| Depth of Seattle-area quake may have helped minimize damage Shock waves dissipated far below surface 04/09/2001 By Kenneth Reich / Los Angeles Times The Feb. 28 earthquake in Washington state demonstrated again to scientists just how the quake menace in the Northwest often differs from California.
Six times since 1939, quakes ranging from magnitude 5.8 to 7.1 have shaken western Washington within 40 miles of last month's epicenter near the state capital, Olympia. Yet none of them has caused near the damage or deaths of several quakes during the same period in California.
Although Washington has done a better job lately of retrofitting its buildings, that was not the primary reason for the comparatively minimal damage from last month's quake, seismology experts said. The main factor, they said, was the depth of the temblor – named the Nisqually quake after the Puget Sound delta under which it occurred.
All six Washington quakes have occurred on a subducting slab of the Juan de Fuca tectonic plate, according to reports in recent weeks by scientists at the University of Washington, the U.S. Geological Survey and the Earthquake Engineering Research Institute.
A subducting slab results from an oceanic plate, over millions of years, "diving" – at about the speed your fingernails grow – under a continental plate. In Washington state, the 60-mile-thick Juan de Fuca plate is diving under the North American continental plate, which runs 30 to 90 miles under the surface.
Periodically, this process generates earthquakes along its upper edge and eventually, 100 miles inland, volcanic eruptions in the Cascade range.
The quakes generally occur in the top three miles of crust of the subducting slab, according to Seth Stein, a subduction expert and geophysicist at Northwestern University.
But because the subduction zone is so deep, much of the energy that's generated is dissipated underground before it can do major damage at the Earth's surface.
Farther off the coast, the plate boundary called the Cascadia Subduction Zone is capable of generating quakes ranging as high as magnitude 9. That would be much more dangerous for Washington state but is not as common as the deep quakes that have occurred throughout the last 60 years.
California – with the exception of Humboldt and Del Norte counties on the North Coast, and the inland volcanoes, Mount Shasta and Lassen Peak – is not affected by the subduction zone.
Most quakes in California are unrelated to subduction, although they may be on plate boundaries, and they happen at far shallower levels than the Nisqually quake, which originated 33 miles below the surface.
The California quakes are more sharply felt on the surface. Their magnitudes may not be any higher than the Washington quakes, but they are closer to where people live.
The 1994 Northridge quake, which rocked California's San Fernando Valley, took place on a sloping plane three to 12 miles beneath the surface, on a "blind" thrust fault. This meant its surface shaking exceeded Nisqually by about 10 times, according to ground motion recordings.
"The Nisqually earthquake caused [only] moderate ground motion throughout the Puget Sound Region," note the authors of the seismology section of the earthquake institute report – Ken Creager and Robert Crosson of the University of Washington, and Thomas Pratt and Craig Weaver of the USGS.
The scientists recognize that "ground motions vary widely from site to site, due in part to the large differences in geologic conditions throughout the Puget Sound," and do not follow simple patterns.
But this was the case in Northridge, too. In both places, scientists frequently are urging the placement of additional recording stations to get a more complete picture. Still, it is clear that Northridge, a magnitude 6.7 quake, was far more violent on the surface than Nisqually, a magnitude 6.8.
Another feature of the Washington state quakes is the paucity of aftershocks, a sharp contrast with most big California quakes.
In the first days following the Nisqually quake, only four small ones were recorded, the largest a mild magnitude 3.4.
A report by the Southern California Earthquake Center notes that "the largest Northridge aftershock ... measured magnitude 5.9, and five other first-week aftershocks exceeded magnitude 5."
In short, the biggest Northridge aftershocks were about 100 times the power of the biggest Nisqually aftershock.
Observing that, in all, hundreds of aftershocks were felt in the first week after Northridge (and thousands have occurred since then), the report says, "Moderate aftershocks are capable of causing further damage to weakened structures, so the lack of significant aftershocks was a factor in mitigating the total loss caused by the Nisqually earthquake."
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