Large earthquakes and Earth rotation: the role of mantle relaxation

By means of a radially stratified spherical model and using the available m oment tensor solutions, we evaluate the contribution of large earthquakes t o polar drift in the last two decades. In contrast to previous analyses, wh ich only considered the elastic response of the Earth to the release of sei smic moment, we account for mantle rheology and thus for the time-dependent postseismic inertia changes driven by global seismicity. We confirm that d uring the last twenty years the global seismic activity has not significant ly altered the rotational parameters of the Earth. However, we find that fo r values of asthenospheric viscosity suggested by recent studies of postsei smic stress diffusion, the delayed relaxation of the mantle amplifies the a verage rate of polar drift by a factor of 1.2 to 1.7 (depending on the widt h of the asthenosphere) even on a decade time-scale.