SEISMIC EVIDENCE FOR SMALL-SCALE HETEROGENEITY THROUGHOUT THE EARTHS MANTLE

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