Seattle Post-Intelligencer

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Feb. 28 quake only a warmup to the 'Big One,' say scientists

Thursday, April 19, 2001

By TOM PAULSON, SEATTLE POST-INTELLIGENCER REPORTER

SAN FRANCISCO -- The Nisqually quake that struck the Puget Sound area in late February was big enough to get our attention, but it wasn't even close to the Big One that scientists predict will some day hit the Northwest.

Yet, because of the magnitude-6.8 earthquake, the Northwest's seismic risks were a hot topic here yesterday at a meeting of the Seismological Society of America.

The Feb. 28 quake, with an epicenter just north of Olympia, was a deep "intra-slab" quake -- the most common type of large earthquake experienced in the Pacific Northwest. The region has had three of these deep quakes since 1949.

The quake produced less ground shaking and damage than expected, given its magnitude, but it still was a big one in terms of data collected -- information that will be of great use for studying the region's overall seismic hazard.

"It was an eye-opener for a lot of people," said Tom Brocher, a geologist with the U.S. Geological Survey in Menlo Park, Calif., and an expert on Northwest quakes. "But we still expect to have a big earthquake on the Seattle fault that's going to be a lot more damaging."

The Seattle fault is a shallow fault, located a few miles down, and shallow faults tend to cause more damage because they cause more severe shaking. A magnitude-6.8 quake on the Seattle fault would have been a major catastrophe.

But another type of quake that could perhaps do as much damage, even though its "fault line" is located hundreds of miles off the Washington and Oregon coast, would be a major event on the Cascadia Subduction Zone -- where a few of the planet's tectonic plates are in slow-motion, subterranean collision.

A seismic event on a subduction zone can produce the Earth's largest quakes, at magnitudes of 8 or 9.

But estimating the potential hazard from earthquakes in "Cascadia" -- the geologists' name for the seismically active western edge of the Pacific Northwest -- isn't an easy task. Geologically, it's a much more complex system than California's. Major quakes are much less frequent in the Northwest. And it's only been within the past decade or so that most experts have taken the seismic risk seriously.

"What we don't know is a lot more than what we know," said Robert Crosson, a geophysicist at the University of Washington.

Crosson said the evidence shows that the Olympic Mountains, the product of millions of years of subduction zone collision of the Juan de Fuca and North American tectonic plates, are likely the driving force behind the Seattle fault.

The folding and crunching of the Earth's crust that pushed up the Olympics, Crosson said, was compensated for by creating depressions in the land due east to create the Seattle and Tacoma basins and the Puget Sound lowlands. That, he noted, adheres to a geological principle of isostatic compensation -- what goes up here, goes down there.

Magnetic and gravity measurements back up his claim, Crosson said, as does the lack of any mountain ranges south of the Seattle fault. If this fault were a normal thrust fault, he said, there should be mountains between Seattle and Tacoma. Now that the Olympics have been eroded over time, Crosson said, these depressed basins "want to pop up." That's likely where these shallow faults come from, he said.

"Bob's really in the minority on that one," said Brocher. Yet, Brocher said, it's a good example of just how many questions still exist about the Northwest's seismic situation. Understanding the type of fault and what drives earthquakes is critical to estimating when another one might happen and how big it could be.

In another talk, the UW's Tony Qamar discussed new tracking of ground movement over time using satellites with the Global Position System (GPS). The idea is to track strains and stresses from tectonics.

All the data confirmed the view that California and Oregon are shoving up against Western Washington. We're getting squeezed, Qamar said, because Canada doesn't budge.

Kathy Troost, a UW geologist who spoke yesterday on the lower-than-expected number of ground failures after the Nisqually quake, showed photos of sinkholes, sand spouts, landslides and liquefaction to show why geologic mapping is important to estimating quake hazards.

"Geology matters," Troost said. She and her UW colleague Derek Booth are leading a team of scientists to develop a predictive "hazard map" for the city of Seattle.

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