Stratification of anisotropy in the Pacific upper mantle

On the basis of the use of broadband (25–150 s) Rayleigh wave group speeds to estimate the 2ψ component of azimuthal anisotropy, we present evidence for a stratification of anisotropy in the uppermost mantle at large scales across the Pacific basin. We confirm previous surface wave studies that established that the fast axis directions of azimuthal anisotropy for intermediate‐ and long‐period Rayleigh waves approximately align with present‐day plate motions. At shorter periods (25–50 s), however, fast axes align nearer to the paleospreading or fossil spreading direction which differs from present‐day plate motion in the old Pacific. These observations, as well as observations of the age dependence of the amplitude of azimuthal anisotropy, imply that azimuthal anisotropy in the Pacific upper lithosphere (