Surface vertical displacements, potential perturbations and gravity changes of a viscoelastic earth model induced by internal point dislocations

Abstract

Using the viscoelastic correspondence principle, we utilize the surface coseismic spheroidal deformation fields (i.e. vertical displacements, potential perturbations and gravity changes) of SNREI earth models caused by four typical types of point dislocation, derived by Sun and Okubo (1993), to deduce the fundamental formulas for spheroidal fields relevant to viscoelastic earth models. In computations, we employ a strike-slip dislocation on a vertical plane buried at the bottom of the lithosphere to estimate the maximal viscous relaxation responses to this kind of source that possibly exist on the surface of the earth. We take the seismic moment as 1022N m, which is characteristic of an average large earthquake. The numerical results demonstrate that, if we take the viscosity as 1019 Pa s in the asthenosphere, and 1021 Pa s in the other mantle layers, the rates of surface vertical displacements and gravity changes within about 2.5°for the 10 postseismic years are respectively 1.5-8.1 cm yr-1 and 4.0-14.9 μgal yr-1: the viscous relaxation for this mantle viscosity profile proceeds much faster than for a constant mantle viscosity of 1021 Pa s.