Near full locking on the shallow megathrust of the central Cascadia subduction zone revealed by GNSS-Acoustic

• We present GNSS-Acoustic seafloor velocities offshore Cascadia. • GNSS-Acoustic velocities require near full locking near the deformation front. • The locked zone is relatively narrow offshore Central Cascadia. • This shallow locking implies a future earthquake in Cascadia could be tsunamigenic. The Cascadia subduction zone represents a seismic hazard to the Pacific Northwest region of North America, yet the state of fault locking near the deformation front, which could cause a devastating tsunami upon rupturing, remains poorly understood due to limited offshore observations along the subduction zone. In this study, we present the first seafloor geodetic measurements of the horizontal deformation rates on the accretionary prism from an array of four GNSS-Acoustic sites surveyed since 2016. These GNSS-Acoustic sites, despite resting on the North American plate, show velocities that are a significant fraction of the subducting Juan de Fuca plate velocity. In contrast, the continuous GNSS stations along the Oregon coast are moving at velocities <1 cm/yr relative to the North American Plate. Locking models constrained by these offshore velocities show that the subduction zone interface near the deformation front must be nearly locked offshore Oregon. To satisfy both the onshore and offshore geodetic observations, the locked zone must be relatively narrow and only minimal aseismic creep is permissible at the deformation front. These results suggest that appreciable elastic strain has accumulated near the deformation front, which elevates the potential for tsunamigenesis along this portion of the subduction zone. There is a large earthquake hazard in the region offshore British Columbia, Washington state, Oregon, and Northern California associated with the Cascadia subduction zone, where the Juan de Fuca oceanic plate subducts beneath the North American continental plate. The lack of seafloor observations of plate movement limits our understanding of the hazard, particularly where a future earthquake could rupture up to the seafloor and cause a large tsunami. We present seafloor measurements using Global Navigation Satellite System (GNSS) and acoustic ranging data collected offshore Oregon from 2016–2022 that show that the seafloor is moving at horizontal rates comparable with the subducting Juan de Fuca plate. This suggests that a future earthquake offshore Oregon is capable of rupturing up to the seafloor and causing a devastating tsunami.