One year after the Nisqually quake, what have we learned?
Seattle Times staff reporter
In midsentence, the floor of her University of Washington office began to shudder and dance. Troost vaulted from her chair and planted her feet to soak up the movement.
"I'm yipping it up, looking up at my ceiling tiles and the lights, laughing with this gentleman that we should really be getting under the table," she said. "But we couldn't. It was such a thrill."
Troost and other scientists have surfed the Nisqually quake's waves ever since.
The 6.8 temblor rocked the Puget Sound region, injuring 410 people and causing damage put at $2 billion to $3.5 billion.
In that time, geologists and seismologists have examined crumbled chimneys, landslides and fissures in the earth. They've visited newly created "sand volcanoes," collected hundreds of soil samples, flown over the region with high-intensity lasers and dug up potential new sections of the Seattle fault.
But their research is as much about framing questions as it is about finding answers, with some areas of particular promise:
• Mapping the region's surface and subsurface geology will help show how underground features — some predating the last glacial period — might affect the way a quake's quivering spreads and how long the shaking might last.
• Strength tests of soil could determine whether certain compositions are more capable of softening ground motion.
"We are trying to determine why sites like the top of a hill in West Seattle shook harder than any other hills in the area," Troost said.
That information will feed other efforts to understand the threat of more explosive earthquakes.
• Attempts to date older and intense earthquakes along crustal faults could reveal the relative frequency of quakes closer to the surface than the Nisqually, which occurred about 37 miles below Anderson Island, northeast of Olympia.
• The discovery of potential new strands of the Seattle fault, from Hood Canal to Port Angeles, may show whether the region is significantly more active than once thought. This is particularly important as the Seattle fault, sitting so close to the surface, holds the potential to be particularly destructive.
"It could be that we've underestimated the risk," said Craig Weaver, Northwest coordinator for the U.S. Geological Survey's (USGS) earthquake program. But, he added, "there's a lot of uncertainty."
Sensors uncover surprises
When the Nisqually quake struck a few minutes before 11 a.m., a wider array of motion sensors than ever captured the shaking regionwide. That was important.
When the World Series quake struck San Francisco in 1989, rescuers were focused on the Bay Area. Their seismic network wasn't large enough to know immediately that significant shaking stretched south to Santa Cruz.
Here, Weaver instantly checked his data and could tell his son, then in ninth grade at Timbercrest Junior High in Woodinville, was safe.
And that data provided a starting point for surprises.
The Puget Sound region has been covered by glaciers at least six times, the last about 14,000 years ago. When the ice receded, hills and deep valleys were deposited with soft, weaker soils that tend to amplify earthquake motions.
Yet one of the most interesting things about the Nisqually quake was what scientists didn't see, said Steve Kramer, a UW civil-engineering professor. Many areas were not as affected by the quake's ground motion as expected.
At Boeing Field, the northern half of the runway partly collapsed. The southern half did not. Harbor Island experienced liquefaction, when seemingly stable soils turn to jelly during subsurface rumbling, but the terminal immediately to the west did not.
"I looked at sites in Puyallup that showed liquefaction in the earthquakes of 1965 and 1949 but didn't here," Kramer said. "I'd like to know why."
The soil factor
To help solve the seismic riddles, scientists pick apart the landscape.
Kramer and his students take vials of soil, encase them in rubber and apply force. As the pressure of water in the soil increases, scientists can measure the soil's strength. After hundreds of tests, they can characterize how soil types might respond to a quake.
"Is there something about (the region's) soils that makes them inherently more resistant?" Kramer asked. "We're a year or two years away from being able to draw firm conclusions."
But those conclusions could fuel new standards for everything from building safety to bridge and road construction.
Scientists did notice that many areas that experienced liquefaction or some ground rupturing — including several tideland areas — had long ago been filled with new dirt that had been poorly compacted and regraded. The northern half of Boeing Field, for example, had been built over old meanders of the Duwamish River.
"Essentially, within the high-risk zone, only the weakest, loosest materials failed," Troost said. "The question is how much more would it take for the rest to fail?"
The material properties of each layer of earth vary tremendously from spot to spot, so Troost has been trying to define that variability. That includes studying what's well below the surface.
Buried deep by the last glacial period are old river valleys, some filled with mudflow deposits. That subsurface configuration could influence ground-water flow, flow to surface streams and, potentially, how the ground shakes.
"One of the big challenges is to use all this to predict how shaking will occur during a shallower quake because these aren't the highest ground motions that are going to be seen here," said Art Frankel of the USGS.
Prevailing wisdom undone
Before 1991, the prevailing wisdom was that Puget Sound didn't have huge crustal quakes — the rip-roaring surface kind that pose the greatest threat to Western Washington. The existing fault system from which crustal quakes stem wasn't known to move. By 1992, large old earthquakes were being documented in these shallower faults.
Only in the past four years have scientists actually touched visible evidence, on Bainbridge Island, of the vast fault system experts suspect could produce a really big earthquake in the region. There they discovered a scarp, or rupture in the ground, on Restoration Point.
Since the Bainbridge discovery, evidence has cascaded. Geologists, using laser-mapping techniques accurate to 10 centimeters, have found a half-dozen new strands of the massive Seattle-fault system — the latest this year at Sunset Beach along Hood Canal. In each case, they've found these faults, possibly independent, nosing up to the north.
Scientists are evaluating finds from a church parking lot in Bellevue to a brushy marsh on Bainbridge to gauge how active the system really is.
"We still don't exactly know how the system works," Weaver said. "If it turns out that we find more of these big crustal faults with multiple events, that could tip us to a higher (risk) assessment. But there's lots of ambiguity."
Craig Welch can be reached at 206-464-2093 or email@example.com.