Effects of mode coupling and location of rotational axis on glacial induced rotational deformation in a laterally heterogeneous viscoelastic earth

Abstract

Lateral viscosity variations introduce two problems in the study of glacial induced rotational deformation. One of them is mode coupling. The other arises because centrifugal deformations are dependent on the location of the rotational axis relative to the lateral heterogeneities in the earth. The effects of mode coupling and displacement of the rotational axis on centrifugal induced potential perturbations are studied on a layered earth with lateral viscosity variations inferred from seismic tomography. The modified coupled Laplace-finite-element method is used to calculate the centrifugal deformations. It is found that mode coupling is weak because the lateral viscosity variation in the deep mantle is relatively small. The effect of displacing the rotational axis is to shift the spectral amplitude from degree 2 and order 0 harmonic to other orders within degree 2 as is required for changing the orientation of the applied centrifugal potential, but again the leakage of spectral amplitude to other harmonics are small. This implies that rotational motion in an earth with lateral viscosity variations can be approximated by the calculation of the corresponding laterally homogeneous earth. © 2006 The Authors Journal compilation © 2006 RAS.