Civil Engineering Reference
In-Depth Information
from an assumed point source. Thus the seismic moment is a more useful measure of the
strength of an earthquake.
Kanamori (1977) and Hanks and Kanamori (1979) introduced the moment magnitude
M
w
scale, in which the magnitude is calculated from the seismic moment by using the fol-
lowing equation:
M
w
- 6.0
0.67 log
M
0
(2.4)
where
M
w
moment magnitude of earthquake
0
seismic moment of earthquake, N
m. The seismic moment is calculated from
Eq. (2.3).
2.4.4
Comparison of Magnitude Scales
Figure 2.15 shows the approximate relationships between several different earthquake
magnitude scales. When we view the data shown in Fig. 2.15, it would appear that there is
an exact relationship between the moment magnitude
M
w
and the other various magnitude
FIGURE 2.15
Approximate relationships between the moment magnitude scale
M
w
and other magnitude
scales. Shown are the short-period body wave magnitude scale
m
b
,
the local magnitude scale
M
L
,
the long-
period body wave magnitude scale
m
B
,
the Japan Meteorological Agency magnitude scale
M
JMA
and the sur-
face-wave magnitude scale
M
S
.
(
Developed by Heaton et al. 1982, reproduced from Idriss 1985.
)
