The Nisqually Earthquake demonstrated again to scientists just how the quake menace in the Northwest often differs from that in 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 Olympia. Yet none of them has caused near the damage or deaths of several quakes during the same period in California.
Although Washington has retrofitted its buildings, that was not the primary reason for the comparatively minimal damage from the Feb. 28 quake, seismology experts said. The main factor, they said, was the depth of the temblor.
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 they generate is dissipated underground before it can do major damage at the Earth's surface.
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