Poloidal‐ and Toroidal‐Mode Mantle Flows Underneath the Cascadia Subduction Zone

Several hypotheses have been proposed to explain intriguing circular shear wave splitting patterns in the Pacific Northwest, invoking either 2‐D entrained flows or 3‐D return flows. Here, we present some hitherto unidentified, depth‐dependent anisotropic signatures to reconcile different conceptual models. At depths shallower than 200 km, the fast propagation directions of seismic waves to the west of the Rocky Mountain are aligned sub‐parallel...