200 seismometers will show Seattle's trouble spots

Thursday, March 23, 2000

By TOM PAULSON
SEATTLE POST-INTELLIGENCER REPORTER

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Dropping the Kingdome will produce only a tiny earthquake Sunday morning, but researchers at the U.S. Geological Survey and University of Washington say it could produce some big findings.

"This is going to give us a much more detailed ground-shaking map for the area," said Craig Weaver, senior scientist for the USGS in Seattle. "We're dropping the Kingdome right in the middle of a big fault."

That's the so-called Seattle Fault -- a seismically active crack in the rock stretching from Bremerton and Bainbridge Island through Seattle to Issaquah. It is believed capable of producing at least a magnitude 7 earthquake.

By comparison, the 1989 Loma Prieta quake that heavily damaged San Francisco registered magnitude 7.1.

The 1995 quake in Kobe, Japan, which killed 6,500 people and caused billions of dollars in damage, was magnitude 7.2.

USGS and UW scientists plan to capitalize on the Dome demolition by installing some 200 seismometers around Seattle to record the miniquake. The study is part of the ongoing Seismic Hazards Investigation in Puget Sound (SHIPS) project.

The implosion of the Dome and some preceding test explosions set off around Seattle should make for a nice wave of seismic data.

Getting a better handle on the Seattle Fault is critical.

It's large enough to devastate the region. Geological evidence indicates that a large quake occurred on this fault about 1,100 years ago. It wreaked havoc on the local landscape, heaving land by tens of feet. Today's Restoration Point on Bainbridge Island was raised 25 feet by that quake.

Tsunamis rolled back and forth across Puget Sound and Lake Washington.

Massive landslides sent coastal forests into the water, submerging plots of trees that still remain standing today in deep water.

Despite the magnitude of the risk, the scientific community only recognized the history of major quakes in the region about a decade ago. The infrequency of large quakes here, combined with the lush Pacific Northwest flora that covers much of the topography, has helped disguise the hazard.

"We've made a lot of progress identifying the nature of that risk," said Bill Steele, coordinator of the UW seismology lab. But there's a lot more to be learned, Steele said, which is why the Kingdome seismic test is needed.

A similar seismic recording test for the SHIPS project was done last September, again involving explosives set off at depth. It caused a fuss when some residents were shaken by what they thought was a real earthquake.

"Our shots traveled more than we expected," said Tom Brocher, a USGS geologist out of Menlo Park, Calif., who with Weaver is lead scientist for this seismic recording project. "We learned our lesson. . . . The shots this time are going to be smaller."

The Dome is scheduled to implode at 8:30 a.m. Sunday. Between 3 and 4 that morning, earthquake scientists will set off explosions about 100 feet deep at four corners of metropolitan Seattle.

The explosives will be detonated deep under Magnuson, Discovery, Seward and Lincoln parks to calibrate the measurement of the seismic wave from the Dome implosion.

The scientists don't expect many people to notice their tests this time around.

The seismic detectors are being placed throughout greater Seattle to record both of the induced seismic events. Many more people than were needed volunteered to allow the detectors to be placed on their property for the experiment, the geologists said.

"The response was just tremendous," Weaver said.

The fact that the explosives used for the seismic testing done in September caused more ground-shaking than anticipated is, in itself, an important finding that demonstrates the need for more study.

"We weren't expecting such a strong response," Weaver said. "We got a big ground roll."

"And that's probably not good news," added Steele.

In general, this means the ground around here is a lot more prone to shaking during an earthquake than was expected.

The September SHIPS study indicated that parts of West Seattle may be as shaky as the Duwamish flats -- an area already considered higher risk during a major quake because it's largely fill material rather than solid rock.

Weaver and Steele said the northern part of Seattle, north of the Ship Canal, also shook more than expected during the September test.

It was already known that areas north of the Seattle Fault (north of the Kingdome) can expect to receive more of the damaging energy from a quake than areas south of the fault. But geologists presumed north Seattle to be relatively stable. The September shots called this assumption into question.

"It wasn't what we expected," Weaver said. It's clear that there's a wide variation in seismic risk across Puget Sound, he said, and that it's not easy to predict how it will vary.

The SHIPS study, including the data gained from knocking down the Dome, will eventually result in a map of seismic risk that should assist in protecting against damage when the next Big One strikes.

"It will give us a really good three-dimensional picture of the geology of downtown Seattle," said Steele.

Art Frankel, a USGS scientist from Golden, Colo., is counting on the data gained Sunday to improve his 3-D computer model of a large quake on the Seattle Fault. He already has crafted a moving visual representation of how a major quake would shake the city based on what's known so far about the geology.

"Our goal is to make detailed maps of Seattle that can describe, block by block, the amount of ground motion shaking expected during an earthquake," Frankel said. "By recording these timed seismic waves, we can get there."

But could the induced Dome quake itself trigger a major quake?

"Well, anything's possible, I guess," said Steele. "But we have quakes bigger than what we'll get from the Kingdome implosion every week and they don't set off the Seattle Fault. . . . Scientists don't like to say something's impossible, but I'd say it's pretty unlikely."

P-I reporter Tom Paulson can be reached at 206-448-8318 or tompaulson@seattle-pi.com