“…The reflector that produces this extra arrival varies globally in depth by several Depth (km) Figure 6 Radial profiles of S-wave velocity in the lowermost mantle: PREM (Dziewonski and Anderson, 1981), SLHE, SLHA (Lay and Helmberger, 1983a), SYL1 (Young and Lay, 1987a), SGHP (Garnero et al, 1988), SWDK (Weber and Davis, 1990), SYLO (Young and Lay, 1990), and SGLE (Gaherty and Lay, 1992). hundred kilometers (e.g., Kendall and Shearer, 1994) and appears to have regional-scale lateral variations of on the order of a hundred kilometers that may produce scattering rather than simple reflections (e.g., Emery et al, 1999;Freybourger et al, 1999;Hutko et al, 2006;Krü ger et al, 1995;Lay et al, 1997Lay et al, , 2004aScherbaum et al, 1997;Thomas et al, 2004a,b;Weber, 1993;Yamada and Nakanishi, 1998). Shear velocity increases of 2.5-3.0% are found in regions under Eurasia (SWDK and SGLE), Alaska (SYLO), Central America (SLHA), the Indian Ocean (SYL1), and the central Pacific (SGHP).…”