A team of geologists says it has captured, for the first time, direct seismic evidence of a subduction zone breaking apart piece by piece rather than shutting down in a single catastrophic rupture. Off the coast of Vancouver Island, where the Juan de Fuca and Explorer plates grind beneath the North American plate, new images of the seafloor's hidden structure reveal a slab tearing into smaller fragments as the ancient system loses strength. The findings, led by Louisiana State University geologist Brandon Shuck, appear in Science Advances.
The images show a fault-bound tear roughly 75 kilometers long cutting through the descending oceanic plate, with one section where the slab has dropped about 5 kilometers relative to its neighbors. Researchers built the picture from the 2021 Cascadia Seismic Imaging Experiment, known as CASIE21, in which the research vessel Marcus G. Langseth towed a 15-kilometer string of underwater listening instruments and beamed sound waves into the seafloor, producing images that work on a principle similar to a medical ultrasound. That imaging was combined with decades of earthquake records to trace where the plate has already snapped and where it remains intact.
A slow, uneven breakup
The Cascadia subduction zone runs roughly 1,000 kilometers from Vancouver Island to Northern California and is capable of producing magnitude-9 megathrust earthquakes, the kind that struck the region in 1700. It has typically been treated in hazard models as one continuous fault. The new study, reported by ScienceDaily and detailed in a Louisiana State University news release, instead suggests its northern end is disintegrating in segments over millions of years, with fragments that have fully separated no longer producing earthquakes of their own.
"This is the first time we have a clear picture of a subduction zone caught in the act of dying. Rather than shutting down all at once, the plate is ripping apart piece by piece, creating smaller microplates and new boundaries."
Brandon Shuck, Louisiana State University
The research builds on seismic data originally gathered by scientists at Columbia University's Lamont-Doherty Earth Observatory, whose earlier surveys of the Cascadia margin established the imaging techniques Shuck's team used to trace the newly identified tears in finer detail than before.
Shuck's team next plans to fold the mapped tears into hazard models for the region, testing whether the fractured segments could host earthquakes of their own or instead act as boundaries that limit how far a future rupture spreads. Better constraints on the fault's true geometry are considered useful input for the seismic hazard assessments that inform building codes and tsunami planning across the Pacific Northwest.
