Researchers have documented an unusually detailed view of a subduction zone beneath the Pacific Northwest in the advanced stages of collapse, according to findings published in Science Advances. The discovery provides the first clear picture of how one of Earth’s major tectonic systems shuts down, answering a long-standing geological question about the end stages of subduction zones, which are responsible for the planet’s largest earthquakes, volcanic eruptions, and continental changes.
The research focused on the Cascadia subduction zone, where the Juan de Fuca and Explorer plates are being forced beneath the North American plate off Vancouver Island. Scientists combined earthquake records with seismic reflection imaging—a technique similar to medical ultrasound—gathered during the 2021 Cascadia Seismic Imaging Experiment. By sending sound waves into the ocean floor and analyzing the returning signals, researchers created detailed images of the seafloor structures and identified large faults and fractures cutting through the sinking plate.
The observations reveal that rather than failing in a single catastrophic event, the subduction zone is terminating through what researchers call episodic or piecewise breakdown. Large tears run through the oceanic plate, including one spanning approximately 75 kilometers where sections continue producing earthquakes in some areas while others have become unusually quiet. The quieter regions suggest portions of the plate have already separated completely, while an enormous fault shows a five-kilometer vertical offset where the slab has dropped significantly.
The breakup process involves transform boundaries that function like geological scissors, cutting across the plate and isolating fragments that can eventually form microplates. As individual sections detach at different times over several million years, the remaining plate loses the downward pull necessary to sustain subduction. This piecewise termination model may explain geological puzzles observed worldwide, including fossil microplate fragments found off Baja California from the ancient Farallon plate, suggesting ancient subduction systems similarly underwent gradual unraveling rather than sudden collapse.
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