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Plate shifts in the Pacific Northwest

The far northern section of the Sumatra-style subduction zone in the Pacific Northwest could be transforming into more of a San Andreas-style seismic zone, according to new research. This geologic reorganization could have implications for the region’s earthquake risk.

In an area known as the Triple Junction, the small Juan de Fuca Plate meets the gigantic Pacific and North American plates. Parts of the Juan de Fuca Plate, including the Explorer microplate on the northern end and the Gorda microplate on the southern end, have been diving beneath the North American Plate, making it a subduction zone for the past 4 million years. Where the plates converge, small earthquakes occur frequently, and lava flows from beneath the seafloor, creating a growing mid-ocean ridge.

In 1995, Kevin Furlong, a geophysicist at Pennsylvania State University in University Park, and colleagues suggested that the subduction of the Explorer microplate may be slowing and may eventually stop altogether. Their hypothesis, Furlong says, was based on seismic reflection data and seafloor topography measurements. However, the supporting data were somewhat scarce.

To get a new picture of the Triple Junction, Robert Dziak, a geophysicist at Oregon State University in Corvallis, and colleagues compiled and mapped 11 years of earthquake recordings from former U.S. Navy hydrophones — previously used during the Cold War to collect information on Russian submarines in the Pacific — and 23 years of measurements of seafloor topography. They used this data to create new models that show that the region is undergoing massive change.

As Dziak’s team reported in the March Geology, the Triple Junction plates are migrating toward the southeast, and the Juan de Fuca subduction zone already looks more like a “transform boundary” — where plates slide past one another — than a subduction zone. The researchers, Dziak says, found a number of examples to back up the models’ suggestions, such as odd magnetic anomalies and the off-kilter orientation of several undersea volcanoes.

If the Triple Junction does indeed become a transform boundary, like the San Andreas farther south, it could change the seismic risk for the Pacific Northwest, Dziak says. Generally speaking, earthquakes generated on transform boundaries tend to be smaller than earthquakes on subduction zones — all of the magnitude-9 or greater earthquakes in human history have been on subduction zones.

But, Dziak cautions, “we could simply be talking about a magnitude-8.9 instead of a magnitude-9 earthquake. Our studies really don’t change the big picture” — that there is a significant seismic risk in the Pacific Northwest. Furthermore, he says, “we’re talking about time frames of millions of years,” before the Juan de Fuca finishes subducting beneath the North American Plate.

“What’s really exciting” about this research, Furlong says, is that at the Triple Junction, “we are catching plate tectonics in action.” Researchers are currently witnessing changes that they usually can see only as the result of millions of years of plate movement. Such research, he says, might help scientists understand some enigmatic features that they see in the geologic record, and “that is pretty cool.”

Megan Sever

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