Anisotropy Variations in the Alaska Subduction Zone Based on Shear-Wave Splitting From Intraslab Earthquakes

Shear-wave splitting observations can provide insight into mantle flow, due to the link between the deformation of mantle rocks and their direction-dependent seismic wave velocities. We identify anisotropy in the Cook Inlet segment of the Alaska subduction zone by analyzing splitting parameters of S waves from local intraslab earthquakes between 50 and 200 km depths, recorded from 2015-2017 and emphasizing stations from the Southern Alaska Lithosphere and Mantle Observation Network experiment. We classify 678 high-quality local shear-wave splitting observations into four regions, from northwest to southeast: (L1b) splitting measurements parallel to Pacific plate motion, (L1a) arc-perpendicular splitting pattern, (L2) sharp transition to arc-parallel splitting, and (L3) splitting parallel to Pacific plate motion. Forward modeling of splitting from various mantle fabrics shows that no one simple model fully explains the observed splitting patterns. An A-type olivine fabric with fast direction dipping 45 degrees to the northwest (300 degrees)-aligned with the dipping slab-predicts fast directions that fit L1a observations well, but not L2. The inability of the forward model fabrics to fit all the observed splitting patterns suggests that the anisotropy variations are not due to variable ray angles, but require distinct differences in the anisotropy regime below the arc, forearc, and subducting plate. Plain Language Summary Mantle flow can cause seismic wave velocity to become directionally dependent (seismic anisotropy) and is related to the flow direction. Using a dense temporary seismic transect along the Cook Inlet segment of the Alaska subduction zone, we identify four distinct regions of anisotropy. This includes a sharp transition from arc-parallel fast directions in the forearc to arcperpendicular fast directions in the backarc. Forward modeling of various mantle fabrics shows that no one simple model fully explains the anisotropy and requires distinct differences in the anisotropy regimes below the arc, forearc, and subducting plate.