Recent seismic imaging off Vancouver Island has revealed something extraordinary: a tear in the subducting oceanic plate beneath the Cascadia Subduction Zone. The finding briefly raised the public's hopes that Cascadia might be "shutting down," potentially lowering earthquake risk in North America's Pacific Northwest.

Fig. 2. Seafloor expression of the NFZ in northern Cascadia.

Map of seafloor scarps associated with active Nootka Fault Zone (NFZ) deformation. Color shading represents the gradient of seafloor bathymetry calculated from an azimuth of 300° clockwise from north using the Global Multi Resolution Topography version 4 grid (89). The scarps of the Northern Nootka Fault (NNF) and Southern Nootka Fault (SNF) have an average trend of ~N28°E. H, Haggis Mound; M, Maquinna Mound. Note that the NFZ is wider in the SW and narrows toward the deformation front in the NE.

Fig. 9. Slab fragmentation and cessation of subduction enabled by the NFZ.

Schematic 3D interpretation of the NFZ-Exp-JdF triple junction region in northern Cascadia where subduction termination is imminent. The paleo-NFZ (thin black lines) developed as a broad shear zone in nascent oceanic lithosphere and reactivated inherited abyssal hill faults formed at the ridge. The weakened lithosphere buckled as a trench-parallel slab tear formed at the northern edge of the Exp slab and migrated across the paleo-NFZ wake. Recent strain localization along the modern NFZ at ~1 Ma dissected the migrating slab tear and effectively isolated the Exp microplate, leading to enhanced tearing, decoupling, and a local reduction in the slab pull force. The NFZ thus serves as a trench-normal segmentation boundary enabling piecewise slab fragmentation and subduction termination.