Near-Trench Seafloor Geodesy: GNSS-Acoustic Plate Motion Measurements at the Northern Cascadia Subduction Zone

The Cascadia Subduction Zone (CSZ) is a convergent margin extending from Northern California to Northern Vancouver Island that is capable of creating M9+ earthquakes. Direct seafloor geodetic observations near the deformation front and the locked zone are required to constrain possible rupture scenarios. Until recently, observations of plate motion have been limited to land-based stations, which are insufficient to resolve the variability of potential rupture scenarios. Consequently, different model scenarios yield varied outcomes for hazard assessments and the development of effective mitigation strategies. Offshore Oregon and Washington (Central Cascadia), this critical data gap is being addressed by GNSS-Acoustic (GNSS-A) observations that recently provided seafloor geodetic evidence indicating near-full locking on the shallow megathrust (DeSanto et al., 2025).

To the north, offshore Vancouver Island, the Northern Cascadia Subduction Zone Observatory (NCSZO) project, primarily funded by the Canada Foundation for Innovation (CFI) and operated by Ocean Networks Canada (ONC), provides seafloor geodetic observations that will constrain model scenarios and offer the opportunity to observe along-strike variations. The NCSZO complements ONC’s NEPTUNE cabled seafloor observatory, which provides real-time data from seismometers, bottom pressure recorders, CORK borehole observatories, and other sensors relevant to seafloor geodesy. The NCSZO is composed of two main offshore components: a GNSS-Acoustic (GNSS-A) seafloor geodesy network with seven stations and a Deformation Front Laboratory providing pressure and tilt measurements across the deformation front. This presentation will provide an overview of the NCSZO and will highlight the first results from the GNSS-A stations, which consist of accurately located seafloor benchmarks. These benchmark locations are monitored via tens of thousands of acoustic interrogations during yearly surveys utilizing an autonomous Wave Glider.

Following the completion of a fourth observation campaign in 2025, we start to see meaningful results from several sites that provide constraints on the locking of the Juan de Fuca plate with the overriding North American plate. These first direct GNSS-A measurements in Northern Cascadia are a significant step towards improving the reliability of regional earthquake and tsunami hazard and mitigation models.