Active Protothrusts and Fluid Highways: Seismic Noise Reveals Hidden Subduction Dynamics in Cascadia

Complex interactions between strain accumulation, fault slip, and fluid migration influence shallow subduction zone dynamics. Using a decade of continuous ambient seismic data from Cascadia seafloor observatories, we identified distinct regional variations in subduction dynamics. Northern Cascadia exhibits a fully locked megathrust with persistent strain accumulation, while central Cascadia displays a slow slip event on protothrusts and rapid fluid migration along fault systems in the overriding plate. Effective fluid transport through the decollement and the Alvin Canyon Fault likely modulates the earthquake behavior but does not cause slow slip events on the megathrust and likely stabilizes large earthquakes, promoting rupture arrest.