Fragment of lost tectonic plate discovered where San Andreas and Cascadia faults meet

A fragment of a long-lost tectonic plate is sliding under the North American continent in the southern part of the Cascadia subduction zone, scientists have discovered. This leftover plate fragment could pose a new earthquake risk to the region.

New research, published Thursday (Jan. 15) in the journal Science, revealed that the Pioneer Fragment — a leftover bit of an oceanic plate that disappeared under the North American Plate some 30 million years ago — is now stuck to the floor of the Pacific Ocean and is moving northwest along with that plate.

This is happening at a spot called the Mendocino triple junction, where California's famous San Andreas Fault abuts the Cascadia subduction zone. Along the San Andreas, the North American and Pacific plates move alongside one another. At Cascadia, which extends from Cape Mendocino, California, to Vancouver Island, British Columbia, the Juan de Fuca and Gorda oceanic plates dive below North America. That tectonic motion is capable of setting off earthquakes of magnitude 9 and above, according to the Pacific Northwest Seismic Network.

Some evidence suggests that earthquakes in the Cascadia subduction zone might trigger earthquakes along the San Andreas, a possibility that would widen the danger from the Cascadia fault.

While the new findings don't make the risk clear, said study first author David Shelly, a geophysicist at the U.S. Geological Survey in Golden, Colorado, they are a step toward understanding this relationship.

The Pioneer Fragment "does increase the area of contact between what’s effectively the Pacific Plate and the subduction zone," Shelly told Live Science.

Shelly and his colleagues probed the Mendocino triple junction using tiny low-frequency earthquakes and tremors — a kind of seismic shiver that originates deep in the crust and can't be felt without sensitive seismometers. "They’re teeny-tiny events but they often occur on the biggest faults," Shelly said.

By analyzing these events, the researchers determined the direction of subtle plate motions. At Mendocino, the Pacific Plate is sliding northwest against the North American Plate, bumping against the Gorda Plate as it pushes under North America. It's a complex situation, and there are competing explanations for exactly where all the pieces are and where the faultlines run.

Shelly and his colleagues found that the situation is even more complex, because a surprise piece of long-gone Farallon Plate still has an influence on the triple junction. This ancient tectonic plate started subducting under North America 200 million years ago, during the breakup of the supercontinent Pangaea. The Juan de Fuca is one remnant of the Farallon. But now, the researchers found that another remnant got stuck to the Pacific plate. This remnant, the Pioneer Fragment, isn't subducting but rather moving sidelong against the continent.

Meanwhile, bits of the Gorda Plate that got scraped off onto the North American Plate as the two ground together have now seemingly been passed back to the Gorda like a "tectonic hot potato" and may be diving back below North America, Shelly said.

This bit of geological messiness may explain why one of the largest triple junction quakes, the 1992 Cape Mendocino earthquake, had a shallower origin than scientists expected. Because of the extra bits and pieces, "the fault may not be following the oceanic crust itself. It may be shallower than that," Shelly said.

Beyond increasing the surface area of the Pacific Plate that interacts with Cascadia, the Pioneer Fragment might have the potential to cause earthquakes itself. Between the fragment and the North American Plate is a nearly horizontal fault, like the icing in a layer cake.

"We don’t know whether that fault can generate large earthquakes, but it is a fault that isn’t currently in the hazard models," Shelly said. "So it’s something we need to consider in the future."