Major earthquakes occurring along the Cascadia subduction zone off the coast of Washington state could strike closer to the state’s urban areas than some models have suggested, a new study notes.

GPS data gathered at dozens of sites throughout western Washington hint that slippage along the interface between the North American and Juan de Fuca tectonic plates could occur as deep as 25 kilometers below the Earth’s surface, says Timothy I. Melbourne, a geodesist at Central Washington University in Ellensburg. That depth, in turn, would place the epicenters of quakes triggered along that portion of the subduction zone — some of which could exceed magnitude 9 —more than 60 kilometers inland, he and CWU colleague James Chapman report online and in the November 28 Geophysical Research Letters.

Seafloor spreading is shoving the eastern edge of the Juan de Fuca tectonic plate, which runs from northern California to southern British Columbia, eastward beneath the North American plate. Long-term observations indicate that the plates are converging at an average rate of between 3 and 4 centimeters each year, says Melbourne. “With GPS, you can see plate tectonics happening on a week-to-week basis,” he notes.

At shallow depths, where Earth’s crust is relatively cool, the tectonic interface is locked, and seismic stress builds up there until it is released during a quake. But deep beneath western Washington, at depths between 25 and 45 kilometers, the two tectonic plates slide past each other for a few days each 18 months or so. These slippage episodes are sometimes described as “silent earthquakes” (SN: 4/27/02, p. 260) but actually do register on seismometers, says Melbourne. “They’re like a magnitude-1 quake but they go on for a couple of weeks,” he notes. The total energy release in each slippage episode, if let loose all at once, would equal that in a quake with a magnitude between 6.3 and 6.7.

The long-term GPS data provide information about where slippage is occurring and how the plates are deforming. For instance, while average plate convergence rates offshore are greater than 3 centimeters per year, those along the coast are about 2.5 cm/yr. Convergence inland, near Seattle, only adds up to about 0.5 cm/yr, says Melbourne. These trends, when combined with previous seismic data, hint that stress is accumulating along the tectonic interface at depths less than 25 kilometers, where the tectonic interface is locked.

GPS data are a more direct way of telling where tectonic slippage is occurring and where it isn’t, says John Vidale, a seismologist at the University of Washington in Seattle. “That data from both GPS and seismic instruments are pointing in the same direction is comforting,” he adds.

A major quake rupturing the 300-kilometer length of the Cascadia subduction zone that runs along the Washington coast would measure magnitude 8.9, Melbourne and Chapman estimate. If the entire 1,100-kilometer subduction zone slipped at once, the quake would be a magnitude-9.2 whopper rivaling the tsunami-spawning quake that slammed Indonesia in December 2004 (SN: 1/8/05, p. 19). Field studies suggest that quakes of such magnitude happen along the Cascadia subduction zone once every 550 years, on average. The last one struck the region in January of 1700 (SN: 11/29/97, p. 348).

Quake hazard analyses for the region, based partially on seismic data, already account for possible tectonic slippage at depths of 25 kilometers, says Garry Rogers, an earthquake scientist at the Geological Survey of Canada in Sidney, British Columbia. Nevertheless, he notes, the new findings provide “more precise measurements than we’ve had before.... This study confirms a lot of what we’ve known about.”

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