Mass distribution and moments of inertia in the outer shell of the Earth

Different mass distributions within the lithosphere and the asthenosphere have been here considered in order to calculate the moment of inertia of the outer shell of the Earth relative to many different axes. In particular, we seek for the maximum moment of inertia, MMI, which represents the theoretical rotation axis that the outer shell of the Earth should attain for mantaining equilibrium. For the present-day distribution of mass at depth, we consider the most updated crustal, lithospheric and asthenospheric models satisfying general geological and geophysical laws. When considering only the lithospheric shell, as if it were fully independent from the asthenosphere (i.e. totally decoupled), our numerical results show the complete lack of equilibrium in terms of moment of inertia with respect to the present-day rotation axis. In further calculations we also included the asthenospheric mantle assuming different density values as well as different compensation depths. Among the numerous tests, the mass distribution models showing theoretical axes of rotation closest to the present-day one are those obtained with a compensation depth of 400 km. The possible implications of these results in terms of westward plates drift and depth of the more likely decoupling layer within the mantle are discussed