Seismic velocity and anisotropy characteristics of active basal accretion: the Olympic Mountains and northern Cascadia Forearc
Manuela Hurtado Betancur, Jonathan Delph, Bin He, & Shihan WuSubmitted August 30, 2026, SCEC Contribution #15219, 2026 SCEC Annual Meeting Poster #TBD
The Cascadia subduction zone shows correlation in subduction-related characteristics along the margin, such as seismogenic behavior, surface uplift and exhumation, and the seismic characteristics of the forearc crust. In particular, the Olympic Accretionary Complex in the northern Cascadia subduction zone shows persistently low shear-wave velocities in the lower crust and correlates with low Bouguer gravity values, elevated topography, elevated non-volcanic tremor and slow slip rates, and low frequency earthquakes. However, the structure and composition of the material comprising the low velocity zone is debated, making it difficult to know how this material is related to the anomalous characteristics of the northern Cascadia margin. In this study, we use Rayleigh and Love waves to constrain the isotropic and anisotropic velocity structure in the region and find persistently slow isotropic shear-wave velocities (∼3.4 km/s) down to ~30 km. The low velocity region is also characterized by positive radial anisotropy (up to ~15%), consistent with subhorizontal laying in a highly anisotropic material beneath the forearc. We show that this signature is not continuous along the margin, but it is spatially concentrated beneath the Olympic Mountains. In addition, this region partially overlaps with some of the highest concentrations of non-volcanic tremor and slow slip along the Cascadia margin, indicating the presence of high pore fluid pressures near the plate interface. We interpret that our low velocity, high positive radial anisotropy zone comprises horizontally-layered mica schists that have been basally accreted to the Cascadia forearc. Particularly efficient basal accretion in this area relative to other portions of the margin may be enabled by higher amounts of fluid along the plate interface, and may be responsible for driving the uplift and exhumation of the Olympic Mountains since at least the mid-Miocene. Thus, this location may represent a region of long-lived material transfer from the downgoing to overriding plate.
Key Words
Anisotropy, basal accretion, Olympic Mountains, Cascadia
Citation
Hurtado Betancur, M., Delph, J., He, B., & Wu, S. (2026, 08). Seismic velocity and anisotropy characteristics of active basal accretion: the Olympic Mountains and northern Cascadia Forearc. Poster Presentation at 2026 SCEC Annual Meeting.
Related Projects & Working Groups
Seismology
