A timescale of true polar wander of a quasi-fluid Earth: An effect of a low-viscosity layer inside a mantle

By means of a simple parameter sensitivity analysis, we demonstrate the effect of a low-viscosity layer inserted inside a mantle of a hypothetical Earth on the timescale of large-scale and long-term true polar wander. Here the timescale in our parameter study means the characteristic scale of viscoelastic readjustment of the rotational bulge in the framework of the quasi-fluid approximation for the long-term reorientation of the Earth. Based on this assumption, we calculate the characteristic timescale and associated viscoelastic tidal Love number with the effect of this layer in order to see the dependences on the viscosity, depth, and thickness of the inserted layer. We also compute the characteristic timescale without this layer for the sake of comparison. Our results indicate that the timescale strongly depends on the existence of this layer: positive dependences on its viscosity and depth and a negative dependence on its thickness. We conclude that the low-viscosity layer has a strong impact on the characteristic timescale, especially if this layer exists at the top of the mantle. Although a few previous studies on the small-scale and short-term true polar wander have also suggested a possible effect of inserting a low-viscosity layer, our study implies that the sensitivity to the low-viscosity layer over a long timespan is not necessarily the same as that over a short timespan.