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Environment

A Gentle Subduction

By Tim FolgerSeptember 1, 1993 5:00 AM

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Seattle may owe its good fortune--it has never been flattened by an earthquake--to the fact that the seafloor off Seattle is so hot.

Where the sea meets the land, the sea washes over the land; where the seafloor and the land collide, though, moving on separate crustal plates, the dense oceanic rock dives under the lighter continent--it gets subducted, as geologists say. Subduction zones are where the largest earthquakes happen: as the two plates rub past each other, they sometimes snag and then spastically break free. The majority of the subduction zones around the world--most of them are around the Pacific--have had very large earthquakes in historical times, says Roy Hyndman, a geophysicist with the Geological Survey of Canada. It is the exceptions to this rule that are puzzling--the Pacific Northwest, for instance, from northern California to southern British Columbia. Although the region has suffered large earthquakes in the past, the most recent one appears to have been about 300 years ago, well before European settlers arrived.

Why have Seattle and Portland been spared the fate of, say, Anchorage, which was devastated by a major quake in 1964? Some geologists have argued that the oceanic and continental plates off the Northwest are temporarily locked, that strain is building up in the locked zone, and that it’s only a matter of time before the strain is released. Hyndman agrees-- but with an optimistic twist. He thinks the high temperature of the oceanic crust along the Northwest coast may allow it to glide under the land with a minimum of friction, thereby reducing the risk of a dangerous quake.

In laboratory experiments, geologists have found that continental and oceanic rocks slide smoothly past one another (without locking up) once their temperature exceeds about 840 degrees Fahrenheit. The rocks don’t melt, but the heat weakens their crystal structure enough for them to deform and flow plastically, at a fraction of an inch per year, without much sticking.

The key question in a subduction zone, then, is how hot the rocks are. The oceanic crust diving under the Pacific Northwest is dying young; it’s probably no more than 8 million years old. That’s because it was created just 150 miles offshore, at a chain of undersea volcanoes called the Juan de Fuca Ridge. Along that ridge two oceanic plates, the Pacific and the Juan de Fuca, are spreading apart, and molten rock is welling up from the mantle to form new seafloor. The tiny Juan de Fuca plate heads east, and almost immediately (geologically speaking) gets subducted under the North American plate.

The seafloor that’s going down beneath the coast is very young, says Hyndman. So it’s still very hot. Also, there’s a thick layer of muds and sands on it that provides an insulating blanket. Most of the mud gets scraped off against the edge of the continent; it’s the volcanic crust that plunges downward. Hyndman and his colleague Kelin Wang used a computer model to estimate the temperature of the crust, based on measurements made by temperature probes dropped into the seafloor mud. The model indicates that the oceanic crust off the Northwest is very hot--nearly 500 degrees-- even before it begins to heat up during its dive toward Earth’s hot interior. The crust entering most other subduction zones is much older, says Hyndman, and thus colder and more rigid.

Five hundred degrees is not hot enough for quake-free gliding, though--the rock has to be at 840 degrees for that. But because the oceanic crust starts out hotter off the Northwest, it reaches the critical temperature earlier in its downward plunge. That means the zone in which the two plates can lock and generate earthquakes is relatively narrow, says Hyndman, and so quakes are smaller.

The locked zone is only 30 to 60 miles wide, he explains, compared with 120 or even 180 miles wide in other subduction zones. So earthquakes are restricted mostly to the outer part of the continental shelf--they don’t come very far under the coast, which is unusual. Large earthquakes can still occur, but not as large and not as far inland as normal. If Hyndman is right, the people of Seattle can feel a bit more comfortable--but not too comfortable.

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