Scientists have observed a subduction zone actively tearing apart off the coast of Vancouver Island for the first time. The process reveals how Earth's tectonic plates fragment into microplates in a gradual collapse. This discovery provides insights into earthquake risks and ancient geological mysteries.
In the Cascadia region off Vancouver Island, the Juan de Fuca and Explorer plates are sliding beneath the North American plate, but new seismic data indicate the system is breaking apart. Researchers used seismic reflection imaging from the 2021 Cascadia Seismic Imaging Experiment (CASIE21), funded by the National Science Foundation, to capture sound waves echoing from the seafloor with a 15-kilometer line of underwater sensors. The images show deep fractures in the oceanic plate, including a 75-kilometer tear where one section has dropped about five kilometers.
"This is the first time we have a clear picture of a subduction zone caught in the act of dying," said Brandon Shuck, a geologist at Louisiana State University and lead author of the study published in Science Advances. "Rather than shutting down all at once, the plate is ripping apart piece by piece, creating smaller microplates and new boundaries."
Earthquake records confirm the findings: seismically active sections along the tear contrast with silent areas where fragments have detached, no longer producing quakes as the rocks separate. This episodic termination process involves transform boundaries acting like scissors, isolating plate fragments while subduction persists nearby. Over millions of years, these breaks reduce the plate's momentum until the zone halts.
The observation explains fossil microplates from the ancient Farallon plate off Baja California and sequences of volcanic activity in the geologic record. "It's a progressive breakdown, one episode at a time," Shuck noted. Slab windows from the tears allow hot mantle material to rise, triggering volcanism.
For earthquake hazards, the fractures may influence seismic energy in Cascadia, which remains prone to large quakes and tsunamis. The study enhances models of fault behavior but does not alter short-term risks.