May 17, 2009
Lori Tobias
The Oregonian
CORVALLIS -- Almost everyone who lives along the Oregon coast knows that if the big one hits, they'll feel some serious shaking.
They also know they will have maybe 10 to 15 minutes to get as far away from the ocean and as high as they can go before a tsunami hits.
Graphic: scientists believe the Juan de Fuca plate and the North America plate are locked in a fierce battle
But what about farther inland? If the Cascadia Subduction Zone ruptures -- and scientists say it's just a matter of time -- will Portlanders feel the jolt? Will rivers flood? Buildings collapse?
A scientist at Oregon State University thinks he has a good idea of what to expect.
Two years ago, Chris Goldfinger, director of the Active Tectonics and Seafloor Mapping Laboratory at OSU, and three other OSU geologists joined a global contingent of scientists in Indonesia to study the 2004 earthquake and tsunami that killed more than a quarter of a million people.
Goldfinger and his colleagues gathered masses of information -- 10,000 years' worth -- which has led to the discovery of some surprising similarities between the Sunda Subduction Zone in the Indian Ocean and the Cascadia Subduction Zone off the Pacific Northwest coast.
They also talked with residents in the city of Medan, which is about 200 miles from the rupture zone in Indonesia.
There, people experienced a rolling motion that made many of them feel seasick. But there was little damage.
The problem for Portland, however, is that it's much closer to the potential rupture zone.
"Portland is only about 85 miles (from the coast), so it will feel the earthquake much more strongly," Goldfinger says. "We won't feel that sharp jolt, but there will be a long rolling that will last three to six minutes."
That's when the damage will happen, he says.
"These buildings are not designed to be swaying for minutes," Goldfinger says of many of the structures in the Portland-metro area.
James Roddey, Earth sciences information officer with the Oregon Department of Geology and Mineral Industries, says Goldfinger is right.
"Nobody knows yet how to really design for a great subduction zone earthquake because we just don't have very many of them," Roddey says.
Roddey says if a subduction zone earthquake does strike, Portland can expect to see damage in all of the brick and mortar buildings because they are not designed to take the sideways motion that would come with such an earthquake.
Wood, steel-reinforced and steel-framed buildings would likely fare better since they can absorb the sideways motion.
"We tell people your homes will do pretty well in these types of earthquakes because the wood is much more flexible."
High-rise buildings, like many in downtown, however, face unique dangers, Roddey says.
"The top begins to move with the earthquake waves independently of the bottom, and now the earthquake is sending shimmers up the building," he says. "Instead of swaying back and forth, now the bottom is moving at a different tempo than the top. It makes it a very dangerous place to be."
As for the rivers, Goldfinger says the tsunami would travel up the Columbia River but it would be more like a big, fast tide, rather than the towering wall of water that coastal dwellers can expect.
"It will slosh like a bathtub, but as far in as Portland, I don't think that much will happen," he says.
Goldfinger is still wading through the information from the 2007 trip -- primarily 100 core samples of turbidites, or underwater landslides -- that may have been caused by ancient earthquakes.
"The question is, when is the next one?" Goldfinger says. "We can't predict that. So the next best thing is to go back in time and see what we can learn about what happened in the past."
So far, Goldfinger knows that the Cascadia Subduction Zone has ruptured its entire length from Northern California to Vancouver Island, B.C., about 19 to 20 times in the past 10,000 years.
Each event measured 9 plus, with the last major earthquake occurring Jan. 26, 1700. He also has learned that the subduction zone has modes, with the southern portion rupturing more often, albeit with less intensity, than the central and northern zones. That tells him that Washington has experienced a major earthquake about every 500 years; central Oregon, about every 450 years; and the southern Oregon/Northern California area, about every 250 years.
"If you are in northern Oregon, there is between a 15 and 25 percent chance of an earthquake in the next 50 years," Goldfinger says. "As you go south, the probability for having one in the next 50 years goes up and up and up."
For all the new research, the Cascadia zone remains something of a mystery. It has the characteristics of other subduction zones, except that it has been very quiet, Goldfinger says.
For a time, one hypothesis held that the Juan de Fuca plate had frozen and ceased to subduct. A second was that the plates were so lubricated with sediment that they had suppressed the earthquakes.
"The hypotheses were hard to argue against because they looked right," Goldfinger says. "Gradually it started to dawn on people that neither made much sense."
Now scientists believe the Juan de Fuca plate and the North America plate are locked in a fierce battle, with the basaltic Juan de Fuca plate pushing the weaker North America plate back 35 millimeters per year -- about 17.5 meters (about 57.5 feet) in the past 500 years.
That could mean trouble is brewing.
"They are compressing like springs, ratcheting tighter and tighter and tighter," Goldfinger says. "Eventually, it will break. When it does, the North America plate will spring 17.5 meters in a minute or so.
"It has been squished for 500 years, and it is going to get all that back all at once. It is a huge amount of energy. The biggest energy release on Earth."
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