1. The most comprehensive global seismic velocity inversions are provided by seismic tomography (13). The characteristic features of all recent tomography models of the lower mantle are faster-than-average linear zones along the circum-Pacific belt and slow regions beneath the central Pacific Ocean and Africa. In many regions the zones of fast velocity anomalies can be traced from the surface to the CMB and are probably associated with subducted slabs. The broad slow anomalies extend from the base of the mantle to at least 1500 km depth and may be indicative of hot upwellings.
2. Lay T., Helmberger D. V., Geophys. J. R. Astron. Soc. 75, 799 (1983).
3. Seismic Evidence for Partial Melt at the Base of Earth's Mantle
4. Lateral Variations in Compressional/Shear Velocities at the Base of the Mantle
5. Long-period S -wave studies allow for a 3% velocity increase over as much as 100 km. However broad-band studies limit the thickness of the transition zone to about 30 km [