SEISMIC ANISOTROPY IN THE D'' LAYER

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 degrees Observed features of S diffracted suggest th at coupling between SVd and SHd can be ruled out as a first order effe ct for this path. Arrivals of SHd are late relative to IASP91 travel-t imes 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 arou nd 3H z(-1) in the range 0.06-0.15 Hz. A transversely isotropic low-ve locity layer in the lowermost mantle with a thickness of 200-300 km ma y account for most of the observed properties of SVd.