All's not quiet on the Northwestern front - earthquakes

ALL'S NOT QUIET ON THE NORTHWESTERN FRONT

The Pacific Northwest has long been thought to be safe from the kind of major earthquake that periodically sends tremors through California. But now researchers think that the region's apparent invulnerability may be a weakness in disguise. A geological area called the Casca dia subduction zone, which extends northward from near where the San An dreas fault veers out to sea, could become the scene of seismic upheavals like those that have devastated some lands along the rim of the Pacific.

The zone consists of several small oceanic plates -- the Juan deFuca, the Gorda, and the Explorer -- that are being subduct ed, or thrust, under the North American plate at a rate of about an inch and a half a year. Geophysicists Thomas Heaton and Stephen Hartzell of the U.S. Geological Survey and the California Institute of Technology noticed that the zone shares a number of characteristics with other seismically active subduction zones, such as those in Colombia, southern Chile, and southwestern Japan: it's geologically young -- millions, rather than tens of millions, of years old; its constituent plates are being subducted at a shallow angle; and it's the site of very few large quakes.

The seismic quiet of the Pacific Northwest may be the tip-off toa tremendous quake in the making. Whereas numerous small earthquakes occur at older subduc tion zones, plates at young zones, Hartzell says, ''tend to be locked . . . they store energy, and then release it all at once.''

Are the plates that meet in the Cascadia zone locked against each other, or are they subducting smoothly? While there's no unequivocal sign of locking, Heaton and Hartzell report in Science, there's some circumstantial evidence for it. The coastline of parts of Washington is gradually ris- ing; some sediments along the coast record sudden changes in the sea level (an indication of past tremors); and there are Indian legends of great earthquakes in the Pacific Northwest.

If the subduction is going smoothly, there may be no major earthquakes in store. But if the plates are locked, say Hart zell and Heaton, the strain that has been built up along the 750-mile-long zone for hundreds of years could be relieved in a series of formidable quakes with a Richter magnitude of eight or a single colossal quake with a magniude of nine, comparable to the giant quake that rocked the Chilean coast in 1960.

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