Geoscience Reference
In-Depth Information
(a)
(b)
8
8
7
7
6
5
6
4
3
5
Earthquakes
Explosions
2
4
1
12
13
14
15
16
17
18
19
20
log M
0
3
3
4
5
7
6
8
m
b
Figure 4.11.
(a) A plot of surface-wave magnitude
M
S
against body-wave
magnitude
m
b
for twenty-six nuclear explosions (solid circles) and ninety-nine
earthquakes (open circles), all in Eurasia. The solid line is the discriminant line
m
b
= 2.87 + 0.60
M
S
. Nuclear explosions are less efficient at generating surface
waves than are earthquakes with the same body-wave magnitude. (After Nowroozi
(1986).) (b) A plot of local magnitude
M
L
against the logarithm of the seismic
moment
M
0
for 128 explosions (solid circles) and 173 earthquakes (open circles).
The explosions were from the Nevada Test Site, U.S.A., and the earthquakes
occurred in California and Nevada, U.S.A., and Baja California, Mexico. (After Woods
and Helmberger, A new seismic discriminant for earthquakes and explosions,
EOS
Trans. Am. Geophys. Un.
,
74
(8), 91. 1993. Copyright 1993 American Geophysical
Union. Modified by permission of American Geophysical Union.)
shown in Fig. 4.12 even seems to extend down to very small earthquakes with
log
M
0
of 9 and source dimensions of only 10 m. Since the drop in stress,
,
for earthquakes is constant over a large range of
M
0
, measurement of
M
0
cannot
yield an estimate for
ς
ς
. The total cumulative seismic moment of earthquakes
10
28
each year is about 3
1.5 MPa) occur for
thrust earthquakes at subduction zones and the highest stress drops (
×
Nm.The lowest stress drops (
<
>
3MPa)
occur for strike-slip earthquakes at mid-ocean-ridge transform faults.
Both the body-wave and the surface-wave magnitude scales saturate (do not
give large enough values) for very large earthquakes and are strongly dependent
on the frequency content of the seismic wave. Therefore, to estimate the size
of large earthquakes, long-period (low-frequency) waves are used. A magnitude
scale, the
moment magnitude M
w
,isobtained from the seismic moment and is
a more reliable measure of the magnitude of an earthquake than is either
m
b
or
M
S
. The moment magnitude is defined as
2
3
M
w
=
log
10
M
0
−
6
.
0
(4.21)
