title of the thesis - Department of Geology - Queen's University

This parameter reflects the magnitude of intermediate principal stress relative to the maximum or
minimum principal stress (Bellier and Zoback, 1995). As the magnitude of the intermediate
principal stress approaches that of the maximum principal stress, R approaches 0 and is indicative
of an extensional stress regime; as it approaches the minimum principal stress, R approaches 1 and
the rock is increasingly confined, indicative of a compressional regime (Bellier and Zoback, 1995;
Bellier et al., 1997). Inversion for events on the 7400 Level yields R = 0.5 which indicates pure
strike-slip failure.
Stress orientations are comparable to the regional stress tensor, discussed in section 3.4.1. Stress
inversion was also performed for localized clusters of events, Cluster 1 and Cluster 2, as
previously defined. This was conducted using the exact solution method (described in Gephart
and Forsyth, 1984) and a two-degree grid to identify perturbations in the local stress field where
dense seismicity occurs. Cluster 3 was omitted from analysis due to insufficient data. Principal
stresses for Cluster 2 approximate those for the 7400 Level stress inversion and the regional stress
(Fig. 3.22). Principal stresses for Cluster 1, however, differ significantly from the regional stress.
The inversion results with the minimal misfit identify a steeply-plunging maximum principal
stress and a near-horizontal minimum principal stress (Fig. 3.23). An additional minimum
principal stress axis that trends north is also identified. A second stress inversion for Cluster 1
events (Fig. 3.24) shows two distinct stress orientations, though this solution has a slightly higher
misfit from the previous solution (8.12 o , as compared to 7.63 o ). R parameter values for Cluster 1
of 0.05 and 0.10, respectively, demonstrate that the rock mass is under extension (uniaxial
eccentric compression). Rotation of the stress tensor in proximity to Cluster 1 may be responsible
for high magnitude, high energy events, though the cause for this change is unclear.
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