Page 326 - Rock Mechanics For Underground Mining

ENERGY, MINE STABILITY, MINE SEISMICITY AND ROCKBURSTS

Figure 10.28 Six possible ways that
mine-induced seismic events can oc-
cur: (A) cavity collapse; (B) pillar
burst; (C) tensional fault; (D) normal
fault; (E) thrust fault; and (F) shallow
thrust faulting (after Hasegawa et al.,
1989).

released by excavation. This suggests that, for the case of seismic events associated
with fault slip, most of the released energy is dissipated by rock comminution during
generation of faults and fractures and subsequent episodes of shear motion along
them.
The ratio of P-wave energy to S-wave energy is an important indicator of the source
mechanism of a seismic event. For natural earthquakes, Boatwright and Fletcher
(1984) showed that the S-wave energy is usually 10 to 30 times the P-wave energy
for a double couple event. Mine scale seismicity shows different E s /E p ratios. For a
mine in the Ruhr Basin, Germany, Gibowicz et al. (1990) found the ratio of S-wave
energy to P-wave energy ranged from 1.5 to 30, with two-thirds of the events hav-
ing E s /E p energy ratios less than 10. Urbancic and Young (1993) obtained similar
results in a study at the Strathcona mine, Ontario, Canada. The proposal is that the
enhanced P-wave energy and reduced S-wave energy can be explained by a non-
double couple source mechanism. Gibowicz and Kijko (1994) proposed that these
results, involving a volumetric component of deformation, are consistent with ten-
sile failures, or at least shear failures with tensile components, that often occur in
mines.

10.10.4 Magnitude
A magnitude scale is an attempt to measure the size of a seismic event, ideally in
real time, and for some scales, in terms of the amplitude of part of the body wave
that it induces. In most cases, the various proposed magnitude scales have been based
on amplitudes recorded over a particular spectral band. The three commonly used
magnitude scales are described below.
The most commonly cited measure of magnitude is Local Magnitude (Richter,
1935). It is based on time domain parameters and therefore requires no spectral
analysis to estimate the magnitude. It is defined by

M L = log[A(D)K w /K)] − logA 0 (D) (10.100)

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