Page 327 - Rock Mechanics For Underground Mining

CHARACTERISATION OF SEISMIC EVENTS

where
M L = local magnitude
K w = the magnification of a Wood-Anderson seismograph at period T
K = instrument magnification factor
A(D) = the maximum trace amplitude at distance D
log A 0 (D) is a calibration factor such that a standard seismograph will have a
trace amplitude of 0.001 mm at a distance of 100 km for a M L = 0 event.
Other than in eastern North America, the Richter local magnitude scale is used to
characterise mine seismic events around the world. For mines in the Canadian Shield,
the Nuttli (Mn) scale is used (Nuttli, 1978). It is defined by the relation

Mn =−0.1 + 1.66 log D + log (A(D)/KT ) (10.101)

where
D = the epicentral distance to source, km
A(D) = half the maximum peak-to-peak amplitude in the S-wave
K = instrument magnification factor
T = time period of ground motion in seconds.
In studies to relate the two magnitude scales, Hasegawa et al. (1989) observed that
over the range of primary interest in mine seismicity (M L = 1.5 to 4.0), for the same
event the Mn scale records magnitudes about 0.3 to 0.6 units greater than the M L
scale.
Moment Magnitude (Hanks and Kanamori, 1979) is based on the seismic moment
derived from parameters in the spectral density plot, and is defined by the equation

2
M = log M 0 − 6.0 (10.102)
3
where
M is the moment magnitude
M 0 is the seismic moment (Nm)
Moment magnitude is the most commonly used measure of source strength.
It has been found that the various body-wave magnitude scales are inadequate
for description of the geomechanical perturbations associated with a seismic event.
Mendecki (1993) provides an example of two seismic events of local magnitude
M L = 5.9 which have seismic moments M o which differ by a factor of 400.

10.10.5 Seismic source mechanisms
In mine seismology, two different types of mine seismic events are observed. The
first types of events are the larger magnitude ones, which occur at some distance from
mining activities and are generally associated with major geological discontinuities
(Gibowicz and Kijko, 1994). This type of source is predominantly associated with a
shear-slip type mechanism, as is commonly recognised in earthquake seismology.
The second type of seismic event occurs in or near the mining domain and is of
low to medium magnitude. The frequency of occurrence of these events is generally a
function of mining activity (Gibowicz and Kijko, 1994). In relating seismic events to
patterns of fracturing in hard rock mines, Urbancic and Young (1993) used fault-plane

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