Page 328 - Rock Mechanics For Underground Mining

ENERGY, MINE STABILITY, MINE SEISMICITY AND ROCKBURSTS

solutions and interpretation of the geological structure to define systematic fracture
development ahead of an excavation face. The observed fracture pattern consisted of
a zone of extensional failures next to the excavation, a transition zone of shear and
shear extensional failures and a zone of shear failures remote from the excavation.
Six possible modes of rock deformation that can induce the patterns of mine seis-
micity have been described by Hasegawa et al. (1989). Of these modes of failure, three
have the focal mechanism of a shear-slip type failure (thrust, normal and reverse fault).
In terms of a mechanical model, rock failures involving slip and shear rupture can
be represented by a double-couple focal mechanism. A double couple consists of
two opposing force couples with no net force or moment. They are equivalent to the
quadrupole force singularities described by Brady and Bray (1978).
The remaining three source mechanisms are represented by non-double-couple
singularities. These are a point force singularity, a dipole singularity and tensional
faulting. Further, some complex non-double-couple mechanisms have been observed
that may be a combination of the simple double-couple and non-double-couple mech-
anisms (Gibowicz and Kijko, 1994).
The radiation patterns from each of these proposed source mechanisms are pre-
sented schematically in Figure 10.29. Such patterns are used in the interpretation of
Figure 10.29 First motions for P
first-motions of waveforms associated with a seismic event. First-motion studies can
and S waves for four source mech-
anisms: (A) single force; (B) dipole; be used in determining fault plane solutions, in the case of a double-couple source,
(C) tension fault; and (D) dip-slip fault or to identify non-double-couple mechanisms.
(after Hasegawa et al., 1989).

Figure 10.30 The nine component
couples (or dipoles) of a moment ten-
sor (after Aki and Richards, 1980).

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