In order to understand the causal relation between postglacial rebound and earthquakes, a realistic ice and water load model is used to (1) calculate stresses induced in the lithosphere and mantle by glacial loading, melting and postglacial rebound and (2) evaluate the effect of glacial loading/rebound on the failure potential for earthquakes in the upper crust. The dependence of both the failure potential and the actual mode of failure on the ambient tectonic stress magnitude, the overburden stress, and lithospheric properties are investigated. Prominent features of this analysis are the inclusion of (1) a viscoelastic mantle and thus the migration of stress, and (2) the ambient tectonic stress and overburden stress contributions in the calculation of the total stress field.
The spatio-temporal calculations, by a finite-element technique, of upper-crustal stresses and the failure potential for earthquakes indicate that fault stability is invariably enhanced directly beneath the load. For the case where stresses induced by the overburden are such that the horizontal component (Sh) is greater than or equal to the vertical component (Sv) (ζ≥ 1, where ζ = Sh/Sv), the model predicts the onset of thrust faulting and maximum earthquake activities soon after deglaciation is complete (when rebound rates are at a maximum). Observational data support this prediction. Since that time, rebound stresses have been decreasing in magnitude, but they continue to act as a trigger mechanism for optimally oriented pre-existing faults that are otherwise on the verge of failure. If one limits the existence of such faults to lie within the pre-weakened zones of eastern Canada, then the spatial distribution of current earthquakes can also be explained.
Perturbations to the magnitude of the tectonic stress components or lithospheric properties do not affect, to any significant extent, the above conclusions.