Page 553 - Rock Mechanics For Underground Mining

TRANSIENT GROUND MOTION

Figure 17.12 (a) Time histories of
ground motion from a coal-mine blast;
(b) frequency spectra for the compo-
nents of the ground waves (after Stagg
and Engler, 1980).

in Table 17.1, due to Housner and Jennings (1982), illustrate the profusion of de-
scriptions. The most commonly used parameters are local magnitude (M 1 ), surface
wave magnitude (M s ), body wave magnitude (M b ), and moment magnitude (M w ).
Moment magnitude is applicable only to earthquakes and mine seismic events, where
the source mechanism involves fault slip. It is observed that the local, surface wave
and body wave magnitude parameters are derived from measurements of maximum

Table 17.1 Magnitude parameters for seismic events (after Housner and Jennings, 1982).
Magnitude Definition Application

Local, M L Logarithm of peak amplitude (in Used to represent size of moderate
microns) measured on a Wood– earthquake or rockburst. More closely
Anderson seismograph at a distance related to damaging ground motion
of 100 km from source, and on firm than other magnitude scales.
ground. In practice, corrections made
to account for different instrument
types, distances, site conditions.
Logarithm of maximum amplitude of Used to represent size of large
Surface wave, M s
surface waves with 20 s period. earthquakes.
Body wave, M b Logarithm of maximum amplitude of Useful for assessing size of large,
P waves with 1 s period. deep-focus earthquakes which do not
generate strong surface waves.
Moment, M w Based on total elastic strain-energy Avoids difficulty associated with
released by fault rupture, which is inability of surface wave magnitudes
related to seismic moment M o (M o = to distinguish between two very large
G · A · D, where G = modulus of events of different fault lengths
rigidity of rock, A = area of fault (saturation).
rupture surface and D = average fault
displacement).

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