The Characteristics of Rayleigh and Love Wave Azimuthal Anisotropy: Observations Across Alaska

Using ambient noise data from 10 s to 50 s period across Alaska, we confirm previous estimates of Rayleigh wave 2ψ azimuthal anisotropy and present the first estimates of Rayleigh wave 4ψ and Love wave 2ψ and 4ψ azimuthal anisotropy, where ψ is the angle of propagation. As in earlier studies, the fast orientations of Rayleigh wave 2ψ are mainly parallel to major faults in Alaska at all periods. We also find that on average the fast orientations of Love wave 4ψ are rotated 45° relative to Rayleigh wave 2ψ, the fast orientation of Rayleigh wave 4ψ aligns with Love wave 2ψ, and the fast orientation differences of Rayleigh and Love wave 2ψ range between 0° and 90° with many between 40° to 60°. These observations are consistent with non-elliptical anisotropy with the ellipticity parameters η_k and η_x considerably smaller than 1. Observations of Love wave 2ψ and Rayleigh wave 4ψ reflect strong Rayleigh-Love coupling, which causes the observed complementary trends with period of the amplitudes of Rayleigh and Love wave 2ψ and Rayleigh and Love wave 4ψ. Recent theories of Rayleigh-Love coupling based on a quasi-degenerate theory allow these observations to be understood and to be used in the future to improve models of the elastic tensor in the crust and mantle.