Large earthquakes and Earth rotation: the role of mantle relaxation

By means of a radially stratified spherical model and using the available moment tensor solutions, we evaluate the contribution of large earthquakes to polar drift in the last two decades. In contrast to previous analyses, which only considered the elastic response of the Earth to the release of seismic moment, we account for mantle rheology and thus for the time‐dependent postseismic inertia changes driven by global seismicity. We confirm that during the last twenty years the global seismic activity has not significantly altered the rotational parameters of the Earth. However, we find that for values of asthenospheric viscosity suggested by recent studies of postseismic stress diffusion, the delayed relaxation of the mantle amplifies the average rate of polar drift by a factor of 1.2 to 1.7 (depending on the width of the asthenosphere) even on a decade time‐scale.