Why Seattle's big quake had little impact

In the end, even the tropical fish were safe.

Here in downtown Seattle, Wednesday's 6.8-magnitude earthquake rattled walls and workers, sending a snowfall of cracking plaster onto the heads of employees and causing a panicked evacuation of the blue-and-cream-colored terra-cotta Medical Dental building.

But when everyone eventually returned to the office later in the day, the ceilings were still sound and the office's collection of tropical fish were still swimming happily in their tanks.

It's an aquatic example of how Seattle escaped the massive damage and widespread loss of life that has accompanied quakes of similar magnitude - from Northridge, Calif., to Kobe, Japan.

Ultimately, damage may reach into the billions of dollars, yet only one person died and few buildings had significant structural damage. By contrast, the Kobe temblor left more than 6,000 people dead and turned the city inside out.

Seattle's relative good fortune came from a combination of factors, including improved building codes and increased preparedness. Perhaps the greatest factor, though, was pure science - the underlying geology and tectonics of the Pacific Northwest.

Unlike the events in Northridge and Kobe, the Seattle-area quake took place deep within the earth - as many as 30 miles down. The result is obvious.

"With a deep earthquake, the shaking tends to last longer, but the vibration you're feeling is [weakened] by the shock waves moving through 30 miles of soil," says Charles Roeder, a professor of civil engineering at the University of Washington here. "The earthquakes we have here tend to be deep - whereas, the earthquakes they have in Los Angeles tend to be fairly shallow."

Different quakes

Indeed, the strongest Seattle earthquakes of the 20th century all have taken place in roughly the same place that this week's event did. Each occurred as one plate of the earth's crust slowly slid farther beneath another.

In California, earthquakes happen as plates simply grind against each other, meaning they're much closer to the surface - and therefore more damaging.

A similar kind of grinding fault, named the Seattle Fault, runs directly through the city, but that remained quiet.

"If it were a 6.8 on the Seattle Fault, we'd be looking at a lot more damage, more like what happened to Kobe," says William Steele, coordinator of the seismology lab at the University of Washington. "We'd be looking at hundreds of billions of dollars of damage - and many casualties."

That's not to say deep earthquakes can't be damaging. Scientists say deep earthquakes could actually be more dangerous for big buildings, causing entire areas of earth to give way. But Wednesday's temblor didn't appear to cross that threshold for intensity.

Moreover, scientists are quick to point out that Seattle's increasing earthquake awareness has led to better buildings.

"The engineers have learned their lessons," says Dr. Steele. "They've studied the California quakes; they've looked at why buildings break apart."

In fact, his colleague Dr. Roeder makes a living by doing just that. His specialty is gauging the effect of earthquakes on steel products used in construction.

"We very much rely on ductility as we design buildings today," he says, defining ductility as any product's "ability to go through severe deformability - the bending and flexing of concrete columns, the bending and flexing of the steel."

Roeder and his fellow engineers put building materials through every imaginable stress, and the result, he says, is that engineers and architects can now design a very large structure that will survive even massive quakes. "It might be damaged," he concedes, "but it's not going to collapse. It's not going to fail."

He also says that general design requirements are more stringent than ever. "If you look at the uniform building code for 1949, at the design specifications for earthquakes, it was probably about two pages," Roeder says. "Today it's probably 50 or 60 pages."

Roeder chooses 1949 because, in that year, the region suffered a 7.1-magnitude earthquake. Its epicenter was virtually in the same location as Wednesday's slow roller: the Nisqually River delta north of Olympia, Wash.

He notes that many buildings were destroyed in '49, so they weren't around to collapse on Wednesday. In addition, "we've had a huge amount of construction in Seattle since 1970, so a lot of our buildings are newer."

Greater awareness

With the 1949 quake - and another in 1965 - people here have become more concerned about seismic activity. Just 10 years ago, scientists predicted that Seattle could be in line for a massive 9.0 quake, and education projects have picked up.

One federally funded program, Project Impact, helps cities prepare citizens for severe earthquakes. During the past three years, Seattle has used this $25 million grant to help retrofit all of Seattle's public schools, as well as to educate homeowners about ways to prepare for quakes.

Ironically, supporters of Project Impact were celebrating its third anniversary at the Phinney Neighborhood Center exactly as Wednesday's earthquake struck.

To be sure, the limited damage estimates don't mean the earthquake didn't cause its share of problems. More than 200,000 residents went hours without electricity. Runways at Boeing Field and Seattle-Tacoma International Airport were shut down. Even the ferry boats didn't run for a time.

Robert Perrson chose 10:55 a.m. to enter a creaky elevator in an old apartment building. "It was freaky," he says. "The thing took about eight seconds to get from one floor to the next. I'd just closed the old metal door. It started moving. I felt it shake - banging side to side - and I knew it was an earthquake.

Rachel Bjork and her co-workers at the Medical Dental building were among thousands forced to evacuate their workplaces. "Before I went outside, I kept trying to push the drawers of the filing cabinets closed," she says, "but they kept opening by themselves."

(c) Copyright 2001. The Christian Science Publishing Society