Pacific Northwest Subduction Zone is Tearing Apart

Scientists have observed a major tectonic system beneath the Pacific Northwest beginning to break apart. This subduction zone, located off Vancouver Island, is actively tearing into pieces. Subduction zones are areas where one tectonic plate slides beneath another. They are responsible for large earthquakes and volcanic eruptions.

The discovery provides insight into how these massive systems end their lifecycles. Researchers previously had limited opportunities to observe this process directly. The findings suggest that subduction zones may die piece by piece, rather than all at once. This observation could help explain ancient plate fragments and volcanic activity.

Researchers combined earthquake records with seismic reflection imaging. This technique uses sound waves to create images of underground structures. The 2021 Cascadia Seismic Imaging Experiment (CASIE21) collected the seismic measurements. Analysis of these echoes revealed large faults and fractures within the sinking plate. Some areas show the plate actively snapping apart.

One significant feature is a 75-kilometer tear through the oceanic plate. Along this tear, some sections continue to produce earthquakes, while others are unusually quiet. The absence of earthquakes suggests that a section of the plate has already separated. This detached area is gradually expanding. The breakup of each individual section can take several million years.

This process, described as “episodic” or “piecewise” termination, involves individual sections tearing away at different times. This slowly dismantles the larger tectonic system. Transform boundaries, which are faults where sections of the Earth's crust move sideways, play a role in this process. They can act like geological scissors, cutting across the plate and isolating fragments. Once a piece separates, it can form a microplate, a smaller piece of the Earth's outer shell that moves independently.

The breakup can also create “slab windows,” openings that allow hotter mantle material to rise. This rising material can alter magma production and contribute to volcanic activity. The findings do not change the immediate earthquake hazard for the Pacific Northwest. The observed breakup occurs over millions of years. However, incorporating these new structures into earthquake models could improve understanding of future ruptures.