UW scientists collect first seismic data from Seattle's Alaskan Way Viaduct

Everyone knows the Alaskan Way Viaduct is not the place to be during an earthquake. Until last month, however, no one actually had measured the way the elevated roadway rattles when the earth rocks.

When a small earthquake struck the region Jan. 30, new seismic sensors on the viaduct recorded the shaking for the first time. The instruments showed that the antiquated structure trembled more intensely than nearby areas. There was no damage from the magnitude-4.5 quake, which originated near Kingston.

But the new sensors revealed that the viaduct and the ground it's built on amplify vibrations, with the top deck shaking 10 times harder than the ground a few dozen yards away. At some frequencies, the shaking was up to 50 times stronger during the quake. The viaduct also shook about twice as long as expected in areas where the ground is firmer.

"It clearly shows us what we already suspected — that the structure shakes strongly," said John Vidale, head of the University of Washington's Seismology Lab. "Now we have an actual number."

Bridges and other structures always shake more at the top than at the foundation, said Jugesh Kapur, state bridge engineer for the Washington Department of Transportation. "It's like a lollipop effect — the mass is up near the top," he said.

But the effect seems a bit more pronounced with the viaduct, he said.

The new data are intriguing, Kapur said, even though they don't add urgency to the state's $3.1 billion plan to replace the viaduct with an elevated section in Sodo and a tunnel through downtown. It's been clear for a long time that the 50-year-old viaduct wasn't built to withstand a major earthquake.

A similar double-decker highway collapsed during the 1989 earthquake in the San Francisco Bay Area, killing more than 40 people. Saturday is the eighth anniversary of the magnitude-6.8 Nisqually quake, which cracked support columns on Seattle's rickety viaduct, leading to costly repairs and traffic closures.

The effects of earthquakes on the viaduct have been simulated many times with computer models. Among the biggest risks are that the loose fill beneath the viaduct will liquefy, or that strong shaking will shatter the brittle structure, said UW civil engineering professor Marc Eberhard, among the first to warn of the viaduct's vulnerability.

UW doctoral student Andrew Delorey installed the first seismic sensors directly on the structure last summer. Working with the U.S. Geological Survey, he put one of the so-called strong motion detectors on a sidewalk at the base and another on a column on the top deck. A third instrument in the basement of a nearby building serves as a reference.

Delorey has been monitoring the way traffic vibrates the roadway. He got lucky with January's small quake. "It was not destructive, but it was big enough that we could see how the viaduct shakes," Delorey said.

The roadway's motions during the earthquake were slight — in the millimeter range. That's similar to vibrations caused by the more than 100,000 vehicles that travel the roadway every day.

Someone driving across the viaduct wouldn't have noticed the earthquake, Delorey said.

"If you were standing there and there were no cars going by, you probably would have felt it," he said.

The viaduct shook so long — more than 50 seconds — because it is located in a large geologic basin that traps seismic energy. "These basins are like Jell-O," Vidale said. "You shake it a little bit and it keeps shaking for a long time."

Eberhard cautioned that it's hard to extrapolate from a small quake to a big one.

But he and Vidale agree that putting seismic instruments on more bridges and buildings would help paint a better picture of the way the structures will respond to earthquakes. Vidale would like to see dozens of new, inexpensive instruments installed in downtown high-rises.

"I'd like to put on one top of the Space Needle, too," he said, with a laugh. "It would be fun to watch it sway."

Sandi Doughton: 206-464-2491 or sdoughton@seattletimes.com

Though the motion was slight, new sensors provide the first view of how different parts of the Alaskan Way Viaduct shake in a real earthquake.