Seismic anisotropy in the D″ layer

Abstract

We present observations of diffracted SV (SVd) for a path between the Fiji‐Tonga islands and the eastern coast of North America at distances greater than 110°. Observed features of S diffracted suggest that coupling between SVd and SHd can be ruled out as a first order effect for this path. Arrivals of SHd are late relative to IASP91 travel‐times by about 10 s, and those of SVd are late relative to SHd by 3 s, for most records. The slope of the log(SVd/SHd) spectral ratio is around 3 Hz −1 in the range 0.06–0.15 Hz . A transversely isotropic low‐velocity layer in the lower‐most mantle with a thickness of 200–300 km may account for most of the observed properties of SVd.