GLOBAL POSTSEISMIC DEFORMATION

We quantify the effects of post-seismic deformation on the radial and horizontal components of the displacement, in the near- and far-field of strike- and dip-slip point dislocations; these sources are embedded in the elastic top layer of a spherical, self-gravitating, stratified viscoelastic earth. Within the scheme of the normal mode technique, w e derive the explicit analytical expression of the fundamental matrix for the toroidal component of the field equations; this component is p ropagated, together with its spheroidal counterpart, from the core-man tle boundary to the earth's surface. Viscosity stratification at 670 k m depth influences the radial and horizontal deformation accompanying viscoelastic relaxation in the mantle over time-scales of 10(3)-10(4) yr, both in the near-field, ranging from 100 to 500 km and in the far- held, from 10(3) to 5x10(3) km. If the upper mantle is differentiated into a low-viscosity zone beneath the lithosphere and a normal upper m antle, faster relaxation is obtained. For an asthenospheric viscosity of 10(20) Pa s we obtain, for a strike-slip dislocation and a seismic moment of 10(22) N m characteristic of an average large earthquake, ho rizontal rates of 1-4 mm yr(-1) in the near-field and 0.05-0.4 mm yr(- 1) in the far-field; these values are maintained over time-scales of 1 0-10(3) yr. Larger rates, with shorter duration, are obtained if the v iscosity is reduced in the low-viscosity channel. As expected, strike- slip dislocations are the most effective in driving horizontal deforma tion in the far-field in comparison with dip-slip ones. It is notewort hy that horizontal velocities are maintained longer in the far-field i n comparison with radial ones, which is not surprising since momentum is propagated in far regions essentially in the horizontal direction; radial deformation is generally lower in the far-field. VLBI technique s, with a precision of a few parts per billion over distances of 10(3) km, can detect global post-seismic deformation induced by large earth quakes. Our results affect the interpretation of the transfer of stres s and seismic activity among different plate boundaries.