Crustal anisotropy beneath P acific O cean‐ I slands from harmonic decomposition of receiver functions

Crustal anisotropy beneath ocean islands can be attributed to preferentially aligned minerals, cracks, or dike structures. Stacked with harmonic weighting, receiver functions from permanent ocean‐island stations display evidence of strong and distinct anisotropy parameters in the underlying crust and underplated layer. We analyze data for 11 IRIS‐GSN stations in the Pacific Ocean. We observe the prevalence of two‐lobed receiver function (RF) amplitude variations with back‐azimuth, consistent with “slow” tilted‐axis anisotropy. In most cases the anisotropy is accommodated in the underplated crust. Synthetic modeling of a representative station indicates that the strength of anisotropy of Vp=10% and Vs=5% is possible. The strike direction of the inferred symmetry axis tends to align with plate motion, with some scatter. At stations in the northwest Pacific i.e., KWAJ, TARA, and WAKE, the strike direction of the symmetry axis aligns with plate motion at the time of volcano emplacement. Beneath station POHA and the closest stations to the present‐day Hawaiian hotspot, alignment of the symmetry axis is almost orthogonal to the plate motion. We attribute the crustal anisotropy to the preferred alignment of dike structures that transported asthenospheric magma toward the seafloor volcanic edifice. Our results suggest that the thermal‐plume origin for ocean islands must be supplemented by tectonic‐stress heterogeneities that allow magma to penetrate the lithosphere via fractures. Magma‐transport fractures should align normal to the least‐compressive direction, which are predicted by theoretical models to align approximately with plate motion at the time of emplacement.