Seismic evidence for small-scale heterogeneity throughout the Earth's mantle

Details in the amplitude structure of short-period precursors to the seismic core phase PKP can be used to constrain the depth extent of mantle scattering. The simplest model consistent with the observations invokes small (similar to 8 km), weak (r.m.s. velocity perturbations of 1%), random heterogeneities uniformly distributed throughout the mantle. The data do not support previously proposed models that place an increase in the concentration of scattering sites near the base of the mantle, and instead place an upper limit on the amplitude of any short-wavelength topography at the core-mantle boundary.