SHIPS in the News

NY Times "Ominous Findings on Seattle Quake Risk", Jan 23, 2001

Kingdome implosion

New York Times
January 23, 2001
Ominous Findings on Seattle Quake Risk
By CAROL KAESUK YOON, BOARD M.V. KITSAP, of Seattle —

In a classic Seattle scene, this morning ferry full of latte-sipping commuters is sailing serenely beneath winter clouds through the labyrinth of islands and mainland that everywhere dissect Puget Sound.

Beneath this tranquillity, however, lies something altogether unnerving. For as Dr. Thomas Pratt and Dr. Brian Sherrod, two earthquake researchers aboard, note, the boat is sailing directly over what some scientists say is the most deadly earthquake threat in the region: the Seattle fault zone. Forty miles long and five miles wide, this huge complex of adjacent faults runs right under the sound and the south end of Seattle itself.

Standing in the rain on the deck, Dr. Sherrod points out a passing terrace of land on Bainbridge Island known as Restoration Point. Once submerged, the terrace appeared when the point was hauled 20 feet up and out of the sound during a violent quake 1,100 years ago on the Seattle fault (one of the faults within the Seattle fault zone and the one that gives it its name). Part of a growing body of evidence for the wreckage wrought by the fault, such sites are potent indicators of what the fault could do again.

"These faults are going to fire up right in the middle of where we all live and work," said Dr. Craig Weaver, regional coordinator for the United States Geological Survey earthquake program. "A magnitude 7 on the Seattle fault will look like those that shook up Kobe, Japan, or Northridge. That's the kind of motion we're predicting here."
In spite of intense interest, the basic architecture of the fault zone has remained elusive to scientists. But this complex geological puzzle, and the threat it may hold for the city above it, is beginning to come into view, thanks to a series of recent experiments that have relied on the implosion of the city's old sports stadium, late-night explosions across the city and the famous friendliness of Seattleites.
"They're seeing things that we've never been able to see before," said Dr. Mark Brandon, structural geologist at Yale, who is not associated with the work. "The resolution is very high quality."
Unfortunately for Seattle, however, the researchers are seeing evidence of more faults, signs of at least one unexpected hot spot of shaking and — most worrisome of all — evidence that the basin upon which most of Seattle is built actually amplifies earthquake shaking.

A huge bowl of bedrock many miles across, the basin lying at the northern edge of the fault zone is thought to have been created when the south side of the fault slipped up and the north side slid down, creating a depression. Over the eons, the basin has filled in with lighter materials, various kinds of mud, silt, sand and muck, creating just the sort of nice flat area by a bay that people like to settle on.

"It's like building on Jell-O," said Dr. Pratt. "You put a bowl of Jell-O on a table and shake the table, that bowl of Jell-O is going to oscillate a lot."

Dr. Tom Brocher, geophysicist with the United States Geological Survey and lead scientist on much of the new research, said that under some conditions he and colleagues were seeing as much as a five- to twelve-fold difference in shaking inside and outside the basin. "That's pretty big," he said. "It's as if all of a sudden you're 10 times closer to the earthquake."

The project, known as Ships for Seismic Hazards Investigation of Puget Sound, was carried out by the Geological Survey along with 10 other institutions, including the University of Washington, University of Texas at El Paso, Oregon State University and the Geological Survey of Canada.

Scientists interested in studying features of the earth, like the Seattle fault zone, which lay almost entirely buried and at great depths, often explore these structures using explosions or other sources of sound or vibration.

Researchers then use seismometers — devices that detect movement of the earth — to monitor the passage of the earthquakelike sound waves. Seeing how quickly waves move through an area, scientists can then begin to visualize not only the structure of what lies beneath the surface but the way in which those rocks and sediments will react to actual temblors.

So Ships researchers have diligently and noisily carried out their studies. They have fired off 33,000 air-gun shots from a research vessel cruising through Puget Sound and beyond. They have detonated 38 underground dynamite explosions late at night in a line running right through the city, causing some Seattle residents to jump out of their beds thinking there was an actual earthquake.

"We do these things in the middle of the night when it's quiet," said Dr. Pratt, a geophysicist with the Geological Survey, to reduce the amount of extraneous noise that could muddle their results. "The trouble is that people are asleep in beds. A bed is a very good motion sensor."

But if the city has been made a giant laboratory, many Seattleites have made good-humored lab assistants. They have continued to volunteer in droves to participate in Ships experiments and have recording devices buried on their property.

"We're in everybody's backyard," said a grateful William Steele, seismology lab coordinator at the University of Washington.

Among the things researchers are now learning from these cumulative experiments is that there appears to be a large fault zone south of Seattle and just north of Tacoma, another city sitting over a sediment-filled basin.

The newly recognized fault zone presents another potential earthquake risk in the region and suggests that more faults are yet to be found.

"The crust beneath Puget Sound is riddled with fractures," said Dr. Sherrod, a geologist with the Geological Survey. "The more we look the more we're going to find. We've just started to look in earnest."

Ships researchers even have some very preliminary data that there may be previously unsuspected faults right at the bottom of the basin underlying Seattle. Scientists note that it remains to be seen whether any discovered faults actively pose earthquake risks, since a fault has to move to create a threat.

Some of the Ships researchers' best experiments have involved seismic serendipity, like the implosion of the 200,000 pound concrete Kingdome, which sat right atop the fault zone.

"That experiment really came out of a bunch of us sitting around saying, `Hey, they're going to blow up the Kingdome, that's going to make a lot of noise,' " Dr. Pratt said.

So in addition to the breathless wonder of Washingtonians who woke up early that Sunday this past March to witness the implosion on television (as well as the countless slow-motion replays), that Seattle mega-event produced a unique experiment: scheduled vibrations on the fault zone and in the midst of this populous city equivalent to a 2.3 magnitude temblor.

The result from the network of 200 seismometers deployed throughout the city is a neighborhood by neighborhood look at how Seattle could shake during an actual quake either from the fault zone or elsewhere. And though the data are just beginning to be analyzed, scientists have already identified one unexpected hot spot of shaking in the city.

"That area south of the University of Washington just lit up," Dr. Brocher said, referring to an area near the Washington Park Arboretum. "We had no idea there would be anything there."

Researchers cautioned against a panic over living near the university, however. They noted that the falling of the Kingdome was a surface vibration that might not perfectly mimic earthquakes. They plan to continue to record in the suspected hot spot to see what happens with other more earthquakelike sources of shaking.

The researchers reaped more data on shaking from the Chi-Chi earthquake in Taiwan. After one of the Ships experiments, researchers had left seismometers in the ground in the hope that they might record just such a real earthquake. The low- frequency vibrations from the Chi- Chi earthquake were particularly valuable because they complemented the findings from higher-frequency vibrations generated by explosions. Of greatest interest is the finding that the Seattle basin is amplifying earthquake shaking as it passes through.

Researchers say they now suspect that the amplification in the basin may be largely due to the topmost layer of gelatinlike sediments.

"They're enormously efficient at capturing and channeling the energy," said Dr. Weaver. "Then there's nowhere for it go, except into buildings, landslides. Any number of things can happen."

In addition to the Ships studies, researchers have used other approaches to study earthquake hazards in the region, like searching for evidence of ancient landslides caused by earthquakes and using aerial laser scans to detect very subtle signs of ancient fault movement. Researchers have even studied differences in gravity over the region to assess what lies beneath.

(The added force of gravity created by the fault zone makes something that would weigh 10,000 pounds in the basin weigh 10,001 over the fault zone.)

One critical piece of information in determining the danger of the Seattle fault zone is knowing how many times it has moved and when.

To answer that question, researchers have gone in search of the faults themselves, seeking out places where they may poke up, providing clues to dates of past temblors.

On the lookout for years, Dr. Sherrod recently spotted one possible site while stuck in traffic. The site, beneath the southbound lanes of Interstate 5 just south of downtown, turned out to be ancient lake beds that have been violently deformed, possibly by the power of the fault zone within which they may lie.

Even more recently Dr. Sherrod and colleagues may have come even closer to the action at a construction site in Bellevue, an affluent suburb east of Seattle. The site, which researchers say is definitely a fault of some sort, lies exactly where researchers would have expected to find the Seattle fault zone exposed.

"This was a major find," said Dr. Sherrod, who is now trying to verify if it is an unrelated fracture or the long-sought Seattle fault itself. "You can't see anything and then someone does an excavation and boom, the lights just come right on. We dropped everything we were doing to run over there and map it."

Despite the increased clarity from Ships and other studies, researchers still do not know exactly which geological forces produced the fault zone, exactly how deep it goes, what angle it tilts at, or just where its north-south or east-west beginnings and ends are — all of which could affect the level of risk the fault zone poses to the area.

And while some scientists say they view the fault zone as the major earthquake risk in the area, there remains debate over what risk it poses relative to the other earthquake-generating features, like the offshore movement of the Juan de Fuca plate beneath the continental plate.

But even as scientists continue to argue over the danger posed by the Seattle fault zone, those working to protect the area's residents are taking these faults increasingly seriously.

"The Seattle fault is absolutely in our design criteria now," said John Hooper, a structural engineer at the firm of Skilling, Ward, Magnusson, Barkshire Inc. in Seattle. The firm worked on the new baseball stadium and the football stadium under construction — both of which sit right atop the fault zone. "I design for it every day."

And while researchers said Seattle was in better shape than other regions faced with high earthquake risks, they noted there were some particular concerns when a major earthquake rolled through.

"So many bridges are likely to fall, and chunks of the Interstate 5 system," said Dr. Weaver, referring to the major north-south highway running through the state. And getting places won't be the only problem. The city's major water supply lines, sewer lines as well as liquid fuel pipelines run right through the fault zone.

But the real question on everyone's mind is, When is the big one coming? Scientists say they cannot say much with precision about the actual timing of the next big quake, however, other than to say it is surely coming.

"Any hopes of predicting earthquakes have really gone out the window," Dr. Weaver said. "It might be 250 years before the next big one — though I doubt it — or it might be tomorrow. I don't think anyone would believe any number that anyone would throw out. There are just too many unknowns."

The destruction of the Kingdome provided vibrations equivalent to a 2.3 magnitude earthquake. Researchers used the event to see how Seattle might shake in a real quake.

geologists on site
Doug Wilson for The New York Times

Dr. Brian Sherrod, left, and Dr. Thomas Pratt, earthquake researchers, under Interstate 5 just south of downtown Seattle. Dr. Sherrod had discovered the site of a possible fault in the immediate vicinity.

geologists on site
Steve Azevedo

Dr. Tom Brocher, seated, a geophysicist with the United States Geological Survey, and William Zamora, a member of the survey team, are researching seismic hazards in the Puget Sound area.

Copyright 2001 The New York Times Company | Privacy Information