Using seismic tomography models to constrain the viscosity structure of the Earth's mantle

Abstract:

Over the past few decades, much work has been done to constrain the viscosity structure of the Earth's mantle using inverse techniques, viscoelastic modelling and post-glacial rebound data. Variations in the Earth's geoid provide constraints on the density structure in the mantle. We begin by deriving density models from a large range of seismic tomography models using appropriate velocity-density scaling factors which vary with depth. We then compute the synthetic geoid to compare with observed gravity anomalies in the Earth. This work improves on past results from geoid inversions by using a larger set of tomographic models, including more recent models. We find there are notable differences between using S- and P- wave tomographic models, with the S-wave models providing a better fit with the observed geoid. We explore the appropriate scaling factor for velocity to density conversion to use for different types of tomographic models. The different tomographic models also result in different viscosity profiles needed to produce the appropriate geoid to fit the observations. A unique viscosity profile corresponding to each tomographic model is not found but rather a family of different viscosity profiles preferred by each model. All models found an increase in the lower mantle viscosity provided the best correlation with the observed geoid but the magnitude of increase varied. Moreover, multiple profiles provide the best correlation when varying both the transition zone and lower mantle viscosity. Using the 3D mantle convection model, Stag-YY (e.g., Tackley, 2008), we are further able to explore the effect of lateral variations in viscosity in addition to the radial variations.

REFERENCES
Tackley, P. J. (2008), Phys. Earth Planet. Int., 171(1-4).