ROTATIONAL INSTABILITIES OF THE EARTH INDUCED BY MANTLE DENSITY ANOMALIES

From the seismic tomographic models, we estimate the offset between the geographic pole and the main inertial axis. This offset, which is large for a static Earth, is drastically reduced when the dynamical behaviour of the mantle is taken into account. For a mantle stratification with a viscosity increase by a factor of about 50, this offset is comparable to what can be estimated on the basis of remanent post‐glacial depression. We also discuss the rate of polar drift induced by both internal mass heterogeneities and Pleistocenic deglaciation. Contributions from internal sources and ice‐sheets disintegration are consistent with the observed secular trend. For high viscosity contrasts, our results are quite insensitive to the phase transition or chemical nature of the upper‐lower mantle interface. This study shows that there is no contradiction between the tomographic models of the Earth and their implications for the rotational stability of our planet; our findings also agree with recent analyses of paleomagnetic data. Copyright 1990 by the American Geophysical Union.