The relative wavelengths of fast and slow velocity anomalies in the lower mantle: Contrary to the expectations of dynamics?

We quantify the spherical harmonic characteristics of shear velocity anomalies as a function of depth for a range of tomographic models of the Earth's mantle. In particular, the amplitude spectra of fast and slow anomalies are analyzed separately. Thus, the higher amplitude harmonic degrees and relative scale lengths of both possible thermal upwellings and downwellings can be determined. Our results are consistent with prior studies in that we find that the amplitude spectra of heterogeneity (fast and slow) in the mid-mantle are essentially flat. However, where heterogeneity is known to be stronger in the deep mantle, slow anomalies are observed to be consistently higher amplitude at harmonic degrees 1–12, while fast anomalies dominate at higher degrees. This result is at odds with many dynamic models in which upwellings tend to be narrow and downwellings tend to be broad. This inconsistency with isochemical dynamic models may be associated with variations in phase, chemistry and/or viscosity in the Earth's lowermost mantle. The technique presented here provides a means for discriminating between these effects when comparing tomographic and dynamic models of the Earth's mantle.