A: On January 19, 1968, a thermonuclear
test, codenamed Faultless, took place in the Central Nevada
Supplemental Test Area. The codename turned out to be a
poor choice of words because a fresh fault rupture some
1200 meters long was produced. Seismographic records showed
that the seismic waves produced by the fault movement were
much less energetic than those produced directly by the
nuclear explosion.
Analysis of local seismic recordings
(within a couple of miles) of nuclear tests at the Nevada
Test Site shows that some tectonic stress is released simultaneously
with the explosion. Analysis of the seismic wavefield generated
by the blast shows the source can be characterized as 70-80
percent dilational (explosive-like) and 20-30 percent deviatoric
(earthquake-like). The rock in the vicinity of the thermonuclear
device is shattered by the passage of the explosions shock
wave. This releases the elastic strain energy that was stored
in the rock and adds an earthquake-like component to the
seismic wavefield. The possibility of large Nevada Test
Site nuclear explosions triggering damaging earthquakes
in California was publicly raised in 1969. As a test of
this possibility, rate of earthquake occurrence in northern
California (magnitude 3.5 and larger) and the known times
of the six largest thermonuclear tests (1965-1969) were
plotted and it was obvious that no peaks in the seismicity
occur at the times of the explosions. This is in agreement
with theoretical calculations that transient strain from
underground thermonuclear explosions is not sufficiently
large to trigger fault rupture at distances beyond a few
tens of kilometers from the shot point.
The Indian and Pakastani test sites
are approximately 1000 km from the recent Afghanistan earthquake
epicenter. The question that has been asked is whether or
not the occurrence of these nuclear tests influenced the
occurrence of the large earthquake in Afghanistan. The most
direct cause-effect relationship is that the passage of
the seismic waves, generated by the thermonuclear explosion,
through the epicentral region in Afghanistan somehow triggered
the earthquake. For example, following the occurrence of
the magnitude 7.3 Landers earthquake in southern California
on June 28, 1992, the rate of seismicity in several seismically
active regions in the western US, as far as 1250 km from
the epicenter, abruptly increased coincident with the passage
of the earthquake generated seismic wavefield through each
site. The abrupt increases in seismicity occurred primarily
in regions of geothermal activity and recent volcanism.
The mechanism by which this occurred remains unknown. The
Afghanistan earthquake occurred at 06:22:28 UT on May 30,
1998 and the thermonuclear test most closely associated
in time occurred at 06:55 UT or after the occurrence of
the earthquake. The other nuclear tests occurred 2-20 days
before the earthquake.
The elastic strains induced in the
epicentral region by the passage of the seismic wavefield
generated by the largest of the nuclear tests, the May 11
Indian test with an estimated yield of 40 kilotons, is about
100 times smaller than the strains induced by the Earth's
semi-diurnal (12 hour) tides that are produced by the gravitational
fields of the Moon and the Sun. If small nuclear tests could
trigger an earthquake at a distance of 1000 km, equivalent-sized
earthquakes, which occur globally at a rate of several per
day, would also be expected to trigger earthquakes. No such
triggering has been observed. Thus there is no evidence
of a causal connection between the nuclear testing and the
large earthquake in Afghanistan and it is pure coincidence
that they occurred near in time and location.
One last point. The largest underground
thermonuclear tests conducted by the US were detonated in
Amchitka at the western end of the Aleutian Islands and
the largest of these was the 5 megaton codename Cannikin
test which occurred on November 6, 1971. Cannikin had a
body wave magnitude of 6.9 and it did not trigger any earthquakes
in the seismically active Aleutian Islands. Suggested reading:
"Nuclear Explosions and Earthquake, the Parted Veil", by
Bruce A. Bolt, W. H. Freeman and Co., San Francisco, 1976.
(UC Berkeley)
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