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

expected to be responsible for a large number of non-double-couple mechanisms observed in
Creighton Mine.
3.4 Stress Tensor Inversion
P-, T- and B-axes derived from focal mechanisms can be used as a crude approximation to
principal stress orientations. Axes represent the ideal stress orientations for a given solution. In
the presence of preexisting fractures, the maximum principal stresses may differ significantly
from P- and T-axes and be oriented elsewhere in compressional and dilatational quadrants
(Gephart and Forsyth, 1984). Indeed in Creighton Mine faults and fractures are oriented obliquely
to principal stress directions, which may cause the P-, T- and B-axes to differ from principal stress
orientations.
One way to resolve the local stress tensor is through stress tensor inversion. Inversion was done
with software developed by Gephart (1990). Details of the inversion method are described in
Gephart and Forsyth (1984) and the program is presented in Gephart (1990). The software makes
use of P- and T-axis measurements and compares different solutions to find the best fit between
the predicted model and actual data with minimal error.
P- and T-axes from 95 focal mechanisms for events located in proximity to the 7400 Level were
used for inversion. Results from this inversion indicate that the maximum principal stress is near
horizontal and oriented E-W and the minimum principal stress trends SSE and has a moderate
plunge (Fig. 3.20). The best model was that with a minimum misfit of 10.62º (Fig. 3.21). The
parameter R ranges between 0 and 1 and is defined as:
2 1
R (Gephart and Forsyth, 1984). (Equation 3.2)
3
1
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