The effect of global seismicity on the polar motion of a viscoelastic Earth

We evaluate the polar drift of a stratified viscoelastic Earth, generated b y 20 years of global seismicity (from the Centroid Moment Tenser catalog, 1 977-1997). Our estimate is better than that of Soldati and Spada [1999] bec ause it accounts for the effects, previously neglected, of earthquakes at d epths larger than 80 km. In fact, we show that deep focus and intermediate- depth focus seismic events tend to affect the Earth's inertia tenser more s ignificantly than shallow ones; specifically, we find that the mean rate of polar drift (evaluated over a 6-year time interval immediately following t he seismic event) due to a vertical dip-slip source at the bottom of the as thenosphere (280 km depth in our model) is 5 times larger than that generat ed by an analogous source, located at the boundary between lithosphere and asthenosphere (80 km depth). We conclude, nevertheless, that global seismic activity is not able to alter significantly the rotational parameters of t he Earth: according to our computations, the changes that it induces in the inertia tenser result in an average polar drift that is still significantl y smaller than the observed one.