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Would a Major Earthquake Sink Portland or Seattle in Liquefied Soil?

April 12, 2012 at 12:00 AM EDT
Though the impact of Wednesday's 5.9-magnitude earthquake off Oregon's coast was minimal, a lesser-known risk of temblors -- a phenomenon called liquefaction where sandy soil turns to liquid and loses its ability to support weight -- has some scientists worried. Tom Bearden reports what's being done to prepare for a major quake.

JUDY WOODRUFF: Next, how scientists are preparing for the dangers of an earthquake in Oregon.

Yesterday, a quake with a 5.9-magnitude struck about 160 miles offshore, with no real impact. But, increasingly, researchers are worried about specific and lesser-known risks of a major tremor there.

NewsHour correspondent Tom Bearden reports.

TOM BEARDEN: Yesterday’s quake off the Oregon coast caused no damage. But, historically, the Pacific Northwest has experienced quakes as powerful as the one that devastated Japan. They just don’t happen as often.

Scott Ashford is the interim dean of engineering at Oregon State University.

SCOTT ASHFORD, Oregon State University: On the average, they occur about every 300 years. The last one was 312 years ago.

TOM BEARDEN: Do the math.


TOM BEARDEN: The National Science Foundation dispatched an American team to Japan shortly after the disaster. Ashford went along to study a lesser known earthquake effect, how sandy soils actually liquefy when the earth shakes, and what happens to structures built on that soil.

Major cities like Portland and Seattle are built on similar soils.

SCOTT ASHFORD: We saw many buildings that just settled and just kept settling. So with a — once the ground liquefied, it lost foundation support, and then with the continued shaking, they just continued to sink into the ground.

TOM BEARDEN: While surface structures settle, underground utilities tend to rise out of the ground.

SCOTT ASHFORD: When the ground turns into almost a viscous fluid, and when that takes place, a lot of our utilities like manholes, like sewer lines, will tend to float up out of the ground, and when something floats like that that’s in the ground, it tends to break.

TOM BEARDEN: Ashford says all this can be mitigated if stronger soils are mixed into the sand and different construction techniques are used.

SCOTT ASHFORD: If you’re able to go in and densify them before you build something on top of it, you can mitigate the liquefaction hazard. There’s also methods of ground improvement, stone columns, deep cement mixing. All are methods to also mitigate that liquefaction hazard, if you don’t have the opportunity to densify the soils ahead of time.

TOM BEARDEN: Scientists also replicate liquefaction by setting off explosives underground, like this test on an island in San Francisco Bay.

SCOTT ASHFORD: One of the things that we learned in the Treasure Island experiments was that the resistance of the soil was much different than what we had learned from the small-scale experiments in the laboratory.

TOM BEARDEN: A much larger test in Japan showed how an earthquake can spawn sand boils, a miniature volcano of sand and water, that can spread huge quantities of sand on the surface, making an area impassable.

Such experiments also showed how liquefaction can cause the ground to shift sideways, a real problem if there’s a bridge on top.

SCOTT ASHFORD: When we’re looking at liquefaction and the subsequent lateral spreading, we typically see that along riverbanks, where there’s enough of a slope that, when the ground liquefies, you essentially have a slope failure.

And you can have several meters of movement of the ground toward the river. Now, when that ground moves, it tends to bring the bridge foundation with it. And that can essentially rip apart the bridge.

TOM BEARDEN: The Oregon Department of Transportation has spent more than 20 years working on ways to minimize earthquake damage and damage from the tsunami that will follow. But they have only managed to retrofit about 100 out of 1,100 bridges at risk.

Highway Division Administrator Paul Mather.

PAUL MATHER, Oregon Highway Division administrator: Virtually every bridge in Western Oregon is at risk. So, that’s the I-5 Corridor. That’s all of 101 and every bridge that connects those corridors in the state highway system, but then there’s also on the local system, as well as private bridges that we have in western Oregon. So it’s virtually every structure in western Oregon.

TOM BEARDEN: Mather says, if those bridges go down, the entire coast will be cut off from outside help . And people won’t be able to evacuate either.

Mather says the state is focusing on shoring up the bridges along several lifeline routes to have a better chance of keeping them open. But even that will take years. And Mather says, at the current rate of bridge replacement, it will be 100 years before every bridge in Oregon is earthquake-resistive.

But those lifeline routes will only be useful if people on the coast actually survive the tsunami.

Pat Corcoran’s job is to preach the gospel of earthquake and tsunami preparedness. He’s a hazardous outreach specialist with Oregon State’s Extension Service. He’s skeptical of purely engineering solutions.

PAT CORCORAN, coastal hazards outreach apecialist: So, in Japan, technology and engineering was a two-edged sword. It was very positive in the structural developments of the roads and infrastructure, so people could evacuate effectively.

And 90 percent of the affected population did evacuate effectively. The other edge of the sword of technology is, the seawalls lulled people into complacency and to develop in places they probably shouldn’t have.

TOM BEARDEN: He says people on the coast will be on their own when the earthquake strikes, and have just minutes to get to higher ground before the tsunami roars ashore. Corcoran says coastal towns must be persuaded to hold regular evacuation drills.

PAT CORCORAN: I think it’s true to say that not everybody will be as enthusiastic about preparing for tsunamis as some. It’s against human nature, really, to spend a lot of time worrying about these things, even though we know it’s going to happen. Again, it’s a matter of bringing alignment with what our heads know and how our feet act on the coast.

TOM BEARDEN: But, after Japan, Corcoran says more Oregonians are finally starting to listen.

JUDY WOODRUFF: On our website, it’s Science Thursday. And on our Science page, Tom has more about how engineers in Oregon are preparing for the worst-case earthquake.

Also there, a story about green burials, using biodegradable caskets and urns and no embalming fluids.