Shear‐Wave Splitting in the Mantle Wedge: Role of Elastic Tensor Symmetry of Olivine Aggregates

Using a 3‐D mantle wedge flow field for a generic oblique subduction system, we calculate elastic tensors of mineral aggregates in the mantle wedge for A‐, B‐, C‐, and E‐type olivine crystal preferred orientations (CPO) and apply the calculated elastic tensor in the forward calculation of shear‐wave splitting (SWS) through the mantle wedge. We find that the hexagonal approximation of the full tensor does not affect the SWS parameters (the fast direction and the delay time) significantly for all CPO types except that the delay time for C‐type CPO becomes shorter. Additionally, we find that despite the 3‐D mantle flow field that results from oblique subduction, the fast direction is margin‐normal for A‐, C‐ and E‐type CPOs and margin‐parallel for B‐type CPO. Plain Language Summary As oceanic plates subduct, they drag the overlying mantle material, inducing mantle wedge flow. As the mantle flows, mineral crystals align in certain directions. Earthquake waves that pass through the aligned crystals travel faster in one direction than another. As a result, the waves split into two parts and are characterized by two parameters: fast direction and delay time. We can simplify how we model this material, but it may impact these parameters. In this study, we test the non‐simplified and simplified material for four different directions of mineral alignment and calculate the fast direction and delay time for a 3‐D mantle flow where the oceanic plate is moving toward the edge of another plate at an angle of 45°. We find that simplifying the material has modest effects on the parameters but simplification can cause smaller delay times for one mineral alignment direction, underestimating how aligned the minerals are. Overall, the mineral alignments result in fast directions that are mainly perpendicular to the edge of the overriding plate, which does not reflect the 3‐D mantle wedge flow pattern. Key Points We calculate elastic tensors of olivine aggregates for four different slip systems and shear wave splitting parameters in the mantle wedge The hexagonal approximation of full elastic tensors is reasonable for most slip systems, particularly the A‐type, and initial polarization The hexagonal approximation results in nearly identical tensors for A‐ and E‐type and reduced delay time for C‐type