March 1, 2001
UW scientists find signs of liquefaction from Wednesday’s earthquake
University of Washington scientists today were finding evidence of liquefaction in areas south of downtown Seattle, some of them heavily damaged in Wednesday’s major earthquake.
“We’ve seen little sand blows, where pressurized sand and water are forced up from the subsurface,” said Kathy Troost, a research scientist in the UW Department of Earth and Space Sciences. “We’re seeing sand blows throughout the area. We’re seeing some that have left a cavity 3 feet deep, we’re seeing some with sand piled 3 feet high. Those are the extremes.”
Liquefaction happens when pressures from an earthquake infuse water into loose subsurface soil, turning the mixture into liquid. That condition is a common cause for structural failure during earthquakes.
On Wednesday, Troost and her team made their observations while driving streets and alleys south of Safeco Field, an area called SODO. Today they were inspecting areas of Harbor Island, where one witness told Troost he watched water shoot some two feet in the air for about 15 minutes after the earthquake. Both Harbor Island and SODO have large areas built on fill.
Scientists and engineers from the UW joined researchers from the United States Geological Survey, other organizations and private companies looking throughout the Puget Sound region for evidence of, and potential problems from, Wednesday’s magnitude 6.8 earthquake. Teams were monitoring landslide dangers in Thurston, Pierce and King counties. Researchers flew over the region on Wednesday and pinpointed areas that required ground-level inspection.
“We’re going to learn a lot from this earthquake,” said Bill Steele, coordinator of the UW’s seismological laboratory.
The earthquake, the strongest to hit the region since 1949, was blamed for one death, more than 200 injuries and more than $1 billion in damage throughout Western Washington. It occurred nearly 33 miles deep in the Earth’s crust, in an area called the Benioff zone, where the Juan de Fuca slides beneath the North American plate.
“This is a best-case scenario. If we were going to get a magnitude 6.8 earthquake, we’re very grateful that it was deep, that the shaking was moderate. That’s why the casualties were so low, why there was so relatively little damage,” Steele said.
“Imagine 10 times the shaking, which is what a shallow earthquake would produce.”
Two unexpected aftershocks jolted the area this morning, a magnitude 3.4 temblor at 1:10 a.m. PST and a magnitude 2.7 at 6:23 a.m. No damage was reported from either, and only the first was widely felt.
Steele said one reason the region came through this quake so well is because government agencies and private companies have worked together for a number of years trying to foster preparation. In additions, citizens have made a commitment through public levies to strengthening roads and bridges and retrofitting public buildings. Much of the recent emphasis on earthquake preparedness came from a Federal Emergency Management Agency program called Project Impact, coordinated locally by the City of Seattle and King, Pierce and Kitsap counties.
“Project Impact has stimulated a lot of direct investment by citizens and local government that has clearly paid off,” Steele said.
For more information, contact Steele at (206) 685-2255 or Troost at (206) 909-9757.