The development of seismic anisotropy below south-central Alaska: evidence from local earthquake shear wave splitting

SUMMARY The Transportable Array in south-central Alaska spans several subduction zone features: backarc, forearc and volcanic arc, making it an ideal tool to study subduction zone anisotropy. Shear wave splitting analysis of 157 local earthquakes of mb ≥ 3.0 that occurred between 2014 and 2019 yields 210 high-quality measurements at 23 stations. Splitting delay times (δt) are generally small (δt ≈ 0.3 s), increasing with distance from the trench. Arc-parallel fast directions, ϕ, are only seen in the forearc, but rotate to arc-perpendicular ϕ in the backarc. Observed ϕ values generally do not parallel teleseismic SKS splitting results, implying that the latter is sensitive primarily to subslab mantle flow, not mantle wedge dynamics. The forearc local-earthquake signal likely originates from anisotropic serpentinite in fractures atop the subducting Pacific Plate, with possible additional signal coming from fractures in the North American crust. Mantle wedge corner flow, potentially with additional arc-perpendicular anisotropy in the subducting slab, explains backarc anisotropy.