A global, high resolution, S (shear) wave velocity model (Zhang and Tanimoto 1992) for the upper 500 km of the Earth's mantle shows images of low velocity anomalies beneath hot spots and ridges which suggest deeper (>200 km) upwellings beneath hot spots than ridges (<100km) and also interactions between ridges and nearby hot spots suggesting that mantle flow from hot spots feeds ridges (Zhang and Tanimoto 1992, Figure 1). However, Su et al. (1992) argue that the Zhang and Tanimoto model underestimates the amplitude and depth of mid-ocean ridge anomalies and is incompatible with SS (an S wave that reflects off the Earth's surface once), SS-S travel time anomalies and mantle waveforms. In particular they find that low velocities beneath ridges extend deeper than 300 km. Thus, while overall tomographic patterns are similar, amplitudes and details of geometry differ (Dziewonski et al., 1993). The controversy is, as yet, unresolved.
Below 100 km the pattern of low velocity beneath the Mid Atlantic ridge is offset to the east (Tanimoto and Zhang, 1992; Anderson et al., 1992) which, according to plate tectonic reconstructions, runs approximately along the boundary where Africa and South America broke up (Tanimoto and Zhang, 1992).
Multiple ScS (S waves that bounce off the core) studies by Revenaugh and Jordan detect greater impedance contrasts at the base of the continental crust than at the base of the oceanic crust (Revenaugh and Jordan 1991a). Revenaugh and Jordan (1991b) identify four reflectors in the upper mantle denoted H (Hales 60 km), G (Gutenberg 60 km), L (Leyman 210km-300km), and X (A new reflector in subducting zones). The reflectors exhibit lateral variability and appear to be related to continental-oceanic deep structure and subduction. The L reflector appears clearest on continental ScS paths and G on oceanic. All reflectors have positive impedance contrast except G, which occurs at a depth of about 60 km in the western Pacific and is identified as the lid-LVZ transition. The authors suggest that L represents a transition from anisotropic lower lithosphere to isotropic asthenosphere in which the anisotropy has been annealed out by the high temperature.