Tacoma poses a quake quandry

THE ASSOCIATED PRESS

SEATTLE -- Tacoma may have been much closer to the epicenter of the 6.8 magnitude earthquake that rattled the Northwest last month, but it had a much easier ride than Seattle, a University of Washington geologist told a physics conference Monday.

It's not time for Seattle residents to pack up and move south, since Tacoma hasn't been immune in previous big quakes, Steve Malone said.

But the mystery of the Feb. 28 Nisqually Quake will keep researchers busy for years.

Citing a "shakemap" that shows how strong the quake actually felt at various surface sites, Malone said "there are hot spots and cool spots that certainly have something to do with local geology, but in many cases it's not obvious that's the dominant factor."

For instance, Tacoma and Seattle both have industrial areas built partly on mushy, muddy former tide flats. But Tacoma's Tideflats area was relatively mildly shaken, while Seattle's Duwamish area was rocked in the quake.

"Down in Tacoma, whether on hard rocks of upper elevations, or in the Tideflats, shaking was comparatively low, even though it was closer to the epicenter," Malone said in an interview.

In major quakes in 1949 and 1965, "there was strong shaking in parts of Tacoma. There was ground failure, all sorts of effects that were seen in those earthquakes that weren't seen in this one," Malone said. "The question now is why?"

Last month's quake occurred at a depth of 33 miles and was centered about 11 miles northeast of Olympia -- about 10 miles south of Tacoma and 35 miles south of Seattle. Some 400 people were injured.

Dozens of buildings in the Seattle area remained red-tagged as uninhabitable Monday, as did buildings in Olympia and elsewhere. More than 5,000 people have applied for federal assistance because of quake losses, with just over half of those from King County.

Damage has been estimated at about $2 billion. Malone said the most remarkable thing about the quake remains how relatively little damage it did.

Some of that can be attributed to earthquake retrofitting of buildings and quake-conscious new construction since the 1970s.

But some of it is baffling.

"This earthquake really did not shake as hard as one might have expected, comparing similar earthquakes at similar depths," Malone said.

Researchers will try to determine whether that has to do with how force was transmitted from the quake site through different types of rock and soil.

It also could be related to the way the quake was caused. It occurred where two massive geologic plates bump and grind in a region known as a subduction zone.

Data still are being collected from the 70 or so seismology instruments placed around the Puget Sound area, Malone said. Analysis of that information, along with earthquake studies already under way, may give some clues to why the quake shook so selectively.

Researchers also hope Congress will approve funding for more instruments to fill holes in the region's seismology mapping, Malone said.

Malone said that, though another major quake is not likely soon, area residents should not get complacent about quake preparations.

Malone's talk was presented on the opening day of the American Physical Society meeting at the Washington State Trade and Convention Center.

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