ABSTRACT:

In mechanics terms, this paper traces the origin of high horizontal stresses in crustal rocks to the viscoelastic compression experienced by them under repeated glacial loading and rigid confinement, acknowledging at the same time the effects of heavy erosion, differential uplift and warping upon unloading, as well as tectonics. Attention is drawn to the general trend of increasing deviator stress with depth both in Ontario and in several other parts of the world, and to the possibility of deep-seated fracturing and shear failure. An attempt is also made to correlate the degree of over-stressing which may occur in some crustal rocks, the consequent release of seismic energy, and the magnitude of the resulting earthquake. With such an approach, an upper bound magnitude of 7 - 8 (on the Richter scale) was deduced for eastern Canadian earthquakes. The application of this mechanistic approach to seismic zoning and prediction is described, along with its inherent limitations.

1.0 INTRODUCTION - OCCURRENCE OF HIGH HORIZONTAL STRESSES IN SOME CRUSTAL ROCKS

A state of high horizontal stresses has been observed in many of the Palaeozoic and Precambrian rocks of Ontario, Canada. The in-situ stresses measured in these rocks (e.g. Coates and Grant 1976; Bielenstein and Eisbacher 1970; Herget 1974; Herget et al 1975; Lo et al 1975; Palmer and Lo 1976) generally correlate with those reported by Hast and others for Fennoscanidan rocks (e.g. Hast 1969; Elfman 1969; and Li 1970). Existence of high horizontal stresses have also been reported in British Columbia (Imrie and Jory 1968; Imrie and Campbell 1976), northeastern United States (e.g. Obert 1962; Hooker and Johnson 1967), in Australia (e.g. Moye 1958; Endersbee and Hofto 1964; Mathews and Edwards 1969), and in southern Africa (e.g. Pallister 1968; Barron 1970; Van Heerdan 1972; Gay 1974; Orr 1975).

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