Seismic Imaging Shows 75 km Tear Fragmenting Juan de Fuca Plate in Cascadia Subduction Zone

Researchers from Louisiana State University combined earthquake catalogs with ship-based seismic reflection profiles collected during the NSF-funded CASIE21 cruise. A 15 km hydrophone streamer recorded reflections from the downgoing slab, revealing large normal faults that cut completely through the oceanic crust and into the mantle. One fault shows an abrupt 5 km drop of the slab, while adjacent segments remain seismically active, indicating stress is still being transmitted across intact portions.

The data overturn the prior assumption that mature subduction zones end in a single catastrophic event. Instead, the plate is rifting into microplates, with the Explorer segment already decoupled. This matches patterns seen in older, fossilized subduction systems in the Mediterranean and western Pacific but had never been imaged in real time. The finding directly affects hazard models because smaller, independent microplates can generate distinct earthquake and tsunami source zones along the Pacific Northwest coast.

A key limitation is that the imaging is two-dimensional and lacks temporal resolution to determine whether the tear is propagating at centimeters or meters per year. Dense ocean-bottom seismometer arrays deployed for three to five years would provide the kinematic constraints needed to forecast when the next segment fully detaches.