Predominance of Large-Scale Heterogeneity and the Shift of Velocity Anomalies between the Upper and Lower Mantle

Long-wavelength seismic velocity studies for the mantle indicate that mantle structure is dominated by very large-scale heterogeneity, shown predominantly by harmonic degrees l=1 and 2 for the whole mantle; near the surface l=5 and 6 produce a secondary peak. Dominance of these anomalies suggests the existence of similarly large-scale convection currents. The qualitative differences between the upper and the lower mantle are evident in the distribution of heterogeneity and in the shift of l=2 velocity variation components. Along with other geophysical observables such as attenuation parameters (Q) and viscosity, different styles of dynamics are implied for the upper and lower mantle. In particular, the shift of the l=2 velocity variation pattern is a fairly robust and common feature among recent three-dimensional Earth models and seems to favor the predominantly layered convection model. There is, however, evidence that the boundary layer between the upper and lower mantle, if it exists, is not as strong as those boundary layers at the top and the bottom of the mantle.

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