Steve Bloom/The Olympian
Rubble from the downtown Washington Federal Savings building rests on the Fifth Avenue sidewalk in Olympia.

Different types create different damage patterns

South Sound was overdue for quake

JOHN DODGE, THE OLYMPIAN

Scientists have said for years that the Puget Sound region was overdue for a damaging earthquake.

Their warnings rang true when the 6.8 magnitude earthquake struck 30 miles beneath the Nisqually Delta 11 miles northeast of Olympia.

The earthquake was the most common of the three types that occur in the Pacific Northwest, deep and situated in the Juan de Fuca tectonic plate that is slowly diving below the North American continental plate.

It was eerily similar in location to the 7.1 temblor that rocked Olympia in 1949 and akin to the last major Puget Sound earthquake, a 6.5 shaker based in the Seattle-Tacoma area in 1965.

"It's in the same place as the 1949 quake," marveled Harry Halverson, an Olympia retiree and co-founder of Kinemetrics, a Pasadena, Calif.-based seismic equipment manufacturer.

Strongest since 1949

This was the seventh earthquake in the Puget Sound area since 1872 that registered magnitude 6 or higher, the first to do so since 1965 and the strongest since 1949.

Based on historic data, magnitude 6 or higher quakes occur about every 30 years.

We were overdue.

The energy from deeper earthquakes has farther to travel and loses some of its force before it reaches the surface.

There is another type of earthquake, a shallow crustal earthquake, that would have caused many times more damage at the same order of magnitude, said Bill Steele, a University of Washington seismologist.

Geologists have uncovered evidence along a shallow fault line of a 7.4 magnitude earthquake in Seattle that triggered major landslides and upheavals on the Earth's surface about 1,100 years ago.

The granddaddy and least frequent of all Pacific Northwest temblors is the subduction zone earthquake.

These occur when the oceanic and continental plates hang up and then break loose with geologic fury -- triggering earthquakes in the 8 to 9 magnitude range.

Logarithmic scale

Magnitude is measured on a logarithmic scale, which means a magnitude 8 quake releases 31 times more energy than a magnitude 7 earthquake.

The last of these great earthquakes struck the state about 300 years ago. They have an irregular recurrence rate, anywhere from 100 years to 1,100 years, according to a state Department of Natural Resources earthquake fact sheet.

While the timing of earthquakes can't be predicted, the areas most prone to damage can.

This earthquake bore that out: The areas with sandy, loosely-packed soils, including the fill material found in downtown Olympia, are the first to lose their strength and collapse during a strong earthquake, a process called liquefaction.

During severe ground motion, these soils act more like a liquid than a solid, thus the term.

"The heaviest damage appears to be in the liquefaction zones," said Jim Strong, a South Puget Sound Community College geology professor who surveyed the Olympia area following the quake.

Research by DNR geologists Steve Palmer and Tim Walsh revealed that ground motion in the Olympia area was twice as severe during the 1965 earthquake as it was in the Seattle area -- despite the fact that Seattle was 10 miles closer to the earthquake's epicenter.

Roughly half of northern Thurston County's 120 square miles features soils and underlying geology with a moderate to high risk of failure during a strong earthquake.

The lack of rain this winter probably helped reduce the severity of damage Wednesday, Strong said. That's because the soils aren't saturated and weakened.

Oddly enough, 1949 was a dry winter as well, Walsh said.

The Olympian Copyright 2001

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