Numerical simulation for the prediction of the plate motions: Effects of lateral viscosity variations in the lithosphere

A numerical simulation of Newtonian viscous flow without inertia terms in a 3-D spherical shell driven by the negative buoyancy due to the slabs has been conducted to understand the effects of weak plate margins on the plate motions. Density loads are inferred from the seismicity and the reconstruction of the subduction history. The toroidal energy of plate motion comparable to the poloidal energy appears, when γ (ratio of the viscosity at margins to that of interiors) becomes O(0.01). For the whole mantle density model, all the plates move too fast relative to the Pacific plate. The direction of major plate motions is generally improved by the inclusion of weak plate boundaries. The density loads in the upper mantle appear to explain the overall plate motions, although some of the plate motions may require hidden and/or deeper density anomalies to be consistent with the observations. As γ decreases, the geoid anomalies associated with the upper mantle slabs change their signs. This reversal affects the long-wavelength components of the geoid anomalies. A considerable part of the horizontal stress field shows a horizontal extension suggesting that another type of density anomalies is necessary to explain the general compressional field of the real Earth.