3D Shear-Wave Velocity and Density Modeling of the Northern Cascadia Subduction Zone

Abstract

In the Northern Cascadia Subduction Zone (NCSZ), we developed a new 3D model for shear-wave velocity (V_S) and density to improve seismic hazard assessments and ground motion simulations. Utilizing seismic data from the past two decades, we measure surface wave dispersion from ambient noise and earthquake recordings and inverted them for V_S perturbations at various depths. Additionally, Bouguer anomaly data were inverted for a 3D density distribution. These models provide direct constraints on shear-wave velocities and density properties across the study region. Our findings corroborate previous research, highlighting: (a) anomalous low velocity zones in the Puget Lowland, attributed to subduction dehydration and trapped fluid-rich sediments; (b) low V_S beneath sedimentary basins, delineating their geometry; (c) deep-seated crustal faults on Vancouver Island indicated by segmented high-V_S zones; (d) a high-velocity anomaly corresponding to the subducting oceanic slab beneath the Olympic Peninsula beneath characterized by an anomalously slow upper mantle velocity; and (e) localized high-velocity layers straddled between low-velocity layers in the upper crust associated with magmatic processes. A novel aspect of our work is the identification of a high-density anomaly rising from the uppermost mantle, aligning with the Puget Sound waterway trajectory. This anomaly, detected at depths exceeding 20 km, provides new insights into the dynamics of oceanic crust-mantle coupling beneath the study area.