregon coast, it will announce itself with minutes of shaking followed by a tsunami as high as 35 feet.

As communities up and down the Oregon coast run through a tsunami-warning drill today, scientists say they're increasingly concerned about the effects such a deadly wall of water would have on the 750 miles of coastline from Northern California to Alaska.

In the wake of a massive Indian Ocean earthquake in December 2004 that killed at least 186,000 people, Northwest researchers realized things could get a lot worse along the offshore Cascadia fault line than they had forecast. Now, as a result, they're scrambling to revise estimates of how far inland flood waters would reach if a tsunami hit communities from Astoria to Brookings.

"It's big," said Yumei Wang, a geotechnical engineer for the Oregon Department of Geology and Mineral Industries, said of the process. "When you throw a pebble into water you know you're going to have ripples. You then have to take into account the ocean floor, near-shore geomorphology and estuaries."

Wang said the most recent inundation maps, developed in 1995, were designed to help coastal communities figure out where to build hospitals, fire stations and community centers. The new maps will focus on tsunami evacuation plans -- pushing evacuees higher up and further inland.

"You want to tell people to go somewhere where they will be safe," she said.

In the meantime, National Weather Service offices from Seattle to San Diego will issue a test warning this morning courtesy of the federal earthquake warning center in Palmer, Alaska. Warning sirens won't generally sound, but many communities will test out their communications between 9-1-1 centers, emergency managers, first responders and schools.

"It doesn't fully test the system, but it's as good as we can get without causing panic," said Tyree Wilde, warning coordination meteorologist for the National Weather Service in Portland.

The warning will be broadcast on National Oceanic & Atmospheric Administration weather radio, through the Emergency Alert System (formerly the Emergency Broadcast System) and will go out over the Internet, pagers, e-mails and fax machines -- a test not so much of their response to a tsunami, but that the message is getting through.

Whether it's a test or the real deal, however, the question is how far up -- and inland -- coastal residents and visitors need to go when they hear the warning.

Officials in Cannon Beach, one of the first Oregon coast communities to adopt a tsunami warning system, have long recommend getting at least 50 feet above sea level. Cannon Beach, like other towns up and down the coast, has developed and marked detailed evacuation routes accordingly.

But preliminary data from a new study of worst-case scenarios in Cannon Beach -- scheduled for release early next year -- show that true safety zones could need to be "tens of feet" higher than original estimates, said Rob Witter, regional coastal geologist for the Oregon Department of Geology and Mineral Industries based in Newport.

So areas considered safe under the old standards could actually be underwater when the Big One hits.

Not every community will have to entirely revamp its emergency plans, however.

Tom Manning, director of Tillamook County Emergency Management, said Tillamook County already added a 25-foot buffer zone to the original inundation maps when it planned evacuation routes and evacuation zones.

"We went beyond the 50-foot zone to 75 feet," he said. "We figured in ten feet of tide and additional room for how high the water would come when it's full of debris."

At least 20 times in the past 10,000 years, a massive rift in the seafloor off the Oregon coast known as the Cascadia subduction zone has torn apart, generating earthquakes anywhere from magnitude 7.0 to 9.0.

It's an area that looks a lot like the seafloor near Sumatra, which generated the 9.0 magnitude quake the day after Christmas in 2004. It was that quake that prompted Oregon's state geologists to take a second, closer look at the 1995 inundation maps.

Oregon scientists, along with colleagues from Oregon State University and the California Geological Survey, are re-mapping the danger zones, starting with Cannon Beach and the northern Oregon coast, later moving to the southern Oregon coast.

Eventually, Witter said, the data will generate a Web-based, interactive map that will allow users to pinpoint inundation hazards in their area with the click of a mouse.

Officials said new, more dire inundation maps probably won't hamper development in towns such as Cannon Beach, which is on a slope, but may have a more chilling effect in towns such as Seaside, which is located on a flatter coastal plain.

"Showing those waves crashing into Seaside in the wave tank last week on TV is not going to go unnoticed," said Alfred Aya Jr., board president of the Cannon Beach Fire District.

Aya said the existing inundation maps were based on what scientists believed was the worst-case scenario. It just turns out that they may not have been "worst" enough.

The real lesson might be for coastal residents not to rely exclusively on any scientific prediction, Aya said.

"The main thing people need to learn is to go to higher ground as soon as the shaking starts," Aya said. "Don't wait for warnings, alerts or sirens. Drop everything and run."

Stuart Tomlinson: 503-294-5940; stuarttomlinson@news.oregonian.com