Mantle layering from ScS reverberations 4. The lower mantle and core- mantle boundary

Long-period, SH-polarized, multiple-ScS phases reflected once from a mantle discontinuity (first-order reverberations) are used to search for abrupt shear-wave impedance contrasts in the lower mantle. Beneath the geographic regions sampled, the depth interval of 1000-2600 km (Bullen's region D′) appears free of any distinct, radial layering, in agreement with the majority of recent seismic models and the notion of near-adiabatic compression of a compositionally homogeneous lower mantle. At greater depths, corresponding to the D″ region of the lowermost mantle, we find evidence for a reflector of long-period seismic energy situated an average of 325 km above the core-mantle boundary (CMB), similar to the discontinuity proposed by T.Lay and coworkers. Our results indicate a 4.4% increase in shear wave impedance and a 1.7% increase in density if the 2.75% increase in shear velocity proposed by Lay and Helmberger (1983) holds true for our study area. A compositional origin is favoured. -from Authors