A newly discovered fragment of an ancient tectonic plate is sliding beneath North America, where the San Andreas and Cascadia fault lines meet. The research, published in Science on January 15th, reveals that this long-lost piece—the Pioneer Fragment—could increase earthquake risks in a region already prone to massive seismic events.
The Hidden Piece of the Puzzle
The Pioneer Fragment is a remnant of an oceanic plate that subducted under the North American Plate approximately 30 million years ago. Instead of fully sinking into the mantle, it became stuck to the floor of the Pacific Ocean and is now moving northwest along with it. This occurs at the Mendocino triple junction, a complex geological zone where the San Andreas Fault (a strike-slip boundary) meets the Cascadia subduction zone (where one plate dives beneath another).
Why this matters: The Cascadia subduction zone is capable of producing earthquakes of magnitude 9.0 or greater. The San Andreas Fault is also a significant seismic hazard. Adding another dynamic element into the mix—a sliding fragment of an old plate—complicates the situation and could potentially increase the risk of large-scale quakes.
A Tectonic Hot Potato
Researchers used tiny, low-frequency earthquakes and tremors to map the subtle movements of these plates. Their analysis revealed that the Pioneer Fragment isn’t subducting, but rather sliding sideways against the North American continent.
Even more surprisingly, bits of the Gorda Plate, which normally dives beneath North America, appear to have been scraped off and are now being passed back to the Gorda Plate like a “tectonic hot potato,” potentially re-subducting. This chaotic interplay explains why some past earthquakes in the region occurred at unexpectedly shallow depths.
The Unknown Hazard
Between the Pioneer Fragment and the North American Plate lies a nearly horizontal fault, like a hidden layer within a geological structure. The critical question is whether this fault can generate large earthquakes. Currently, it isn’t included in existing hazard models, meaning its potential contribution to seismic risk remains unknown.
“We don’t know whether that fault can generate large earthquakes, but it is a fault that isn’t currently in the hazard models,” said David Shelly, a geophysicist at the U.S. Geological Survey. “So it’s something we need to consider in the future.”
The discovery of the Pioneer Fragment underscores the intricate and unpredictable nature of plate tectonics. While the exact implications for earthquake hazards are still unclear, this finding emphasizes the need for ongoing research and updated risk assessments in the Pacific Northwest.
