THE VERIFICATION OF A COMPREHENSIVE NUCLEAR TEST BAN

• Document Type: CREST
• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP84B00148R000300800006-0
• Release Decision: RIFPUB
• Original Classification: K
• Document Page Count: 9
• Document Creation Date: December 20, 2016
• Document Release Date: January 16, 2008
• Sequence Number: 6
• Publication Date: October 1, 1982
• Content Type: OPEN SOURCE

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Establishe;i 1845 ~ ~~ October 1982 Volume 247 Number 4
e Verifieatiom of a Comprehensive
uclear 'lest fan
Networks. of seismic instruments could monitor a total test ban
Even small' clandestine explosions could be
even ifextreme measures were taken to evade detection
by Lynn R;..-Sykes and Jack F. Evernden
Two treaties put into eeect over um -- ,..
past 20 years have set limits on the the verification of a treaty banning ex- for them is P.(for primary) .waves' The
testing of nuclear weapons.. The " plosions larger than about one kiloton slower body vibrations are shear waves,
L iraited Test Ban Treaty of 1963, which may have been arguable, but it no long- which are similar to the waves on a vi-
We address this question as seis- - brating string; they are called S (for
i
s.
has been signed. by more than 120 na- er
ohs,, prohibits nuclear, explosions in mologists who have been concerned for shear or secondary}waves. An under--:
the atmosphere, the oceans.and space, . many years with the detection of under- - ground explosion is a source of nearly ":
a'.iowing them only underground. The ground explosions by seismic methods pure P waves because it applies a uni-
Threshold Test Ban Treaty of 1976, a ' and with means of distinguishing under- form pressure to the walls of the cavity.
trilateral agreement between the U.S. ground explosions from earthquakes. it creates. An earthquake, on the other -
-
.nd the U.S.S.R., prohibits underground We are certain that the state of knowl- hand, is generated when two blocks- of
-tests of nuclear weapons with a yield edge of seismology and the techniques the earth's crust rapidly slide past each
,greater than, 150 kilotons. In the present for monitoring seismic waves are suffi- other along the plane of a fault. Because
climate of widespread pressure for more cient to ensure that a feasible seismic of this shearing motion an earthquake
e1'ective control of nuclear arms the network could soon detect a clandes- radiates predominantly S waves.
idea of a comprehensive ban on all nu- tine underground testing program in- A result of the spherical symmetry of
clear testing is receiving renewed atten- volving explosions as small as one kilo- the explosion source is that all the seis-
tion. Such an agreement would be an ton. In short, the.technical capabilities mic waves it generates have a nearly ra-
important measure. It might inhibit the needed to police a comprehensive test dial symmetry around the focus of the
development of new weapons by the ban down to explosions of very small explosion. In contrast, the highly direc-
major nuclear powers, and it might also size unquestionably exist; the issues to tional character of an earthquake source
help to prevent the spread of nuclear- be resolved are political. gives rise to seismic waves with strong
weapons technology to other countries. ly asymmetric patterns. The asymmetry
A halt to all testing was the original An underground explosion sets up in the amplitude of the waves received
goal of the negotiations that led to the !A elastic vibrations that propagate as at seismometers throughout the world
1963 Limited Test Ban. New talks with seismic waves through the earth and provides the means whereby seismolo-
the aim of achieving a total ban were along its surface. The waves travel great gists can determine the faulting mech-
begun in 1977 by the U.S., the U.S.S.R. distances, and seismic monitoring in- anism of a given earthquake.
and Britain, but the talks were suspend- struments in common use are sensitive In addition to the P and S body waves
ed in 1980. In both cases the main im- enough. to record even those generat- there are also two types of seismic waves
pediment to a comprehensive treaty was ed by very small explosions. Once the that propagate . only over the surface
the contention by the U.S. and Britain waves have been detected the main task of the earth. They are called Rayleigh
that compliance with the treaty could is to distinguish the seismic signals of waves and Love waves, and they result
not be verified because sufficiently small explosions from those of earthquakes. from complex reflections of part of the
underground nuclear explosions could. This can be done with a network of sev- body-wave energy in the upper layers of
not be reliably detected and identified.. eral widely separated seismometers. the earth's crust. A simple explosion can
In July the Reagan Administration an- Two types of elastic vibrations can 'generate Rayleigh waves but not Love
nounced that the test-ban negotiations propagate through the solid body of the waves, whereas an earthquake generates
with the U.S.S.R. and Britain will not be. earth, that is, through the crust and the waves of both types. .
resumed. Once again the primary reason mantle. The first waves to arrive at a Seismologists characterize the size of
al leis .. event by means of magnitudes.
given was a iacK of connuence m inch,- ~~,~?~??~?-? ~?- --
ods of verifying compliance. which are similar to sound waves in A: given event ca~- be assigned several
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 the logarithm of the amplitude of a par-
5o - ticular type of wave normalized for dis- t .
/-., NNIUVCU r-L) r\CICCIJC LUDO/U I/ IV . l~IF1-rCUrO'Fl7UU I'FOrCUUUJUUODUUUV-U
U.S. magnuuuca, GOa.,. -_ed on a differ- :..; ..
~o ent kind of seismic wave. A magnitude is
tance and depth of focus. VI the numer-
ao --1 i-- ous magnitudes that can be defined for
.- -hall di
sc ss
30r- omy -wvr wu.1. .., -- - -.
M., and m, ThP
latter on one-second P waves. The mag-
l is ultimatel
i
i
i
0
customary measure of energy release is
the yield in kilotons, where one kilo-
10
ton is the energy released by detonating
11. 1 It
y
gna
c s
sm
10
F-I nitude of a se
- related to the enemy released at the site
of the event. For a nuclear explosion the
20
1,000 tons of TNT.
Every year there are numerous earth-
quakes whose magnitudes are in the
range corresponding to the -yields of un-
derground explosions. Several methods
can be applied to several types of waves
to distinguish the seismic waves of ex-
plosions from these of earthquakes. The
location of a seismic event and its depth
below the surface are important criteria;
indeed, the great majority of routine-
ly detected events can be classified as
earthquakes simply because they are ei-
ther too deep or not at a plausible site
for an explosion. The remaining events
can be reliably classified by the amount
of energy radiated in the several kinds
of waves at various frequencies.
The location of an event in latitude
and longitude isa powerful tool for clas-
sification. The position is determined by
recording the arrival time of short-peri-
od P waves at several seismographic sta-
1_L
tions in various parts of the world. The
network monitoring a test-ban treat} '
travel time of the P waves to each sta-
tion is a function of distance and depth
of focus. From the arrival times it is
possible to determine the location of the
source with an absolute error of less
than 10 to 25 kilometers if the seismic.
data are of high quality.
The identification of seismic events a!
sea is quite simple. It is assumed that the
ple hydroacoustic stations around the
shores of the oceans and on a few crit-
ical islands to measure pressure waves
in seawater. The hydroacoustic signal of
tied out at a rate of about 50 per year, prin-
chart shows, the main effect of the Limited
Test Ban Treaty of 1963 (broken rertical lint)
was not to reduce the number of test expli -
sions but merely to drive most of them and r: -
ground. Nuclear test explosions in the atmo? -
phere and underwater are represented by co -
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from that of an earthquake and can be
detected at such long range that the
identification of a seismic event at sea as
an explosion or an earthquake is simple
and positive. Hence any event whose
calculated position is at least 2S kilome-
ters at sea (a margin allowing for errors)
can be classified as an earthquake on
the basis of its location and the charac-
ter of its hydroacoustic signal.
The accuracy with which the position
of a seismic event can be determined in
an area offshore of an-island arc has
been tested with an array of ocean-bot-
tom seismometers oil the Kamchatka
Peninsula and the KtriWIslands in the
U.S.S.R. The tests indicate that the ac-
curacy of a seismic network under these
circumstances is much better than 25
kilometers. Holding to that standard,
however, one finds that well over half
of the world's seismic events are defi-
nitely at sea and are therefore easily
identi?ed as earthquakes.
Another large group of detected
events have their epicenters on land but
in regions where no nuclear explosions
are to be expected; these events too can
be safely classified as earthquakes. In-
deed, almost all the world's seismic ac-
tivity is in regions that are of no concern
for monitoring compliance with a com-
prehensive test ban. Thus the simple -
act of locating seismic events classifies
most of them as earthquakes.
f'alculating the depth of focus pro-
vides a means of identifying a large
fraction of the remaining earthquakes.
From 55 to 60 percent of the world's
earthquakes are at depths of more than
30 kilometers; at least 90 percent are
more than 10 kilometers deep. Any seis-
mic event as deep as 15 kilometers is
certainly an earthquake. No one has yet
drilled into the earth's crust as far down
as 10 kilometers, and the deepest nucle-
ar explosions have been at a depth of
about two kilometers.
Several seismological procedures can
be employed to determine an event's
depth of focus. In most cases the depth is
calculated at the same time as the loca-
tion. When a seismic event is detected at
20 stations or more, a routinely calculat-
ed depth of 30 kilometers or more en-
sures with a 95 percent degree of confi-
dence that the event was at least 15 kilo-
meters below the surface.
A powerful technique for estimating
depth can be applied if at least one seis-
mological station is within a few hun-
dred kilometers of the detected event.
(A monitoring network for a compre-
hensive test ban would be quite likely to
meet this condition in areas where nu-
clear testing might be expected.) A pair
of P and S waves generated at the same
instant and recorded by a station near
the event follow identical paths but
propagate at different speeds. The dif-
ference in their times of arrival, or in
FOUR TYPES OF SEISMIC WAVE are illustrated. The two waves at the top propagate
through the solid body of the earth; the two at the bottom propagate only near the surface. The
compressional body waves called P (for primary) waves travel fastest and are the first ones to
arrive at a seismometer; they are the predominant type of body wave produced by an under-
ground explosion. The slower body waves called S (for shear or secondary) waves vibrate in a
plane transverse to their direction of Propagation; they are the predominant type of body wave
produced by an earthquake. The surface waves called Rayleigh waves and Love waves result
from complex reflections of the P and S body waves in the upper layers of the earth's crust.
other words the difference in their phas-
es, therefore serves to determine the
time of origin of the event. With experi-
ence the seismograms of a station near
the event can be successfully analyzed
to detect at least one pair of such P and S
phases. Given the time of origin deter-
mined in this way and the arrival times
of the P waves at only a few distant
receivers, an accurate estimate of the
depth of focus can be made.
There may remain critical seismic re-
gions where nearby stations do not exist.
.Data from large events can then be em-
ployed to refine the calculated depth
and location of smaller events. The es-
sence of the technique is to correct the
observed times of small events by noting
the differences between the observed
and the calculated times for a large
event in the same area. The procedure is
in routine use by several networks.
The combined effectiveness of loca-
tion and depth in distinguishing earth-
quakes from explosions is impressive.
More than 90 percent of all earthquakes
either are under oceans or are at least 30
kilometers deep (or both). Most of the
remaining earthquakes are of little inter-
est because they are in countries that are
unlikely to be testing nuclear weapons
or in countries where clandestine testing
would be impossible. For the U.S., of
course, the U.S.S.R. is the country of
prime interest. About 75 percent of the
earthquakes in and near the U.S.S.R. are
in the eastern part of the country near
the Kamchatka Peninsula and the Ku-
rile Islands. Almost all of the shocks in
these areas either have a focal depth
greater than 50 kilometers or are well
offshore. It turns out that seismic events
whose calculated position is on land in
.the U.S.S.R. or less than 25 kilometers at
sea and whose calculated depth is less
than 50 kilometers constitute only about
.5 percent of the world's earthquakes.
This amounts to about lot) earthquakes
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 Jt
PATHS OF SEISMIC WAVES are traced on a cross section of the earth. Body waves from an
earthquake or an explosion travel through the crust and mantle along the curved paths labeled
P: S, pP and pS. A pP wave is a compressional wave that is produced by the reflection of a P
wave from the surface of the earth just above an earthquake or an explosion; a pS wave is a
shear wave that results from the conversion of part of the compressional energy of an upward
P wave into transverse energy as the P wave is reflected from the surface. Surface waves such
as Rayleigh waves and Love waves diminish rapidly in amplitude with increasing depth. The
hypocenter is the focal point of an earthquake or an underground explosion from which the
waves radiate. The epicenter is the point on the earth's surface directly over the hypocenter.
per year with an mb magnitude greater
than 3.3 for which other seismic dis-
criminants must be employed.
None of, the measures we have dis-
cussed so far relies on the detailed char-
acteristics of the waves radiated by
earthquakes and explosions. Several
powerful discriminants are based on
those characteristics, in particular on
the relative amounts of energy in waves
of different types and periods. For ex-
ample, a shallow earthquake generates
20-second Rayleigh waves with ampli-
tudes at least several times greater than
those of an explosion that releases the
same amount of energy. In the notation-
al practice of seismology the compari-
son of the two magnitudes is referred to
as the Ms: mb ratio, that is, the ratio
of long-period to short-period waves.
second spectral discriminant is
~ based on the observation that long-
period P and S waves are rarely or nev-
er seen in association with explosions
but one type or the other is routinely
detected today by simple seismometers
for most earthquakes that have a one-
second P-wave magnitude of at least
4.5. More sophisticated seismic stations
and more sophisticated analysis of the
signals could lower the magnitude at
which such waves can be detected.
A third distinction is that surface
waves of the Love type are generated far
more strongly by shallow earthquakes
than. they are by underground explo-
sions, including even abnormal explo-
sions. Still another characteristic feature
of the seismic signal from explosions is
that the first motion of the earth stimu-
lated by P waves is always upward be-
cause the explosion itself is directed
outward; the first P-wave motion in
an earthquake can be either upward or
downward.
An important factor contributing to
the separation of earthquakes from ex-
plosions on an MS : Mb diagram is that P
waves from the two kinds of events have
different radiation patterns. Explosions
radiate short-period P waves equally in
all directions, whereas earthquakes have
very asymmetric patterns. Hence most
earthquake sources show a decrease of
from .4 to one magnitude unit from the
peak values when the P-wave ampli-
tudes are averaged over pertinent radia-
tion angles. A simple explosion does not
initially radiate any shear wr.ves; earth-
quakes typically generate large shear
waves. As a result Rayleigh waves gen-
erated by many types of earthquakes
have a larger amplitude than the corre-
sponding waves generated by under-
ground explosions of the same Mb.
There is a characteristic time for the
formation of the source of a seismic
event; the time is equal to the maxi-
mum source dimension divided
by the velocity of source formation. The
source dimension for earthquakes is
the length of the break where most of
the short-period energy is released; it is
from three to 20 times greater, depend-
ing on the state of stress in the rocks,
than the radius of the cavity and shatter
zone of a comparable explosion. The
velocity of source formation for earth-
quakes is from somewhat less to much
less than the velocity of shear waves in
the rocks surrounding the fault, where-
as the relevant velocity for explosions
is the velocity of shock waves in the
rock, which is essentially the velocity
of compressional waves. As a result
vi rvv v ......... .11 U1C SIZC 01 LI1C
source and the velocity of source forma-
tion the characteristic times for earth-
quakes and explosions differ by a factor
of from six to 40. It is therefore not sur-
prising that differences are observed be-
tween the short-period P-wave spectra
of earthquakes and explosions.
Observations of several U.S. explo-
sions have demonstrated the existence
of a phenomenon called overshoot. It is
related to shock waves in strong rock,
but it can be thought of as the equivalent
of cavity pressure rising to high values
followed by a decrease in pressure by a
factor of four or five; the lower pres-
sure is then maintained for many tens
of seconds Overshoot, when it occurs,
provides additional P-wave spectral dish
crimination and augments discrimina-
tion by means of the Mg: Mb ratio for
larger events
jt was once thought that an explosion
1 could not give rise to any Love waves
at all. A phenomenon that was of great
significance in thwarting President Ken-
nedy's effort to achieve a comprehen-
sive test-ban treaty in 1963 was the ob-
servation that many underground nucle-
ar explosions at the U.S. testing site in
Nevada, particularly those in hard rock,
generated unmistakable Love waves.
The failure of the qualitative criterion
"No Love waves from explosions" (at
a time when such quantitative criteria
as the comparison of the magnitudes
of long-period and short-period waves
were not adequately established) left
seismologists unable to guarantee their
ability to distinguish the seismic waves
of underground explosions from those
of earthquakes
The presence of Love waves in the
Nevada tests has since been explained.
What was not considered in the earlier
analyses was the influence of the natural
stressed state of the earth on the waves
generated by an explosion. The creation
of a cavity and its surrounding shatter
cone by an underground explosion leads
to the release of some of the natural
stress, which in turn generates seismic
waves equivalent to those of a small
earthquake, including Love waves. The
observed waves are a superposition of
the waves from the explosion and from
the release of the stress.
The release of natural stress also al-
ters the amplitude of Rayleigh waves.
The perturbation has never been large
enough, however, to put in doubt the
nature of an event identified by the ratio
of long-period to short-period waves.
Only rarely does the perturbation signif-
icantly affect the amplitude of P waves;
it is not known ever to have changed the
direction of their first motion. More-
over, if the magnitude Ms is determined
from Love waves rather than Rayleigh
waves, the ratio method (MS: Mb) pro-
vides an excellent discriminant.
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=In short, if seismologists had done the records of other available stations
their homework thoroughly by 1963, were examined, the events ceased to
the nations of the world might well have be "anomalous."
achieved a comprehensive test-ban trea- For the remaining problem events
.ty then. Today the release of-natural Ms : Mb measurements based on 20-sec-
stresses in the earth is significant only as and Rayleigh waves gave values in the,
h .perturbing factor that must be taken range characteristic of explosions. Sev-
into account when the yield of an explo- eral of these events were at depths of
sion is estimated from. Rayleigh waves.. from 25 to 50 kilometers, where the pos-
Reports that earthquakes occasional- sibility of nuclear testing can be exclud-
ly have Ms?: mb values like those of ex- . ed in any case, but the magnitude ratio
plosions have been cited as a factor that nonetheless demanded explanation. It is
might impede. the effective monitoring known from seismological theory that
of a comprehensive test ban. In.analyz certain types. of earthquakes at these
in-'a large set of earthquakes mall parts depths excite long-period Rayleigh
of. the world: and of: underground: explo- waves.,noorly. The theory predicts, how-
world.
in the: U.'S." and the U.S.S.R. we"'-ever,, that Love ? waves and vibrations
'fund only one example of.this kind of.-..: called: higher-mode Rayleigh waves are
ambiguity..-The, focus of the event was.:in many instances vigorously generated
far from the a'rea.in which the seismom in these circumstances. An analysis of
titer network 'gave its best results. recordings of the Love waves and the
in 1972, at a meeting of the UN. Com-: 'higher-mode Rayleigh waves identified
mittee on Disarmament, the U.S. sub- . several more of the problem events as
mitted a list of 25. "anomalous" events.. earthquakes.
that were said to be indicative of a prob Only-:a-single sequence of events at
lem in discrimination. In 1976 the. 25 one place in Tibet remained as a prob-
events were reanalyzed by: one of us 1em. In . that region underground nuclear
-(Sykes) and two other seismologists, testing is unlikely, but the nature of the
'Robert Tathum and Donald Forsythe. It events could not be determined with cer-
was established that about half of the tainty from the magnitude ratios. We
events had MS: Mb values that put them think the reason is that with the seismo-
Aearly in, the earthquake population. graphic networks.of the 1960's, when
I:;ost of the original magnitudes had the events were recorded, Love waves
been determined from only one or, two could not be detected for small-magni-
stations, and much existing informa- tude events because they were obscured
tion had not even been consulted. When. by background earth noise. New instal-
lations and new modes of data process-
ing have greatly reduced the problem. If
series were to take place today, we think
they would be identified unambiguous-
ly. Long-period seismographs in bore-
holes and routine digital processing of
seismograms lead to a suppression of
background noise and increase the de-
tectability of, many types of waves, in-
As it happened, the nature of the Tt
betan problem sequence was resolved in
spite of the inadequacies of the long-pe-
riod data of the time. At several stations
the first motion. of the P waves was.
downward, which is not possible for an
explosion. Hence the events must have
been small earthquakes.::..:::,
It seems reasonable to say that for the
..networks we shall describe below there
should no longer be any problem events
asian earthquake with a one-second P-
20 years whose waves are classified as
those of an explosion- (Of course, nu-
merous smaller Eurasian earthquakes
during that period went unidentified be-
.cause of inadequate data.) Furthermore,
to our knowledge not one out of sever-
al hundred underground nuclear explo-
sions set off in the same period-radiated
seismic waves that could be mistaken
for those of an earthquake. Our experi-
RADIATION PATTERNS of the P waves resulting from an under- earthquake fault In the comparatively simple case of a vertical strike-
ground explosion (left) and an earthquake (right) are compared. The . slip fault, shown here, the four-lobed radiation pattern observed at
first motion of the P waves from an explosion is uniformly outward. ' the surface for both P waves and Rayleigh waves is a simple projec-
and hence is generally observed as an upward displacement at all seis- tion of the three-dimensional P-wave configuration emanating from
mic stations. The first motion of the P waves from an earthquake is. the hypocenter of the earthquake. The radiation pattern of the Love
outward in some directions and inward in others; the pattern of the waves emitted by the same source is rotated by 45 degrees with re-
waves at the surface depends on the orientation of the plane of the ' spect to the surface pattern of the P waves and the Rayleigh waves.
51
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 bility that an event will remain unidenti-
fied when all the available techniques of
discrimination are brought to bear.
No monitoring technology can offer
an absolute assurance that even the
smallest illicit explosion would be de-
tected. We presume that an ability to
detect and identify events whose seismic
magnitude is equivalent to an explosive
yield of about one kiloton would be ad-
equate. It is often assumed that for the
U.S. to subscribe to a comprehensive
test ban it would require 90 percent con-
fidence of detecting anyviolation by an-
other party to the treaty:.. Developing a
new nuclear weapon.. however, general-
ly requires a series of tests and the prob-..
ability that at least one explosion will be
detected rises sharply as the number of
the tests is increased. Moreover, a 90
percent level of confidence for the detec-
tion of even a single explosion probably
is not needed. For a country seeking to
evade the treaty the expected probabili-
ty of detection would certainly have to
be less than 30 percent. and perhaps
much less, even if only one illicit test
were planned. The test-ban agreements
that have been considered over the years
all include an "escape clause" through
which a.country could renounce its trea-
ty obligations. Unless the probability
of detection were very low, a country
whose national interest seemed to de-
mand a resumption of testing would pre-
sumably invoke the escape clause rath-
er than risk being caught cheating.
Given these standards of reliability
for a monitoring system, it is possible to
specify the size and the sensitivity of the
ARCTIC EARTHQUAKE
SEPTEMBER 8, 1972 mp=5.9
TUP II
DOWN
seismic network that would be needed to was not set off during or soon after a
verify compliance with a comprehen- large earthquake. If one must consider
sive test ban. Two kinds of network can the possibility that a country would try
be considered for maintaining seismic to evade a test-ban treaty by decoupling,
surveillance. of the U.S.S.R. One net- or muffling, an explosion and thereby
work consists of 15 stations outside the reducing the amplitude of the emitted
borders of the U.S.S.R. In the second seismic signals, an improved network
network the 15 external stations are sup- would be required. In principle such
plemented by 15 internal ones. muffling could be done by detonating
The ultimate limit on the detection of the explosion in a large cavity or by
seismic signals is imposed by micro- using energy-absorbing material in a
seisms, or random vibrations of the smaller cavity. The former stratagem
earth's surface. Most microseisms are might reduce the seismic signal of an
induced by the earth's atmosphere and explosion by 1.9 magnitude units as
oceans. In order to detect a one-kiloton measured by one-second P waves (that
explosion. in much of the U.S.S.R. a,-' is, by mb). The latter stratagem might
monitoring network would have to be bring a reduction of one unit.
able to recognize above the background The use of an oversize cavity is clearly
noise any event with a short-period P- the more worrisome possibility, but it
wave magnitude of- 3.8 or more. In or-
der to distinguish an explosion from an
earthquake by comparing the long-peri-
od magnitude with the short-period one,
the network would also have to be able
to detect surface waves with an Ms mag-
nitude.of 2.5 or more. The network of 15
external ..stations could achieve these
goals. Indeed, since almost all the seis-
mic areas of the U.S.S.R. are along its
borders, the external network would be
sensitive to events of even smaller mag-
nitude there. The mere detection of a
seismic event in most areas of the interi-
or would constitute identification of the
event as an explosion.
The lower limit of one kiloton on the
yield of an explosion that could be de-
tected by an external network is based
on the assumption that the coupling be-
tween the explosion and the seismic ra-
diation is efficient and that the explosion
could be attempted only in certain geo-
logic formations: a salt dome or a thick
sequence of bedded salt deposits. Few
areas of the U.S.S.R. have deposits of
salt in which the construction of a cavity
large enough for decoupling a several-
kiloton explosion would be possible.
The maximum size of a cavity that
could reasonably be constructed and
maintained sets a limit of two kilotons
on explosions that might be muffled in
this way and escape detection by the
15-station external network.
Another way to reduce the amplitude
of radiated seismic waves is by detonat-
ing an explosion in a low-coupling medi-
um such as dry alluvium. The maximum
thickness of dry alluvium in the U.S.S.R.
sets a limit of 10 kilotons on explosions
that might be concealed by this. means,
again, assuming that only the 15 exter-
nal stations were installed.
NUCLEAR EXPLOSION. U.S.S.R.
AUGUST 28, 1972 mp=6.3
TuP
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1 MINUTE
- l 1-
SEISMMOGRAMIS OF LONG-PERIOD WAVES from an earth-
quake in the Arctic near the US.S.R. (top) and an underground nu-
clear explosion in the U.S.S.R. (bottom) were recorded at a seismic
station in Elath, Israel, roughly equidistant from the two events. The
short-period body waves generated by the two shocks were observed
to have almost the same magnitude. The magnitude of the long-peri-
od Rayleigh waves recorded in these traces, in contrast, is clearly
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H~ ~1'il ,
much greater for the earthquake than it is for the explosion. The ra-
tio of long-period surface waves to short-period body wares has been
shown to be a reliable criterion for distinguishing the seismic waves
of earthquakes from those of explosions. In addition the P wave of
the explosion has more high-frequency energy than the P wave of
the earthquake. The S wave of the earthquake is large, whereas that
of the explosion is small and not easily identified in the seismogram.
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'Another possible drawback of an ex
clusively external network should- be
mentioned. Confusion could arise when
signals from.. two or more earthquakes
reached a station simultaneously. The
effect would be most troublesome when
the long period waves from a small
event in the U.S.S.R. arrived. at the same
time as similar waves from a much larg-
er earthquake elsewhere in the world.-
Under these circumstances it might be
difficult to establish with certainty by
comparing his with mb the identity of :. g ,6
of 15 external stations.-there would be a
few events per year inwhich the smaller- >
U.S.S.R. or within 2Skilometers of its Q
borders and at a depth of less than 50 u-5
kilometers... D
A monitoring network made tip of 15. o
L3 seismographic stations outside.the . w
U.S.S.R. and 15 inside it would large o
ly eliminate the problem of coincident LL:. 4
earthquake signals and would greatly 0
reduce the maximum yield of an explo- n
sion that might escape, detection, even if L_
decoupling were attempted. The inter-
nal monitoring stations would besimple? <
unattended ones, with the capability of . 3
measuring vertical ground motion and
.two orthogonal components of horizon-
tal motion, so that the distance and di-
rection of a nearby event could be esti
mated from the data of a single short-
range station. With such a network in
EXPLOSIONS
place, and assuming that muffling was 5.5 6 6.5
. attempted in the presence of normal MAGNITUDE OF ONE-SECOND BODY WAVES (mb)
earth noise, the largest explosion that CLEAR DISTINCTION between earthquakes and explosions is evident in this plot of the
would have a 30 percent chance of es- magnitude of long-period surface waves (Ms) against that of short-period body, waves (mb)-
caping detection in any setting except a The 383 earthquakes represented by the black dots were compiled from a set of all the earth-
salt dome would be .5 kiloton. quakes recorded worldwide in a six-month period that had an mb value of 4.5 or more and a
For salt domes the main area of con- focal depth of less than 30 kilometers. (There are fewer dots than earthquakes because the
cin the U.S.S.R. is the region north magnitudes of some of them coincided.) The colored squares designate underground explosions
cern e the Caspian Sea. Our hypothetical in the U.S. and the colored crosses underground explosions in the U S S.R Only one earthquake
falls within the explosion population, as defined by the straight line separating the two groups
network has three stations there. Even of events. This single event, which had the smallest magnitude of any of the earthquakes in the
a small explosion in a large salt-dome survey, took place in the southwest Pacific Ocean, a region where the sensitivity of the net-.
cavity would emit certain P and S waves work of seismic stations is poorer than it is in most of the Northern Hemisphere. The mb values
with an amplitude large enough to be were adjusted to take into account regional variations in the amplitudes of short-period waves.
detected by nearby stations. Further-
.
more, detection by even one of the sta-
tions would immediately identify the
event as an explosion because the area ' DEPTH 3 50 KM.
has no natural seismic activity. As a re- LOCATION > 25 KM. AT SEA
DEPTH . 30 KM.
sult evasion would not be likely to be Ms:mb
attempted at a yield greater than one
kiloton even in the salt-dome area. DEPTH 50 KM.
A possible strategy for evasion that Ms:mb
has been mentioned from time to time is LOCATION > 25 KM. AT SEA
the one of hiding the seismic signal of a DEPTH D, 30 KM.
nuclear explosion in the signal of a large Ms:mb
earthquake, which might be near the site LOCATION > 25 KM. AT SEA
of the explosion or far from it. For the
U.S.S.R. the only credible possibility is
a distant earthquake because the only
possible testing sites where earthquakes METHODS OF DISTINGUISHING earthquakes from explosions were tested by applying
are frequent enough to make the effort the methods to all the earthquakes with a magnitude of 4.5 or more recorded during a 162-day
worth while are on the Kamchatka Pe- period in 1972. The sample consisted of 948 events. Many of them could be classified as earth-
quakes (rather than explosions) by their location or their depth. The remaining events could be
ninsula and in the Kurile Islands. Clan- classified by comparing the magnitude of long-period surface waves with the magnitude of
destine testing there is not likely because short-period body waves (the ratio SIS: mb). The sequence in which the tests were applied of-
seismic activity in the area can be moni- . fected the efficiency of the procedure, but all events were identified regardless of the sequence.
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*Tbled, in detail from stations in Japan ones the only effective evasion schemes
and tk Aleutian Islands. Indeed, ocean- at yields of one kiloton or more would
Bottom,seismometers and hydroacous- require both decoupling and hiding the
tic sensors could be placed just offshore. explosion signal in an earthquake.
The first defense against evasion by
the masking of a test in a large earth- he. issues relating to the monitor-
uake is the questionable feasibility of Ting of a comprehensive test ban can
q
the subterfuge. Unless the evader main-
tained several testing sites the number of
opportunities per year for clandestine
testing would be quite Limited. In addi-
tion the evader would have to maintain
his weapons in constant readiness for
firing. To attain the evasion capability
comprehensive test ban would actually
establish the equivalent of a very low
threshold, since weapons of extremely
low yield could be tested underground
"without the certainty of being detected
` and identified. A treaty that imposed a
be summarized as follows. The under- threshold near the limit of seismological
standing of seismology and the testing...: monitoring capability might therefore
of seismometer networks are sufficient- be considered an alternative to a com-
ly complete to ensure that compliance prehensive test ban. Such a treaty. might
with a treaty could be verified with a. be preferable to the present quite high
high level of confidence. The only explo- threshold, but it would have the dis-
sions with a significant likelihood of es- advantage that arguments could arise
given below he would''have: to set off an caping detection would be those of very over the exact yield of tests made near
explosion within 100 seconds of the time small yield: less than one kiloton provid- the threshold. Indeed, the judgment of
of arrival; of the short-pertod waves of.:,-;:;.ed. the monitoring system includes sta-
mate the maximum am,plitude.and the .:`._.It is important to=view the question of
decay rate of the earthquake, waves with.. yield in the context of the nuclear weap
high accuracy, and he would have to be 'ons that have been tested up to now.
certain of the amplitude of the. P waves Theories that. ushered in the nuclear
always be less. equivocal than an exact..,
edly violated the 1976 treaty by testing.
generated by'the explosion to within .1 age in 1945 had a yield of from 15 to devices with a yield greater than the
magnitude unit. Even after taking these' 20 kilotons.. Yields increased rapidly to 150-kiloton limit Such reputed viola-
precautions the evader would have to the -point where the -U.S.S.R. tested.-. tions were recently cited as evidence
accept a high probability that the.event a 53,000-kiloton. weapon.in 1961. The that the threshold treaty, which has not
would be detected by at. least one men- largest 'underground explosion. had a been ratified by. the U.S. Senate, is not
itoring station and a small probability yield of almost 5,000 kilotons, Unclassi- verifiable and should be renegotiated.
that three stations would detect it. He fied reports place the yield of the weap- On the basis of our analysis we conclude
would also have to install and operate ons carried by intercontinental missiles that the reports are erroneous; they are
his testing site (including a large cavity) in the range from 40 to 9,000 kilotons. based on a miscalibration of one of the
and his own seismological network in,- The yields of underground explosions curves that relates measured seismic
total secrecy over a period of years.
. In contrast to these daunting require
tnents for successful evasion, the only
requirements for a monitoring nation
that might go undetected or unidentified magnitude to explosive yield. When the
under a comprehensive test ban are. correct calibration is employed, it.is ap
therefore . much smaller than those of parent that none of the Russian weapons
the first nuclear weapons. If .the thresh-. tests exceed 150 kilotons, although sev-
are to operate a network of high-quality old of 'reliable detection and identi-
seismic stations and to process the data . fication is one kiloton, that is only
-,vith.determination. Against a network one-150th of the limit specified by the
of 15 external stations and 15 internal Threshold Test Ban Treaty of 1976.
7
THRESHOLDS OF DETECTION for seismic events in the Eastern
Hemisphere are delineated !,y the two sets of contours drawn on this
pair of maps for a proposed network of 15 seismic stations established
outside the U.S.S.R. Colored dots give the location of 12 of the 15 sta-
tions; three others are off the maps. The number on each contour in-
dicates that an event of that magnitude'-or larger has at least a 90
percent probability of being detected by four or more stations. The
contours on the map at the left represent the detection thresholds for
short-period body waves and those on the map at the right the detec-
tion thresholds for long-period surface waves. On these maps and the
ones on the opposite page the only seismic noise taken into account
is the microseismic noise, generated by the atmosphere and oceans.
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eral come close to it.
- ? Observations at the Nevada Test Site
(NTS), where American nuclear-weap-
ons tests are held, indicate there are lin-
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IMPROVED DETECTION THRESHOLDS for seismic events in
the Eastern Hemisphere are delineated by the two sets of contours
drawn on this pair of maps for a proposed network of 30 seismic sta-
tions: 15 outside the U.S.S.R. and 15 inside. For most of the U.S.S.R.
the effect of adding the 15 interior stations would be to lower the de-
ear correlations between the logarithm
of the explosive. yield and the two mag-
nitude values, MS and mb, for explo-
sions with yields greater than 100 kilo-'
tons. When the measured Ms and Mb
values of explosions at the Russian test.
site near Semipalatinsk are inserted into
the NTS formulas, however, the result-
ing estimates of yield given by Mb are
more than four times as great as those
given by Ms. For explosions in hard
rock at many test sites estimates of yield
based on the NTS Ms formula have
invariably agreed with actual yields,
whereas estimates based on the NTS Mb
formula have sometimes been in drastic
disagreement with the actual yield.
A strong correlation has been found
between Mb values measured at individ-
ual stations and P-wave travel times to
these stations. The U.S.S.R. routinely
publishes seismological bulletins that
include P-wave arrival times of earth-
quakes, and it is straightforward to in-
terpret the times for stations in central
Asia in terms of the expected pattern of
Mb values near. Semipalatinsk. From an
analysis of the P-wave signals it is pre-
dicted that the mb value for an explosion
at Semipalatinsk is 40 percent greater
than an equivalent explosion at NTS.
This is the same correction that must be
applied to the curve relating Mb to yield
at NTS to make the Mb estimates of the
yield of Russian explosions consistent
with the NIS estimates. Thus two modes
of analysis lead to the conclusion that
there is an essentially universal relation
between hIs and yield whereas the curve
relating Mb to yield must be calibrated
for each test site.
tection threshold for short-period body waves from a magnitude of
3.8 to one of 3.4 (left); the corresponding effect for long-period sur-
face waves would be to lower the detection threshold from a. magni-
tude of 2.6 to one of 2.3 (right). The interior stations would also pro.
vide more accurate information on the focal depth of a seismic event.
A comprehensive treaty would have
an additional advantage over a low-
threshold treaty: all technological un-
certainties -would. work against the po-
tential evader. A country planning a sur-
reptitious nuclear test could not know
the exact seismic-detection capability of
other nations or the exact magnitude of
the seismic waves that would be gener-
ated by his test. A ban on nuclear explo-
sions of all sizes would also have the
important conceptual value that nucle-
ar weapons, no matter what their size,
would be recognized as inherently dif-
ferent from conventional weapons.
It is sobering to consider how the
state of the world would differ if a
full test ban had been achieved in 1963.
The number of nuclear weapons has
grown tremendously since then and is
now estimated at from 50,000 to 100,-
000. The loss of life and the social dam-
age that would be inflicted in a ma-
jor nuclear exchange are vastly greater
than they were in 1963. Furthermore,
both the U.S. and the U.S.S.R. are less
secure now than ever before, not be-
cause of any failure to develop arms but
because of the growing stockpiles of
weapons and the inability of any nation
to defend itself against nuclear attack.
A comprehensive test-ban agreement.
should not be regarded as a substitute
for disarmament. Meaningful reduc-
tions in the nuclear threat must include a
continuing and serious process of arms
control; in this process, however, a com-
prehensive test-ban treaty could have an
important part. The problems of negoti-
ating such a treaty are overwhelmingly
Before the suspension of. negotiations
between the U.S., Britain and the
U.S.S.R. in 1980 tentative agreement
had been reached on a number of issues.
All three nations agreed that a test-ban
treaty would include a prohibition of
all'tests of nuclear weapons' in all en-
vironments, a moratorium on peaceful
nuclear explosions until arrangements
for undertaking them could be worked
out, provisions for on-site inspections,
a mechanism for the international ex-
change of seismic data and the installa-
tion of tamperproof seismic stations by
each country in the territory of the oth-
ers. The proposed treaty would have
a term of three years. The agreements
on the long standing issues of on-site
inspection, peaceful explosions and
the placement of monitoring stations
in each country represented important
breakthroughs. It would be a setback for
the cause of international security if this
hard-won ground were now lost.
For many years the stated policy of
the U.S. has emphasized the desirability
of a complete test ban if.verification
could be ensured. The policy was not
fundamentally altered by the recent de-
cision of the Reagan Administration to
put off further negotiations on the test
ban. On the contrary, it. was reported
that the Administration still supports
the ultimate goal of a comprehensive
ban on nuclear testing but has doubts
about the efficacy and reliability of seis-
mic`methods of verification. As we have
attempted to show here, there can be no
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