INDUCED STRESSES AND FAULT POTENTIAL IN EASTERN CANADA DUE TO A DISC LOAD - A PRELIMINARY-ANALYSIS

In order to understand the causal relation between postglacial rebound and earthquakes, a simple disc load model is used to: (1) calculate s tresses induced in the lithosphere and mantle by glacial loading, melt ing and postglacial rebound; and (2) evaluate the effect of glacial lo ading/rebound on the failure potential for earthquakes in the upper cr ust. The dependence of the failure potential and the actual mode of fa ilure on the coefficient of friction, the ambient tectonic stress magn itude/direction the stress due to the overlying rocks, and lithospheri c thickness are investigated. Prominent features of this paper are the inclusion of: (1) a viscoelastic mantle and thus the migration of str ess; and (2) the ambient tectonic stress and overburden stress contrib utions in the calculation of the total stress field. It is assumed tha t, throughout the Earth, there are optimally oriented pre-existing vir tual faults that are initially close to but not at failure; thus, a ti me-dependent quantity called dFSM (related to the Coulomb-Mohr failure criterion) can be defined such that a negative value of dFSM would ad vocate faulting or earthquake activities whereas a positive value of d FSM would promote stability. The results indicate that, under all comb inations of tectonic stress magnitude and overburden stress, crustal l oading promotes fault stability directly underneath the load. Upon the removal of the load, thrust faulting is predicted within the ice marg in if the horizontal stress (S-h) induced by the overburden is greater than or equal to the vertical component (S-v) of the overburden stres s (zeta greater than or equal to 1, where zeta = S-h/S-v). Under this condition, theory predicts that faulting or earthquake activity should have reached a maximum immediately after deglaciation. If the horizon tal stress induced by the overburden is less than the vertical compone nt of the overburden stress (zeta < 1), then theory predicts fault sta bility within the ice margin. The theory predicts fault instability bo th north and south of the ice margin. The mode of failure, however, is completely determined by the value of zeta. The trade-off between the tectonic stress magnitude and the overburden stress parameter (zeta) is also investigated. It is shown that a larger tectonic stress magnit ude can be used to compensate a smaller value of zeta. The results of this analysis show that variations in the coefficient of friction, lit hospheric thickness and a ductile zone below the upper crust do not si gnificantly affect the above conclusions.