Viscosity stratification of the lower mantle as inferred by the j₂ observation.

We have employed a layered model with 2 viscoelastic layers in the mantle for the purpose of examining the degree of viscosity stratification in the lower mantle. We have used the rotational data involving the secular variation of the gravitational harmonic coefficient J₂ in constraining the viscosity structure of the deep mantle, since we can demonstrate that the predictions of j₂ are affected by at most 15% due to the presence of the 670 km density discontinuity. This fact then allows us to lower the interface depth f separating the upper and lower portions of the mantle and to conduct an extensive sensitivity analysis of the lower mantle viscosity solutions to changes of f. This study indicates that the resolving power of mantle viscosity profiles from the j₂ data may deteriorate for depths >2000 km.-from Authors