13 A strategy described as “In God we trust, all others we verify.”
more surface wave energy (M s ), from the explosions, which
generate more body (P) wave energy (m b ).
The challenge of seismic monitoring has increased in recent
years. Since 1996 the USA has abided by the Comprehensive
Test Ban Treaty (CTBT), which bans all nuclear testing, preventing the development of new nuclear weapons. Thus the
focus of US monitoring efforts has expanded to include smaller
countries around the world. 13 There is also the need to identify
possible smaller nuclear tests, including those by terrorists.
Hence seismic monitoring must identify explosions less than
1 kt, which have a magnitude of 4–4.5 (Eqn 2). This requires
locating and identifying more than 200,000 earthquakes and
additional mining explosions every year.
1.2 Seismology and society 27
0
60
120
180
240
Time (s)
May 11, 1998, Indian nuclear test
(magnitude 5.1)
April 4, 1995 Indian earthquake
(magnitude 4.8)
P
Surface wave
Fig. 1.2-19 Seismograms showing the differences
between an earthquake and an explosion. For shallow
earthquakes, in this case an m b 4.8 shock in India, the
P wave is much smaller than the surface waves. By
contrast, the initial P wave is the largest arrival for
explosions like this Indian nuclear test. Data recorded
at Nilore, Pakistan. (Courtesy of the Incorporated
Research Institutions for Seismology.)
m b = C + 0.75 log Y,
(2)
but the constant differs for Nevada (C = 3.95) and Kazakhstan
(C = 4.45). With these corrections, it appears that the Soviet
Union complied with the treaty.
Monitoring nuclear tests requires distinguishing them from
earthquakes. Examples of the differences are shown in Fig. 1.219 for an earthquake and an explosion in India. Earthquakes
occur by slip across a fault, generating large amounts of shear
wave energy and hence large surface waves. By contrast, explosions involve motions away from the source, and so produce
far less shear wave energy. Hence, for bombs the surface waves
are dwarfed by the initial P wave. This difference is the basis for
discrimination between earthquakes and explosions. A plot of
M s vs m b (Fig. 1.2-20) separates earthquakes, which generate
Fig. 1.2-18 Yields of underground nuclear tests carried
out by the Soviet Union, determined through seismically
observed m b magnitudes. After the Threshold Test Ban
Treaty (TTBT), seismology verified that the Soviet
Union was in general compliance with the 150-kiloton
limit. Data courtesy of P. Richards (personal
communication).
YIELD (kiltons of TNT)
YEAR
150 kiloton TTBT Limit
10,000
1,000
100
10
1
1960
1965
1970
1975
1980
1985
1990
150-kiloton TTBT limit
Yield (kilotons of TNT)
10,000
1,000
100
10
1
1960
1965
1970
1975
1980
1985
1990
Year
more surface wave energy (M s ), from the explosions, which
generate more body (P) wave energy (m b ).
The challenge of seismic monitoring has increased in recent
years. Since 1996 the USA has abided by the Comprehensive
Test Ban Treaty (CTBT), which bans all nuclear testing, preventing the development of new nuclear weapons. Thus the
focus of US monitoring efforts has expanded to include smaller
countries around the world. 13 There is also the need to identify
possible smaller nuclear tests, including those by terrorists.
Hence seismic monitoring must identify explosions less than
1 kt, which have a magnitude of 4–4.5 (Eqn 2). This requires
locating and identifying more than 200,000 earthquakes and
additional mining explosions every year.
1.2 Seismology and society 27
0
60
120
180
240
Time (s)
May 11, 1998, Indian nuclear test
(magnitude 5.1)
April 4, 1995 Indian earthquake
(magnitude 4.8)
P
Surface wave
Fig. 1.2-19 Seismograms showing the differences
between an earthquake and an explosion. For shallow
earthquakes, in this case an m b 4.8 shock in India, the
P wave is much smaller than the surface waves. By
contrast, the initial P wave is the largest arrival for
explosions like this Indian nuclear test. Data recorded
at Nilore, Pakistan. (Courtesy of the Incorporated
Research Institutions for Seismology.)
m b = C + 0.75 log Y,
(2)
but the constant differs for Nevada (C = 3.95) and Kazakhstan
(C = 4.45). With these corrections, it appears that the Soviet
Union complied with the treaty.
Monitoring nuclear tests requires distinguishing them from
earthquakes. Examples of the differences are shown in Fig. 1.219 for an earthquake and an explosion in India. Earthquakes
occur by slip across a fault, generating large amounts of shear
wave energy and hence large surface waves. By contrast, explosions involve motions away from the source, and so produce
far less shear wave energy. Hence, for bombs the surface waves
are dwarfed by the initial P wave. This difference is the basis for
discrimination between earthquakes and explosions. A plot of
M s vs m b (Fig. 1.2-20) separates earthquakes, which generate
Fig. 1.2-18 Yields of underground nuclear tests carried
out by the Soviet Union, determined through seismically
observed m b magnitudes. After the Threshold Test Ban
Treaty (TTBT), seismology verified that the Soviet
Union was in general compliance with the 150-kiloton
limit. Data courtesy of P. Richards (personal
communication).
YIELD (kiltons of TNT)
YEAR
150 kiloton TTBT Limit
10,000
1,000
100
10
1
1960
1965
1970
1975
1980
1985
1990
150-kiloton TTBT limit
Yield (kilotons of TNT)
10,000
1,000
100
10
1
1960
1965
1970
1975
1980
1985
1990
Year