November 2, 1999
Web posted at: 3:25 p.m. EST (2025 GMT)

Intraslab earthquakes "are the most common to rattle the Seattle area," said Arizona State University geologist Simon Peacock.

Copyright 1999, Environmental News Network, All Rights Reserved

The earthquakes, known as intraslab, register magnitude 7-7.5 and occur approximately 30 miles directly below metropolitan areas in the Pacific Northwest, said Simon Peacock, a geologist at Arizona State University.

Intraslab earthquakes "are the most common to rattle the Seattle area," he said. "The growth of the Seattle area in the last 30 years makes the risk posed by these earthquakes much greater."

Like the more feared, and better-understood, great thrust earthquakes, intraslab earthquakes occur in subduction zones, where oceanic crust dives beneath the edge of a continent.

Great thrust earthquakes occur at shallow depths of zero to 30 miles along the sloping boundary between the descending plate and the continental margin. In contrast, intraslab earthquakes occur within the descending oceanic crust at depths of 30 to 190 miles beneath the surface and are caused by different processes.

In 1996, Stephen Kirby and colleagues at the U.S. Geological Survey proposed that intraslab earthquakes occur when intense heat and pressure in subduction zones change the descending oceanic crust into denser rock.

Subduction zone earthquakes occur where oceanic crust dives beneath the edge of a continent.

These mineralogical changes cause the subducting oceanic crust to liberate water stored in the original minerals and to reactivate pre-existing faults. "If you pump up the water pressure in that fault, you can cause an earthquake to occur," said Peacock.

Peacock and colleague Kelin Wang, a geoscientist at the Geological Survey of Canada's Pacific Geoscience Center, tested Kirby's theory by comparing two subduction zones in Japan, carefully calculating the temperatures of the subducting oceanic crust and comparing their seismic and volcanic records.

Their results confirmed Kirby's theory and showed that oceanic crust subducting in warm subduction zones, such as the one beneath southwest Japan, liberate water at shallow depths. In cold subduction zones the mineralogical changes take place much deeper, thus water is liberated into pre-existing faults much deeper.

Like southwest Japan, the Pacific Northwest and southern Mexico are underlain by warm subduction zones. This makes cities like Seattle, Washington, susceptible to an intraslab earthquake just 30 miles beneath the surface.

"The Vancouver-Seattle-Tacoma area may be more at risk from an intraslab earthquake than from a larger earthquake along the offshore trench," Peacock said in a statement. "We're starting to realize that we have to worry about a magnitude 7-7.5 intraslab earthquake located 50 km [30 miles] beneath Seattle or Vancouver, as well as a magnitude eight or nine on the coast."

To highlight the risk posed by intraslab earthquakes, Peacock pointed out that a 7.4 magnitude intraslab earthquake shook Oaxaca, Mexico, on Sept. 30 and killed at least 27 people.

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