I-5 Bridge’s quake vulnerability worries experts

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Would sponsors of the Columbia River Crossing project continue to push for the multibillion-dollar project even without fretting about it falling down in the next major earthquake?

Anne Pressentin, a spokeswoman for the bistate CRC office in Vancouver, had a straightforward answer.

“Yes,” she said.

That’s because there are five other major purposes for the project besides making the crossing more resistant to earthquakes: To reduce congestion, improve the movement of freight, improve pedestrian and bicycle access, offer transit alternatives to commuters, and improve safety in the one the most accident-prone stretches of Interstate 5 in the Northwest.

However, earthquake vulnerability is not a trivial matter among project planners, emergency managers and earth scientists.

“If I’m sitting on that bridge in a traffic jam, and I’m underneath one of those counterweights, I really try to get out from under it,” said Evelyn Roeloffs, a geophysicist with the U.S. Geological Survey in Vancouver. “It makes me nervous to be parked under there.”

Each of the concrete counterweights tips the scale just shy of 700 tons.

The current twin three-lane spans, built in 1917 and 1958, are perched atop concrete piers and timber piles. Those piles extend only into the muddy river bottom rather than all the way down to the underlying rock, making the spans susceptible to liquefaction to a severe quake.

A seismic vulnerability study conducted for the crossing project in 2006 identifies two major sources of especially worrisome earth movement for the I-5 Bridge. One involves the offshore Juan de Fuca tectonic plate slipping suddenly beneath the North American plate in a monster quake that last occurred on Jan. 26, 1700. This Cascadia subduction zone quake could generate a magnitude of up to 9.0, resulting in several minutes of strong shaking in the Portland-Vancouver area. The other concern would be major quake on a fault zone along Portland’s West Hills, which could generate a magnitude-6.8 earthquake.

Those kind of severe quakes might occur today, tomorrow, or 300 years from now.

U.S. Sen. Patty Murray, D-Wash., last week highlighted the safety hazard when she met with local business and political leaders in Vancouver.

“It is unsafe,” she said, “and it’s holding back this region’s economy right now.”

In an interview afterward, Murray said she was concerned about the long-term seismic vulnerability. But she was just as worried about the day-to-day traffic hazard caused by the lack of shoulders and tightly spaced interchanges.

Erik Robinson: 360-735-4551 or erik.robinson@columbian.com.