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


Riccardo Caputo, Dipartimento di Fisica e Scienze della Terra, Università di Ferrara, via Saragat 1, 44122 Ferrara, Italy. Tel.: +39 0532 974688; e-mail:


Terra Nova, 25, 38–47, 2013


Different mass distributions within the lithosphere and the sub-lithospheric mantle have been considered here 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 maintaining equilibrium. For the present-day distribution of mass at depth, we consider the most updated crustal, lithospheric and sub-lithospheric models satisfying general geological and geophysical laws. When considering only the lithospheric shell, as if it were fully independent from the sub-lithospheric mantle (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 sub-lithospheric 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 possible decoupling layers within the mantle are discussed.