Seismic Verification of Nuclear Testing

Treaties

May 1988

NTIS order #PB88-214853
Recommended Citation:

U.S. Congress, Office of Technology Assessment, eism”c Verification of Nuclear

Testing Treaties, OTA-ISC-361 (Washington, DC: U.S. Government Printing Office,

May 1988).

Library of Congress Catalog Card Number 88-600523

For sale by the Superintendent of Documents

U.S. Government Printing Office, Washington, DC 20402-9325

(order form can be found in the back of this report)
Foreword

Since the advent of the atomic bomb there has been interest from both an

arms control and environmental perspective to restrict the testing of nuclear weapons. Although the debate over nuclear testing has many facets, verification is a

central issue to the consideration of any treaty. At the requests of the Senate

Select Committee on Intelligence, the House Committee on Foreign Affairs, and

the House Permanent Select Committee on Intelligence, OTA undertook an assessment of seismic capabilities to monitor underground nuclear explosions.

Like an earthquake, the force of an underground nuclear explosion creates

seismic waves that travel through the Earth. A satisfactory seismic network to

monitor such tests must be able to both detect and identify seismic signals in

the presence of “noise,” for example, from natural earthquakes. In the case of

monitoring a treaty that limits testing below a certain size explosion, the seismic

network must also be able to estimate the size with acceptable accuracy. All of

this must be done with an assured capability to defeat adequately any credible

attempt to evade or spoof the monitoring network.

This report addresses the issues of detection, identification, yield estimation,

and evasion to arrive at answers to the two critical questions:

● Down to what size explosion can underground testing be seismically monitored with high confidence?

● How accurately can the yields of underground explosions be measured?

In doing so, we assessed the contribution that could be made if seismic stations

were located in the country whose tests are to be monitored, and other cooperative provisions that a treaty might include. A context chapter (chapter 2) has been

included to illustrate how the technical answers to these questions contribute to

the political debate over:

● Down to what yield can we verify Soviet compliance with a test ban treaty?

● Is the 1976 Threshold Test Ban Treaty verifiable?

● Has the Soviet Union complied with p-resent testing restrictions?

In the course of this assessment, OTA drew on the experience of many organizations and individuals. We appreciate the assistance of the project contractors

who prepared background analysis, the U.S. Government agencies and private

companies who contributed valuable information, the project advisory panel and

workshop participants who provided guidance and review, and the many additional reviewers who helped ensure the accuracy and objectivity of this report.

u JOHN H. GIBBONS

Director

 

I l l
Seismic Verification of Nuclear Testing Treaties Advisory Panel

Howard Wesley Johnson, Chair

Honorary Chairman of the Corporation

Massachusetts Institute of Technology

Walter Alvarez

Professor

Department of Geology and Geophysics

University of California, Berkeley

Thomas C. Bache

Manager, Geophysics Division

Earth Sciences Operation

Science Applications International Corp.

William E. Colby

former Director of Central Intelligence

Agency

F. Anthony Dahlen

Professor

Department of Geological and Geophysical

Sciences

Princeton University

H.B. Durham

Distinguished Member of Technical Staff

Sandia National Laboratories

John R. Filson

Chief, Office of Earthquakes, Volcanos,

and Engineering

U.S. Geological Survey

W. J. Hannon, Jr.

Assistant Program Leader

Analysis & Assessment

Verification Program

Lawrence Livermore National Laboratory

University of California

Roland F. Herbst

Member of President’s Staff

RDA Associates

Eugene T. Herrin, Jr.

Professor

Department of Geological Sciences

Southern Methodist University

Franklyn K. Levin

former Senior Research Scientist, Exxon

Raymond J. McCrory

former Chief, Arms Control Intelligence

Staff

Central Intelligence Agency

Karen McNally

Director, Charles F. Richter Seismological

Laboratory

University of California, Santa Cruz

John R. Murphy

Program Manager and Vice President

S-CUBED

Wolfgang K.H. Panofsky

Director Emeritus

Stanford Linear Accelerator Center

Stanford University

Paul G. Richards

Professor

Department of Geological Sciences

Lamont-Doherty Geological Observatory

Columbia University

Jack Ruina

Director, Defense & Arms Control Studies

Program

Center for International Studies

Massachusetts Institute of Technology

Lynn R. Sykes

Higgins Professor of Geological Sciences

Lamont-Doherty Geological Observatory

Columbia University

Thomas A. Weaver

Deputy Group Leader, Geophysical Group

Los Alamos National Laboratory

University of California

NOTE: OTA appreciates and is grateful for the valuable assistance and thoughtful critiques provided by the advisory panel members. The panel does not, however, necessarily approve, disapprove, or endorse this report.

OTA assumes full responsibility for the report and the accuracy of its contents.

iv
OTA Project Staff—Seismic Verification of Nuclear Test Ban Treaties

Lionel S. Johns, Assistant Director, OTA

Energy, Materials, and International Security Division

Peter Sharfman, International Security and Commerce Program Manager

Gregory E. van der Vink, Project Director

Administrative Staff

Jannie Home Marie C. Parker Jackie Robinson

Contractors

Steven M. Day Zoltan A. Der Frederick K. Lamb Robert P. Masse ́

Acknowledgments

In addition to the advisory panel and contractors, OTA gratefully acknowledges

the valuable contributions made by the following:

Ralph W. Alewine, III, Defense Advanced Research Projects Agency

Shelton S. Alexander, Pennsylvania State University

Charles B. Archambeau, University of Colorado

Robert R. Blandford, Defense Advanced Research Projects Agency

John S. Derr, United States Geological Survey

Don Eilers, Los Alamos National Laboratory

Jack Evernden, United States Geological Survey

Frederick E. Followill, Lawrence Livermore National Laboratory

Robert A. Jeffries, Los Alamos National Laboratory

James F. Lewkowicz, Air Force Geophysics Laboratory

J. Bernard Minster, Scripps Institute of Oceanography

Otto W. Nuttli, St. Louis University

Mary Peters, Naval Research Laboratory

Robert A. Phinney, Princeton University

Keith Priestley, University of Nevada

Alan S. Ryan, Center for Seismic Studies

Robert H. Shumway, University of California

Stewart Smith, Incorporated Research Institutions for Seismology

Sean Solomon, Massachusetts Institute of Technology

Jack Rachlin, United States Geological Survey

Irvin Williams, Department of Energy

Robert Zavadil, Air Force Technical Applications Center

Department of Energy, Office of Classification
Workshop I: Network Capabilities

Stewart Smith, Chair

President, Incorporated Research Institute for Seismology

Charles B. Archambeau

Professor

Cooperative Institute for Research in

Environmental Science

University of Colorado, Boulder

Thomas C. Bache

Manager, Geophysics Division

Earth Sciences Operation

Science Applications International Corp.

John S. Derr

Geophysicist

U.S. Geological Survey

Jack Evernden

Research Geophysicist

U.S. Geological Survey

Willard J. Hannon

Assistant Program Leader for Analysis &

Assessment

Verification Program

Lawrence Livermore National Laboratory

University of California

Keith Nakanishi

Assistant Program Leader for Seismic

Research

Lawrence Livermore National Laboratory

University of California

Paul G. Richards

Professor

Department of Geological Sciences

Lamont-Doherty Geological Observatory

Columbia University

Frederick E. Followill Robert J. Zavadil

Seismologist Chief, Evaluation Division

Lawrence Livermore National Laboratory Directorate of Geophysics

University of California Air Force Technical Applications Center

Workshop II: Identification

J. Bernard Minster, Chair

Visiting Professor of Geophysics

Scripps Institute of Oceanography

University of California, San Diego

Charles B. Archambeau

Professor

Cooperative Institute for Research in

Environmental Science

University of Colorado, Boulder

Thomas C. Bache

Manager, Geophysics Division

Earth Sciences Operation

Science Applications International Corp.

Jack Evernden

Research Geophysicist

U.S. Geological Survey

Willard J. Hannon

Assistant Program Leader for Analysis &

Assessment

Verification Program

Lawrence Livermore National Laboratory

University of California

Robert R. Blandford John R. Murphy

Program Manager for the Nuclear Program Manager and Vice President

Monitoring Office S-CUBED

Defense Advanced Research Projects

Agency

vi
Keith Priestley

Professor

Seismological Laboratory

University of Nevada

Jack Rachlin

Geologist

U.S. Geological Survey

Paul G. Richards

Professor

Department of Geological Sciences

Lamont-Doherty Geological Observatory

Columbia University

Robert H. Shumway

Professor

Division of Statistics

University of California, Davis

Lynn R. Sykes

Higgins Professor of Geological Sciences

Lamont-Doherty Geological Observatory

Columbia University

Robert J. Zavadil

Chief, Evaluation Division

Directorate of Geophysics

Air Force Technical Applications Center

Workshop III: Evasion

Sean C. Solomon, Chair

Professor, Department of Earth, Atmospheric & Planetary Science

Massachusetts Institute of Technology

Charles B. Archambeau

Professor

Cooperative Institute for Research in

Environmental Science

University of Colorado, Boulder

Thomas C. Bache

Manager, Geophysics Division

Earth Sciences Operation

Science Applications International Corp.

Robert R. Blandford

Program Manager for the Nuclear

Monitoring Office

Defense Advanced Research Projects

Agency

Jack Evernden

Research Geophysicist

U.S. Geological Survey

Eugene T. Herrin, Jr.

Professor

Department of Geological Sciences

Southern Methodist University

Keith Priestley

Professor

Seismological Laboratory

University of Nevada

Jack Rachlin

Geologist

U.S. Geological Survey

Lynn R. Sykes

Higgins Professor of Geological Sciences

Lamont-Doherty Geological Observatory

Columbia University

Willard J. Harmon

Assistant Program Leader for Analysis &

Assesmsent

Verification Program

Lawrence Livermore National Laboratory

University of California

vii
Workshop IV: Yield Determinations

Robert A. Phinney, Chair

Chairman, Department of Geological & Geophysical Sciences

Princeton University

Ralph W. Alewine, III

Director of Nuclear Monitoring Office

Defense Advanced Research Projects

Agency

Charles B. Archambeau

Professor

Cooperative Institute for Research in

Environmental Science

University of Colorado, Boulder

Thomas C. Bache

Manager, Geophysics Division

Earth Sciences Operation

Science Applications International Corp.

Donald Eilers

Associate Group Leader

Los Alamos National Laboratory

University of California

Willard J. Hannon

Assistant Program Leader for Analysis &

Assessment

Verification Program

Lawrence Livermore National Laboratory

University of California

Eugene T. Herrin, Jr.

Professor

Department of Geological Sciences

Southern Methodist University

Robert A. Jeffries

Program Director for Verification &

Safeguards

Los Alamos National Laboratory

Frederick K. Lamb

Professor

Department of Physics

University of Illinois, Urbana

Keith Priestley

Professor

Seismological Laboratory

University of Nevada

Paul G. Richards

Professor

Department of Geological Sciences

Lamont-Doherty Geological Observatory

Columbia University

Alan S. Ryall

Manager of Research

Center for Seismic Studies

Robert H. Shumway

Professor

Division of Statistics

University of California, Davis

Lynn R. Sykes

Higgins Professor of Geological Sciences

Lamont-Doherty Geological Observatory

Columbia University

Robert J. Zavadil

Chief, Evaluation Division

Directorate of Geophysics

Air Force Technical Applications Center

Vlll
Contents

Page

I. Executive Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2. Seismic Verification in the Context of National Security . . . . . . . . . . . . . . . . . 23

3. The Role of Seismology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

4. Detecting Seismic Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

5. Identifying Seismic Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

6. Methods of Evading a Monitoring Network . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

7. Estimating the Yields of Nuclear Explosions . . . . . . . . . . . . . . . . . . . . . ......113

Appendix. Hydrodynamic Methods of Yield Estimation . ...................129
Chapter 1

Executive Summary
CONTENTS

Page

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

The Test Ban Debate.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

The Meaning of Verification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Aspects of Monitoring Underground Nuclear Explosions . . . . . . . . . . . . . . . 6

Detecting Seismic Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Identifying Seismic Events as Nuclear Explosions.. . . . . . . . . . . . . . . . . . 8

Evading a Seismic Monitoring Network. ....... . . . . . . . . . . . . . . . . . . . 11

How Low Can We Go?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Leve 1–Above 10 kt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Level 2–Below10 kt but Above 1-2 kt . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Level 3–Below 1-2 kit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Level 4–Comprehensive Test Ban . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Estimating the Yield of Nuclear Explosions. . . . . . . . . . . . . . . . . . . . . . . . . . 15

Soviet Compliance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Verification of the TTBT and the PNET. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

A Phased Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Boxes

Box Page

l-A. NRDC/Soviet Academy of Sciences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

1-B. CORRTEX. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Figures

Figure No. Page

l-1. Signal From Semipalatinsk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

l-2. Explosion and Earthquake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Chapter 1

Executive Summary

Technologies that were originally developed to study earthquakes may now

enable the United States

limit

to verify a treaty with the Soviet Union to further

the testing of nuclear weapons.

INTRODUCTION

Seismology now provides a means to monitor underground nuclear explosions down to

low yields, even when strenuous attempts are

made to evade the monitoring system. By doing so, seismology plays a central role in verifying arms control agreements that limit the testing of nuclear weapons. Seismology, however,

is like any other technology: it has both strengths

and limitations. If the capabilities of seismic

monitoring are to be fully realized, it is necessary to understand both how the strengths can

be used and how the limitations can be avoided.

To a great extent, the capabilities of any

given seismic monitoring network are determined by how the monitoring task is approached

and what supplementary provisions are negotiated within the treaty. If agreements can be

negotiated to reduce uncertainty, then seismology can be very effective and extremely low

yields could be monitored with high confidence.

This report addresses two key questions:

1. down to what size explosion can underground testing be seismically monitored

with high confidence, and

2. how accurately can the yields of underground explosions be measured seismically?

The answers to these questions provide the

technical information that lies at the heart of

the political debate over:

1. how low a threshold test ban treaty with

the Soviet Union we could verify,

2. whether the 1976 Threshold Test Ban

Treaty is verifiable, and

3. whether the Soviet Union has complied

with present testing restrictions.

Seismic monitoring as discussed in this study

is evaluated without specific references to the

particular treaty regime to which it is to apply. There will always be some limit to the capability of any given monitoring network, and

hence there will always be a threshold below

which a seismic network could not monitor

with high confidence. Consequently, should a

total test ban be enacted there will be a very

low threshold below which seismic methods

cannot provide high confidence monitoring.

Such a treaty could still be considered to be

in the national interest if, taking both seismic

and nonseismic verification methods into account, the significance of undetected violations

(if they were to occur) would be outweighed

by the benefits of such a treaty.

THE TEST BAN DEBATE

Test ban treaties are a seemingly simple ap- advantages in one area must be weighed against

preach to nuclear arms control, yet their im- advantages in another. Consequently, all aspects

pact is complex and multi-faceted. The deci- of a new treaty must be considered together

sion as to whether a given test ban treaty is and the cumulative impact evaluated in terms

in our overall national interest is dependent of a balance with the Soviet Union. Finally,

on many questions concerning its effects. Dis- the total net assessment of the effects of a

3
4

Photo credit: Department of Energy

Signal cables and test device being lowered down test hole.

treaty on our national security must be weighed of a nuclear deterrent and the extent to which

against the alternative: no treaty. arms control can contribute to national security. It is perhaps because test ban treaties go

One’s opinion about the effects of a test ban, to the very heart of nuclear weapons policy that

and thus its desirability, is largely dependent the debate over them remains unresolved. Three

on one’s philosophical position about the role decades of negotiation between the United
5

States and the U.S.S.R. have produced only

three limitation treaties, two of which remain

unratified:

1.

2.

3.

1963 Limited Nuclear Test Ban Treaty

(LTBT). This treaty bans nuclear explosions in the atmosphere, in outer space,

and under water. It was signed by the

United States and the U.S.S.R. on August

5, 1963 and has been in effect since October 10, 1963. Over 100 other countries

have also signed this treaty.

1974 Threshold Test Ban Treaty (TTBT).

This treaty restricts the testing of underground nuclear weapons to yields no greater

than 150 kilotons (kt). It was signed by

the United States and the U.S.S.R. on

July 3,1974. Although the TTBT has yet

to receive the consent of the U.S. Senate,

both nations consider themselves obligated

to adhere to it.

1976 Peaceful Nuclear Explosions Treaty

(PNET). This treaty is a complement to

the TTBT and restricts individual peaceful nuclear explosions (PNEs) to yields no

greater than 150 kt, and aggregate yields

in a salvo to no greater than 1500 kt. It

was signed by the United States and the

U.S.S.R. on May 28,1976. Although PNET

has yet to receive the consent of the U.S.

Senate, both nations consider themselves

obligated to adhere to it.

Nuclear explosions compliant with these

treaties can only be conducted by the United

and the U.S.S.R. underground, at specific test

sites (unless a PNE), and with yields no greater

than 150 kt. Although they have had important positive environmental and arms control

impacts, these treaties have not prevented the

development of new types of warheads and

bombs. For this reason, public interest in a

complete test ban or a much lower threshold

remains strong, and each year a number of

proposals continue to be brought before the

Congress to limit further the testing of nuclear

weapons.

THE MEANING OF VERIFICATION

For the United States, the main national

security benefits derived from test limitation

treaties are a result of the Soviet Union being

similarly restricted. In considering agreements

that bear on such vital matters as nuclear

weapons development, each country usually

assumes as a cautious working hypothesis that

the other parties would cheat if they were sufficiently confident that they would not be caught.

Verification—the process of confirming compliance and detecting violations if they occur—

is therefore central to the value of any such

treaty.

“To verify” means to establish truth or accuracy. Yet in the arena of arms control, the

process of verification is political as well as

technical. It is political because the degree of

verification needed is based upon one’s perception of the benefits of a treaty compared with

one’s perception of its disadvantages and the

likelihood of violations. No treaty can be considered to be either verifiable or unverifiable

without such a value judgment. Moreover, this

judgment is complex because it requires not

only an understanding of the capabilities of

the monitoring systems, but also an assessment as to what is an acceptable level of risk,

and a decision as to what should constitute significant noncompliance. Consequently, people

with differing perspectives on the role of nuclear weapons in national security and on the

motivations of Soviet leadership will differ on

the level of verification required.l

‘This issue is discussed further in chapter 2, Seism”c Verification in the Context of National Security.
6

ASPECTS OF MONITORING UNDERGROUND

NUCLEAR EXPLOSIONS

Like earthquakes, the force of an underground

nuclear explosion creates seismic waves that

travel through the Earth. A seismic monitoring network must be able both to detect and

to identify seismic signals. Detection consists

of recognizing that a seismic event has occurred and locating the source of the seismic

signals. Identification involves determining

whether the source was a nuclear explosion.

In the case of a threshold treaty, the monitoring network must also be able to estimate the

yield of the explosion from the seismic signal

to determine if it is within the limit of the

treaty. All of this must be done with a capability that can be demonstrated to adequately

defeat any credible attempt to evade the monitoring network.

If the seismic signals from explosions are not

deliberately obscured or reduced by special efforts, seismic networks would be capable of detecting and identify with confidence nuclear

explosions with yields below 1 kt. What stops

this capability from being directly translated

into a monitoring threshold is the requirement

that the monitoring network be able to accomplish detection and identification with high

confidence in spite of any credible evasion scenario for concealing or reducing the seismic signal from a test explosion.

Demonstrating that the monitoring capability meets this requirement becomes complex

at lower yields. As the size of the explosion

becomes smaller:

●

●

●

there are more opportunities for evading

the monitoring network,

there are more earthquakes and industrial

explosions from which such small clandestine explosions need to be distinguished,

and

there are more factors that can strongly

influence the seismic signal.

The cumulative effect is that the lower the

yield, the more difficult the task of monitoring against possible evasion scenarios.

The threat of evasion can be greatly reduced

by negotiating within a treaty various cooperative monitoring arrangements and testing

restrictions. However, there will eventually be

a yield below which the uncertainty of any

monitoring regime will increase significantly.

The point at which the uncertainties of the monitoring system no longer permit adequate verification is a political judgment of the point at

which the risks of the treaty outweigh the benefits.

Determining the credibility of various evasion methods requires subjective judgments

about levels of motivation and risk as well as

more objective technical assessments of the

capability of the monitoring system. To separate the technical capabilities from the subjective judgments, we will first describe our capability to detect and identify seismic events

and then will show how this capability is limited by various possible evasion methods. All

considerations are then combined to address

the summary question: How low can we go?

Detecting Seismic Events

The first requirement of a seismic network

is that it be capable of detecting a seismic

event. If the Earth were perfectly quiet, this

would be easy. Modem seismometers are highly sophisticated and can detect remarkably

small motions. Limitations are due not to the

inherent sensitivity of the instrument but rather

to phenomena such as wind, ocean waves, and

even rush hour traffic. All of these processes

cause ground vibrations that are sensed by

seismometers and recorded as small-scale background motion, collectively referred to as

“noise.” Seismic networks, consisting of groups

of instruments, are designed to distinguish

events like earthquakes and explosions from

this ever-present background noise. The extent

to which a seismic network is capable of detecting such events is dependent on many factors. Of particular importance are the types

of seismic stations used, the number and distribution of the stations, the amount of back-
7

ground noise at the site locations, and the efficiency with which seismic waves travel from

the source to the receiving station through the

surrounding geologic area.

Detecting seismic events can be accomplished

with high certainty down to extremely small

yields. A hypothetical seismic network with

stations only outside the Soviet Union would

be capable of detecting well-coupled2 explosions with yields below 1 kt anywhere within

the Soviet Union and would be able to detect

2A “well-coupled” explosion is one where the energy is well

transmitted from the explosion to the surrounding rock.

even smaller events in selected regions. The

existing seismic array in Norway, for example, has easily detected Soviet explosions with

yields of a fraction of a kt conducted 3,800

kilometers away at the Soviet test site in Eastern Kazakhstan (figure l-l).

Seismic stations within the Soviet Union

would further improve the detection capability of a network. In principle, almost any

desired signal detection level could be achieved

within the Soviet Union if a sufficient number

of internal stations were deployed. In this

sense, the detection of seismic events does not

provide a barrier for monitoring even the lowest

threshold treaties. From a monitoring standFigure 1-1 .-Signal From Semipalatinsk

Noress

array

beam

I I I I I I I I I

0 10 20 30 40 50

Time (seconds)

Seismic signal from a 0.25 kt explosion at the Soviet test site near Semipalatinsk. Recorded 3,800 km away at the NORESS

seismic array in Norway on July 11, 1985. The signal to noise ratio is about 30 indicating that much smaller explosions could

be detected even at this great distance.

SOURCE: R.W. Alewire Ill, “Seismic Sensing of Soviet Tests,” Defense 85, December 1985, pp. 11-21.
8

point, stations within the Soviet Union are

more important for improving identification

capabilities than for further reduction of the

already low detection threshold.

Identifying Seismic Events as

Nuclear Explosions

Once a seismic signal has been detected, the

next task is to determine whether it was created by a nuclear explosion. Seismic signals

are generated not only by nuclear explosions,

but also by natural earthquakes, rockbursts

in mines, and chemical explosions conducted

for mining, quarry blasting, and construction.

Every day there are many earthquakes

around the globe whose seismic signals are the

same size as those of potential underground

nuclear explosions. Several methods can be applied to differentiate earthquakes from underground nuclear explosions. Note, however, that

no one method is completely reliable. It is the

set of different identification methods taken

as a whole and applied in a systematic fashion

that is assessed when summaries on capability are given. In this sense, identification is

a “winnowing” process.

The most basic method of identification is

to use the location and the depth of the event.

Over 90 percent of all seismic events in the

U.S.S.R. can be classified as earthquakes simply because they are either too deep or not in

a plausible location for a nuclear explosion. For

seismic events that are in a location and at a

Photo credit: Department of Energy

Craters formed by cavity collapse in Yucca Flat, Nevada Test Site.
9

depth that could bean explosion, other methods of discrimination based on physical differences between earthquakes and explosions are

used.

When a nuclear device explodes underground,

it applies uniform pressure to the walls of the

cavity it creates. As a result, explosions are

seen seismically as highly concentrated sources

of compressional waves, sent out with approximately the same strength in all directions from

the point of the detonated device. An earthquake, on the other hand, occurs when two

blocks of the Earth’s crust slip past each other

along a fault. An earthquake generates shear

waves from all parts of the fault that rupture.

These fundamental differences between earthquakes and explosions are often exhibited in

their seismic signals. As a result, seismologists

have been able to develop a series of methods

to differentiate the two sources based on the

different types of seismic waves they create.

The combination of all methods, when applied

in a comprehensive approach, can differentiate with high confidence between explosions,

down to low yields, and earthquakes.

As the size of the seismic events gets smaller,

nuclear explosions must be distinguished not

only from earthquakes, but also from other

kinds of explosions. Industrial chemical explosions (e.g. in a quarry operation) pose a particularly difficult problem because their seismic

signals have physical characteristics similar

to those of nuclear explosions. Consequently,

the seismic methods that are routinely used

to differentiate earthquakes from explosions

cannot distinguish between some legitimate

chemical explosions for mining purposes and

a clandestine nuclear test explosion. Fortunately, industrial explosions in the range of

1 to 10 kt are rare (less than one a year). Large

explosions are usually ripple fired so as to minimize ground vibration and fracture rock more

efficiently. Ripple firing is often accomplished

with bursts spaced about 0.2 seconds apart

over a duration of about a second. Recent work

suggests that this ripple-firing has an identifiable signature apparent in the observed seismic signals, and therefore can be used to identify such chemical explosions. However, the

absence of evidence for ripple firing cannot be

taken as evidence that the event is not a chemical explosion. Because of the size consideration, industrial explosions are not an identification problem for normal nuclear explosions

above 1 kt. The difficulty, as we will see later,

comes in distinguishing between a small decoupled nuclear test and a large salvo-fired chemical explosion. This difficulty can be limited

through such treaty provisions as options for

inspections and constraints on chemical explosions. 3

Because a seismic signal must be clearly detected before it can be identified, the threshold for identification will always be greater

than the threshold for detection. As described

in the previous section, however, the detection

threshold is quite low. Correspondingly, even

a hypothetical network consisting of stations

only outside the Soviet Union would be capable of identifying seismic events with magnitudes corresponding to about 1 kt if no attempts were made to evade the monitoring

system.4 Seismic stations within the Soviet

Union would further improve the identification capability of a network.

It has been argued that the use of high frequency seismic data will greatly improve our

capability to detect and identify low-yield nuclear explosions.56 Recent experiments conducted by the Natural Resources Defense

Council together with the Soviet Academy of

Sciences are beginning to provide high frequency seismic data from within the Soviet

Union that shows clear recordings of small explosions (see box l-A). There remains, however,

a lack of consensus on the extent to which the

use of higher frequency data will actually im3See chapter 6, Eva&-ng a Monitoring Network, for a discussion of treaty constraints.

‘See chapter 5, Identifying Seism”c Events.

‘For example, much lower identification thresholds have been

defended by J.F. Evernden, C.B. Archarnbeau, and E. Cranswick, “An Evaluation of Seismic Decoupling and Underground

Nuclear Test Monitoring Using High-Frequency Seismic Data,”

Reviews of Geophysics, vol. 24, May 1986, pp. 143-215.

‘See chapter 4, De&ctingSeismz”cE vents, and chapter 5 Identifying Seism”c Events, for discussions of high frequency monitoring.
10

Box 1-A.—NRDC/Soviet Academy of Sciences

New seismic data from the Soviet Union is becoming available through an agreement between

the Natural Resources Defense Council and the Soviet Academy of Sciences. The agreement provides for the establishment of a few high-quality seismic stations within the Soviet Union around

the area of the Soviet test site in Kazakhstan. The agreement also included experiments in which

the Soviet Union detonated chemical explosions of known yield near the test site so that the test

site could be calibrated.

The seismograph below is from a 0.01 kt chemical explosion detonated near the Soviet test site

and recorded 255 km away. The signal of the explosion can be clearly seen along with the coincidental arrival of seismic waves caused by a large earthquake that occurred south of New Zealand. The

three components of ground motion are east-west (E), north-south (N), and vertical (Z).

Start of explosion

recording

Start of earthquake

record 1

SOURCE: Natural Resources Defense Council.

prove monitoring capabilities. The lack of con- til more extensive data can be collected at resensus is due to differences in opinion as to gional distances from areas throughout the

how well U.S. experience and the limited ex- Soviet Union. Nevertheless, there is general

perience near the Soviet test site can be ex- agreement among seismologists that good

trapolated to an actual comprehensive moni- data is obtainable at higher frequencies than

toring system throughout the Soviet Union. those used routinely today, and that this data

Consequently, the debate over improved ca- offers advantages for nuclear monitoring that

pability will probably remain unresolved un- should continue to be explored.
11

Photo credit: Sand/a National Laboratories

An example of what an internal seismic station might look like.

Evading a Seismic Monitoring Network

To monitor a treaty, nuclear tests must be

detected and identified with high confidence

even if attempts are made to evade the monitoring system. As mentioned earlier, it is the

feasibility of various evasion scenarios that

sets the lower limit on the monitoring capability. The major evasion concerns are:

●

●

●

that the signal of an explosion could be

hidden in the signal of a naturally occurring earthquake,

that an explosion could be muffled by detonating it in a large underground cavity,

or

that a nuclear test could be disguised as

or masked by a large legitimate industrial

explosion.

The hide-in-earthquake scenario assumes

that a small nuclear test could be conducted

by detonating the explosion during or soon after an earthquake. If the earthquake were sufficiently large and the small explosion properly

timed, the seismic signal of the explosion would

be hidden in the seismic signal of the earthquake. However, it is not practical for an

evader to wait for an earthquake that is in the

immediate vicinity of the test site. Therefore,

the masking earthquake would have to be large

and at some distance. The smaller nuclear explosion will produce higher frequency signals

than the earthquake, and filtering the signals

will reveal the signals from the explosion (figure 1-2). For this and other reasons (chapter

6), the hide-in-earthquake scenario need no

longer be viewed as a credible evasion threat

for explosions above 5-10 kt. To counter this

threat for explosions below 5-10 kt may require

access to data from seismic stations within the

Soviet Union, because the higher frequency

seismic signals from explosions below 5-10 kt
12

Figure l-2.—Explosion and Earthquake

Conventional Recording Using Low Frequencies

50 100 150

Large 1⁄2 kiloton

earthquake explosion Time (seconds)

Same Recording But With High Frequency Passband

o

Both the upper and

is the conventional

ends, the arrival of

50 1(

Large

earthquake

I I

150 200

1⁄2 kiloton

explosion

Time (seconds)

lower seismograms were recorded in Norway and cover the same period of time. The upper seismogram

recording of low-frequency seismic waves. Both it and the lower recording show, at the time of 30 secwaves from a large earthquake that occurred in the eastern part of the Soviet Union. About one minute

after the earthquake, at a time of 100 seconds, the Soviet Union conducted a very small (about 1⁄2 kt) underground nuclear

explosion at their Kazakhstan test site. With the standard filter (upper seismogram), the signal of the explosion appears hidden by the earthquake. Using a passband filter for higher frequency seismic waves (lower seismogram), the explosion is revealed.

SOURCE: Semiannual Technical Summary for Norwegian Seismic Array for 1984, Royal Norwegian Council for Scientific and Industrial Research, Scientific Report

No. 1-84/85.
13

may not always be picked up by stations outside the Soviet Union.

Decoupling appears to be potentially the

most effective of all evasion methods. It involves detonating a nuclear device in a large

underground cavity so as to muffle the seismic signal. If the explosion occurs in a large

cavity, the explosive stresses are reduced because they are spread over the large area of

the cavity wall. At a certain cavity size, the

stresses will not exceed the elastic limit of the

rock. At such a point, the explosion is said to

be “fully decoupled” and the seismic signal be

comes greatly reduced. At low seismic frequencies, a fully decoupled explosion may have a

signal 70 times smaller than that of a fully coupled explosion. At high frequencies the decoupling factor is probably reduced to somewhere

between 10 and 7.

Above 10 kt, decoupling is not considered

to be a credible evasion scenario because: 1)

the clandestine construction of a cavity large

and strong enough to decouple a 10 kt explosion is not feasible; and 2) even if such an explosion were somehow fully decoupled, the seismic signal would stand a good chance of being

detected and possibly identified. Therefore,

decoupling could be most effective for small

explosions up to a few kt, particularly when

done in conjunction with a legitimate industrial explosion. For example, a potential evasion method would be to secretly decouple a

small nuclear explosion in a large underground

cavity and mask or attribute the muffled signals to a large chemical explosion that is simultaneously detonated under the guise of legitimate industrial activity.

As discussed in the section on identification,

differentiating a small nuclear explosion from

a legitimate industrial explosion associated

with mining and quarry blasting is difficult

because both produce similar seismic signals.

This is not a problem for identification under

normal circumstances because industrial explosions in the 1-10 kt range occur less than

once a year. However, industrial explosions

may create a problem when considering the

decoupling evasion scenario. That is, some lowyield decoupled explosions might produce seismic signals comparable to those observed from

large chemical explosions and no routine capability has yet been developed to differentiate,

with high confidence, between such signals.

None of the evasion scenarios poses any serious problem for monitoring explosions above

10 kt. However, to provide adequate monitoring capability below 10 kt, efforts must be

made to limit decoupling opportunities. This

would include an internal seismic network and

provisions within the treaty (such as pre-notification with the option for on-site inspection)

to handle the large numbers of chemical explosions. At a few kt, decoupling becomes possible when considered as part of an evasion sce

nario. Arranging such a test, however, would

be both difficult and expensive; and it is not

clear that the evader could have confidence

that such a test (even if it were successful)

would go undiscovered. However, the possibility remains that a few tests of small magnitude could be conducted and remain unidentified as nuclear explosions.

HOW LOW CAN WE GO?

Given all the strengths and limitations of

seismic methods in detecting and identifying

Soviet nuclear explosions, combined with the

credibility of the various evasion scenarios, the

ultimate question of interest for monitoring

any low-yield threshold test ban treaty is essentially: How low can we go? The answer to

that question depends largely on what is negotiated in the treaty. As we have seen, the

challenge for a monitoring network is to demonstrate a capability to distinguish credible

evasion attempts from the background of frequent earthquakes and legitimate industrial

explosions that occur at low yields.
14

The monitoring burden placed on the seismic network by various evasion scenarios can

be greatly lessened if seismology gets some

help. The sources of help are varied and numerous: they include not only seismic monitoring

but also other methods such as satellite surveillance, radioactive air sampling, communication intercepts, reports from intelligence

agents, information leaks, interviews with

defectors and emigres, on-site inspections, etc.

The structure of any treaty or agreement should

be approached through a combination of seismic methods, treaty constraints, and inspections

that will reduce the uncertainties and difficulties of applying seismic monitoring methods to

every conceivable test situation. Yet with these

considerations in mind, some generalizations

can still be made about monitoring at various

levels.

Level l–Above 10 kt

Nuclear tests with explosive yields above 10

kt can be readily monitored with high confidence.7 This can be done with external seismic

networks and other national technical means.

The seismic signals produced by explosions of

this size are discernible and no method of evading a seismic monitoring network is credible.

However, for accurate monitoring of a 10 kt

threshold treaty it would be desirable to have

stations within the Soviet Union for improved

yield estimation, plus treaty restrictions for

handling the identification of large chemical

explosions in areas where decoupling could

take place.

Level 2—Below 10 kt but

Above 1-2 kt

Below 10 kt and above 1-2 kt, the monitoring network must demonstrate a capability to

defeat evasion scenarios. Constructing an underground cavity of sufficient size to fully

decouple an explosion in this range is believed

to be feasible in salt, with dedicated effort and

7The United States and the Soviet Union presently restrict

their testing to explosions with yields no greater than 150 kt.

The bomb dropped on Hiroshima was estimated to have a yield

of 13 kt.

resources. Consequently, the signals from explosions below 10 kt could perhaps be muffled.

The seismic signals from these small muffled

explosions would need not only to be detected,

but also distinguished from legitimate chemical industrial explosions and small earthquakes.

Demonstrating a capability to defeat credible

evasion attempts would require seismic stations throughout the Soviet Union (especially

in areas of salt deposits), negotiated provisions

within the treaty to handle chemical explosions, and stringent testing restrictions to limit

decoupling opportunities. If such restrictions

could be negotiated, most experts believe that

a high-quality, well run network of internal stations could monitor a threshold of around 5

kt. Expert opinion about the lowest yields that

could reliably be monitored ranges from 1 kt

to 10 kt; these differences of opinion stem from

differing judgments about what technical provisions can be negotiated into the treaty, how

much the use of high frequencies will improve

our capability, and what levels of monitoring

capability are necessary to give us confidence

that the Soviet Union would not risk testing

above the threshold.

Level 3–Below 1-2 kt

For treaty thresholds below 1 or 2 kt, the

burden on the monitoring country would be

much greater. It would become possible to

decouple illegal explosions not only in salt

domes but also in media such as granite, alluvium, and layered salt deposits. Although it

may prove possible to detect such explosions

with an extensive internal network, there is

no convincing evidence that such events could

be confidently identified with current technology. That is, additional work in identification

capability will be required before it can be determined whether such small decoupled explosions could be reliably differentiated from the

background of many small earthquakes and

routine chemical explosions of comparable

magnitude.

Level 4—Comprehensive Test Ban

There will always be some threshold below

which seismic monitoring cannot be accom-
15

plished with high certainty. A comprehensive weigh the significance of any undetected clantest ban treaty could, however, still be consid- destine testing (should it occur) below the monered adequately verifiable if it were determined itoring threshold.

that the advantages of such a treaty would outESTIMATING THE YIELD OF NUCLEAR EXPLOSIONS

For treaties that limit the testing of nuclear

weapons below a specific threshold, the monitoring network not only must detect and identify a seismic event such as a nuclear explosion

but also must measure the yield to determine

whether it is below the threshold permitted by

the treaty. This is presently of great interest

with regard to our ability to verify compliance

with the 150-kt limit of the 1974 Threshold

Test Ban Treaty.

The yield of a nuclear explosion maybe estimated from the seismic signal it produces.

Yield estimation is accomplished by measuring from the seismogram the size of an identified seismic wave. When corrected for distance

and local effects at the recording station, this

measurement is referred to as the seismic magnitude. The relationship between seismic magnitude and explosive yield has been determined

using explosions of known yields. This relationship is applied to estimate the size of unknown explosions. The problem is that the relationship was originally determined from U.S.

and French testing and calibrated for the Nevada test site. As a result, Soviet tests are

measured as if they had been conducted at the

Nevada test site unless a correction is made.

No correction would be needed if the U.S. and

Soviet test sites were geologically identical,

but they are not.

The Nevada test site, in the western United

States, is in a geologically young and active

area that is being deformed by the motion between the North American and Pacific tectonic

plates. This recent geologic activity has created an area of anomalously hot and possibly

even partially molten rock beneath the Nevada

test site. As a result, when an explosion occurs at the Nevada test site, the rock deep beneath Nevada absorbs a large proportion of

the seismic energy. The Soviet test site, on the

other hand, is more similar to the geology

found in the eastern United States. It is a geologically old and stable area, away from any

recent plate tectonic activity. When an explosion occurs at the Soviet test site, the cold,

solid rock transmits the seismic energy strongly.

As a consequence, waves traveling from the

main Soviet test site in Eastern Kazakhstan

appear much larger than waves traveling from

the Nevada test site. Unless that difference

is taken into account, the size of Soviet explosions will be greatly overestimated.

The geological difference between the test

sites can result in systematic error, or “bias,

in the way that measurements of seismic waves

are converted to yield estimates. Random error is also introduced into the estimates by the

measurement process. Thus, as with any measurement, there is an overall uncertainty associated with determining the size of an underground nuclear explosion. This is true whether

the measurement is being made using seismology, hydrodynamic methods, or radiochemical

methods. It is a characteristic of the measurement. To represent the uncertainty, measurements are presented by giving the most likely

number (the mean value of all measurements)

and a range that represents both the random

scatter of the measurement and an estimate

of the systematic uncertainty in the interpretation of the measurements. It is most likely

that the actual value is near the central number and it is increasingly unlikely that the actual number would be found towards either end

of the scatter range. When appropriate, the uncertainty range can be expressed by using what

is called a “factor of uncertainty. ” For example, a factor of 2 uncertainty means that the

best estimate of the yield (the “measured central value”) when multiplied or divided by 2,

defines a range within which the true yield will

fall in 95 out of 100 cases. This is the high de-
76

gree of certainty conventionally used in seismology and may or may not be appropriate

in a verification context.8

The uncertainty associated with estimates

of the systematic error can be greatly reduced

by negotiating provisions that restrict testing

to specific test sites and by calibrating the test

sites. If such calibration were to be an integral

part of any future treaty, the concern over systematic errors of this kind would become minimal. The majority of errors that would remain

would be random. As discussed in chapter 2,

a country considering cheating could not take

advantage of the random error, because it

would not be possible to predict how the random error would act on any given evasion attempt. In other words, if a country attempted

to test above the threshold, it would have to

realize that with every test the chances of appearing in compliance would decrease and at

the same time the chance that at least one of

the tests would appear in unambiguous violation would increase. For this reason, the range

of uncertainty should not be considered as a

range within which cheating could occur.

Most of the systematic error associated with

estimating the yields of Soviet nuclear explosions is due to geological differences between

the U.S. and Soviet test sites and in the coupling of the explosion to the Earth. Therefore,

the single most important thing that can be

done to reduce the uncertainty in yield estimation is to calibrate the test sites. Calibration could be accomplished through an exchange of devices of known yield, or through

independent measures of the explosive yield

such as can be provided by radiochemical or

hydrodynamic methods (See box l-B).

Our present capability to estimate seismically the yields of Soviet explosions is often

cited as a factor of two.9 While this may reflect present operational methods, it is not an

accurate representation of our capability. Our

‘See chapter 2, Seismic Verification in the Context of National

Security.

‘U. S. Department of State, “Verifying Nuclear Testing Limitations: Possible U.S.-Soviet Cooperation, Special Report No.

152, ” Aug. 14, 1986.

capability could be greatly improved by incorporating new methods of yield estimation.

Most seismologists feel that if new methods

were applied, the resulting uncertainty for

measuring explosive yields in the range of 150

kt at the Soviet test site would be closer to

a factor of 1.5 than a factor of two.10 Present

methods are stated to be accurate only to a

factor of two in part because they have not yet

incorporated the newer methods of yield estimation that use surface waves and Lg waves. 11

The uncertainty of this comprehensive approach could be further reduced if calibration

shots were performed and testing were restricted to areas of known geologic composition. It is estimated that through such measures,

the uncertainty in seismically measuring Soviet

tests could be reduced to a level comparable to

the uncertainty in seismically measuring U.S.

tests. An uncertainty factor of 1.3 is the current

capability that seismic methods are able to

achieve for estimating yields at the Nevada test

site.

As with detection and identification, yield

estimation becomes more difficult at low yields.

Below about 50 kt, high-quality Lg-wave signals can only be reliably picked up by stations

within the Soviet Union less than 2,000 km

from the test site. For explosions below 10 kt,

the uncertainty increases because small explosions do not always transmit their signals efficiently to the surrounding rock. For small explosions, the uncertainty could be reduced by

restricting such tests to depths below the water

iOThis reduction of uncertainly derives from using More than

one statistically independent method. Consider as an example

the situation where there are three independent methods of calculating the yield of an explosion, all of which (for the sake of

this example) have a factor of two uncertainty in a log normal

distribution:

# of Methods Resulting Uncertainty

1 2.0

2 1.6

3 1.5

By combining methods, the uncertainty can be reduced below the uncertainty of the individual methods. This methodology, however, can only reduce random error, Systematic error,

such as differences between the test site, will remain and limit

the extent to which the uncertainty can be reduced unless calibration is performed.

‘‘See chapter 7, Estimating the Yields of Nuclear Explosions.
17

table, so that their signal will be transmitted the capability of seismic methods could be ineffectively. proved to a point comparable to the accuracy

Our capability to estimate explosive yields

would depend to a large degree on the approach

that was taken and what was negotiated in future treaties. If new methods of yield determination were incorporated into the measurements and calibration shots were performed,

of other methods, such as CORRTEX, that require a foreign presence and equipment at the

test site. In any case, if the objective of reducing the uncertainty is to reduce the opportunities for cheating, small differences in random

uncertainty do not matter.

SOVIET COMPLIANCE

The decision as to what constitutes adequate

verification should represent a fair assessment

of the perceived dangers of non-compliance.

This necessarily involves a weighing of the advantages of the treaty against the feasibility,

likelihood, and significance of noncompliance.

Such decisions are subjective and in the past

have been influenced by the desirability of the

treaty and the political attractiveness of particular monitoring systems.12 Specific concern

over compliance with test ban treaties has been

heightened by findings by the Reagan Administration that:

“Soviet nuclear testing activities for a number of tests constitute a likely violation of legal obligations under the Threshold Test Ban

Treaty.”13

Although the 1974 Threshold Test Ban

Treaty and the 1976 Peaceful Nuclear Explosions Treaty have remained unratified for over

10 years, both nations have expressed their intent to abide by the yield limit. Because neither the United States nor the Soviet Union

has indicated an intention not to ratify the treaties, both parties are obligated under international law (Article 18, the 1969 Vienna Convention on the Law of Treaties) to refrain from

acts that would defeat their objective and

purpose.

“%e chapter 2, Seisnn”c Verification in the Context of National Security.

“’’The President’s Unclassified Report on Soviet Noncompliance with Arms Control Agreements, ” transmitted to the

Congress, Mar. 10, 1987.

In examining compliance with the 150-kt

threshold, seismic evidence is currently considered the most reliable basis for estimating

the yields of Soviet underground nuclear explosions.14 The distribution of Soviet tests indicates that about 10 (out of over 200) Soviet

explosions since the signing of the Threshold

Test Ban Treaty in 1974 could have estimated

yields with central values above the 150 kt

threshold limit, depending on how the estimate

is made.15 These 10 tests could actually be at

or below the 150 kt limit, but have higher yield

estimates due to random fluctuations in the

seismic signals. In fact, when the same methods of yield estimation are applied to U.S. tests,

approximately the same number of U.S. tests

also appear to be above the 150 kt threshold

limit. These apparent violations, however, do

not mean that one, or the other, or both countries have cheated; nor does it mean per se that

seismology is an inadequate method of yield

estimation. It is inherent in any method of

measurement that if several tests are performed at the limit, some of these tests will

have estimated central values above the yield

limit. Because of the nature of measurements

(using any method), it is expected that about

half the Soviet tests at 150 kt would be measured as slightly above 150 kt and the other half

would be measured as slightly below 150 kt.

“Conclusion of the Defense Intelligence Agency Review Panel

as stated in a letter from Roger E. Batzel, Director of Lawrence

Livermore National Laboratory to Senator Claiborne Pen on

Feb. 23, 1987.

*’See chapter 7, Estimating the Yields of Nuclear Explosions.
18

I

Box 1-B.–CORRTEX

CORRTEX (Continuous Reflectometry for Radius versus Time Experiments) is a technique that

was developed in the mid-1970s to improve yield estimation using non-seismic (hydrodynamic) methods. In the CORRTEX technique, a satellite hole is drilled parallel to the emplacement hole of the

nuclear device and an electrical sensing cable is lowered down the hole (see figure). When the explosion occurs, a shock wave moves outward crushing and electrically shorting the cable.

By measuring with electronic equipment at the surface the rate at which the cable is shorted

out, the rate of expansion of the shock wave can be calculated. From the rate of expansion of the

shock wave and the properties of the surrounding medium, the yield of the nuclear device can be

estimated. A full assessment of this method is presented in the Appendix, Hydrodynamic Methods

of Yield Estimation.

CORRTEX CORRTEX

recorder recorder

r 1 t

Surface 1 t Surface

-

Sensin g

cabl e

osti c

point d

Typical cable emplacement

in satellite hole

‘TSensing

cable

/

/I

\

Y

point

\

\

\

/

\

\

Shock front /

progression

Moving shock wave fro m

nuclear detonation

crushes and shortens cable
19

All of the estimates of Soviet and U.S. tests

are within the 90 percent confidence level that

one would expect if the yields were 150 kt or

less. Extensive statistical studies have examined the distribution of estimated yields of explosions at Soviet test sites. These studies have

concluded that the Soviets are observing a yield

limit consistent with compliance with the 150

kt limit of the Threshold Test Ban Treaty.16 17

‘81bid.

ITone of the fir9t ~oupS to carry out such statistical studies

was Lawrence Livermore National Laboratory. Their conclusion was reported in open testimony before the Senate Armed

Services Committee on Feb. 26, 1987 by Dr. Milo Nordyke,

Leader of the Treaty Verification Program.

VERIFICATION OF THE TTBT AND THE PNET

As noted above, the 1974 Threshold Test

Ban Treaty and the 1976 Peaceful Nuclear Explosions Treaty have not been ratified. Most

recently, the Senate failed to consent to ratification at least in part because of concerns that

the size of Soviet explosions cannot be measured with adequate confidence.18 As a result,

for over 10 years the United States has continued to abide by the treaties, yet refused to

ratify them, ostensibly because they cannot

be adequately verified. Note that if the treaties were ratified, the protocols would come

into force calling for an exchange of data which,

if validated, would then improve the verification.19 20 However, the treaty contains no provisions for an independent verification of the

data, most importantly, the yields of the calibration shots. Therefore, many experts question the value of such data, unless the data can

be validated.

As a solution to the problem of uncertainty

in yield estimation, the administration has recently proposed the use of an on-site measurement system called CORRTEX for all tests

above 50 kt. The CORRTEX system is stated

‘“Threshold Test Ban Treaty and Peaceful NucJear Explosions

Treaty, Hearings before the Senate Committee on Foreign Relations, Jan. 13 and 15, 1987, S. Hrg. 100-115.

‘The protocol of the TTBT calls for an exchange of geological data plus two calibration shots from each geophysically distinct testing area.

‘“Monitoring would also be improved by the data that would

be obtained from all PNE shots that have occurred in the Soviet Union since 1976.

to have a factor of 1.3 uncertainty for measuring yields greater than 50 kt at the Soviet test

site.21 22 The drawbacks are that it requires prenotification and cooperation of the host country to the extent that foreign personnel and

their equipment must be allowed at the test

site for each test. Also, CORRTEX has limited

application for monitoring a low-yield treaty

and none for detecting clandestine testing, and

so it would not improve our ability to monitor

low-yield testing thresholds.

Alternatively, advanced seismic methods

could be used. The advantage of seismic methods is that a continued presence of foreign personnel at the test site would not be necessary.

Additionally, our ability to monitor all Soviet

testing (not just testing above 50 kt) would

be improved. If the Soviet test site was

calibrated and advanced seismic methods were

utilized, the uncertainty in seismic yield estimation could be reduced to a level comparable to

CORRTEX. In fact, CORRTEX could be used

to confirm independently the yields of the calibration shots. The Soviet Union has already

agreed to the use of CORRTEX for one or two

such explosions at the Soviet test site.23

*’U. S. Department of State, Bureau of Public Affairs, “U.S.

Policy Regarding Limitations on Nuclear Testing, Special Report No. 150, ” August 1986.

“see appendix, Hydrodynm”c Methods of Yield Estimation.

‘

s

Statement of the Secretary of State of the United States

and Minister of Foreign Affairs of the Soviet Union, Dec. 9, 1987.

A PHASED APPROACH

If the policy decision were made that treaties further restricting or eliminating the testing of nuclear weapons are in our national interest, then from a verification viewpoint there

is much to be said for a phased approach to

this goal. Conceptually, it would begin with
20

a limit that can be monitored with high confidence using current methods, but would establish the verification network for the desired

lowest level. The threshold would then be lowered as information, experience, and confidence increase.

For example, the United States and the Soviet Union could begin with a treaty that prohibited testing above 10 kt. This is a level that

can currently be well monitored seismically .24

The verification network negotiated within the

treaty, however, would be designed with the

goal of monitoring down to the 1-2 kt level.

This would include a network of advanced seismic stations throughout the Soviet Union to

detect off-site testing plus negotiated provisions to reduce evasion possibilities.25 Principal

among these would be to restrict testing to below the water table and to a specified test site

where decoupling opportunities would be limited, to require some special handling (such as

pre-notification and on-site inspection) for the

detonation of large chemical explosions, and

to institute measures to confirm a prohibition

on decoupling. To reduce the systematic uncertainty in yield estimation, a series of calibration shots would need to be conducted at

each test site using either an independent

method such as CORRTEX or the exchange

of devices of known yield.

24some exWrts be~eve that decoupling is not feasible above

5 kt and consequently, that a 5 kt threshold could be well monitored with existing methods and facilities; while others would

place the threshold somewhere between 5 and 10. However, virtually all experts agree that tests above 10 kt can be well monitored, even assuming the monitored country is intent on

cheating.

26CJ& chapter G, Eva&”ng a Mom”toring Network.

After such a network had been in operation

for some time, many of the disagreements concerning hypothetical networks would be resolved. It would be known how well seismic

waves travel through the geology of the Soviet Union and what noise levels exist in various areas. Through such a process, experience

with large-scale monitoring would be gained

and there would be more accurate knowledge

of what level of monitoring effort is needed.

After this information and experience had been

obtained, provisions within the treaty could

call for the further reduction of the threshold.

At that point, it would be better known down

to what level monitoring could be accomplished. If it were determined that a lower

threshold could be verified, the testing threshold could be set to the new verifiable limit. By

the continuation of periodic reviews, the treaty

would always be able to use developments in

seismology to maximize the restriction of nuclear testing.

This procedure, however, does not take into

account any considerations other than seismic

verification. It simply presents the maximum

restrictions that could be accomplished from

a seismic verification standpoint. Considerations other than seismic verification may result indifferent thresholds being more desirable. Lower thresholds, or even a complete ban

on testing, may be chosen if the political advantages are seen to outweigh the risk, and

if the significance of minor undetected cheating is seen to be small when all monitoring

methods are considered. Higher thresholds

may be chosen to permit certain types of testing or to avoid placing a threshold at a boundary where particularly significant tests could

occur at yields only slightly above the

threshold.
Chapter 2

Seismic Verification in the Context

of National Security
CONTENTS

The

The

Are

Page

Role of Verification. . . . . . . . . . . . . . . . . . . . . ... , . . . . . . . . . . . . . . . . . . 23

Definition and Value Judgment of “Verification” . . . . . . . . . . . . . . . . . 24

Test Ban Treaties in Our National Interest?. . . . . . . . . . . . . . . . . . . . . . 25

Evaluating the Capability of a Verification Network. . . . . . . . . . . . . . . . . . . 29

Uncertainty and Confidence Levels, , . . . . . . . . . . . . . . . . . . . . . . . . . . . , , . 29

The Relationship Between Uncertainty and Cheating Opportunities . . . . 33

What Constitutes Adequate Verification? . . . . . . . . . . . . . . . . . . . . . . . . . . 35

The Question of Determining Compliance . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Boxes

Box Page

2-A. First Interest in Test Ban Treaties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

2-B. Recent Interest in Nuclear Test Limitations. . . . . . . . . . . . . . ., . . . . . . 28

Figures

Figure No. Page

2-1. Nuclear Testing, July 16, 1945-December 31, 1987 . . . . . . . . . . . . . . . . . 27

2-2. Measurements of 150 kt With Factor-of-2 Uncertainty . . . . . . . . . . . . . 32

2-3. Fifty-Percent Confidence Level for Measurements of 150 kt With

Factor-of-2 Uncertainty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Table

Table No. Page

2-1. Confidence Intervals for an Explosion Measured at 150 kt . . . . . . . . . 32
Chapter 2

Seismic Verification in the Context of

National Security

Seismic monitoring is central to considerations of verification, test ban

treaties, and national security.

THE ROLE OF

For an arms control agreement to be successful, each participating country must feel that

the provisions of the treaty will enhance its

own national security. This requires an evaluation by each country of the costs and benefits to its national security of the treaty’s

restrictions. In the case of nuclear test ban

treaties, the cost is accepting restrictions on

the ability to test nuclear weapons. In return

for paying this price, each country gains the

direct benefit of similarly restricting the other

country. In addition to the direct benefits of

the agreement, participating countries can also

gain the political and non-proliferation benefits of working for arms control, as well as the

environmental benefits of reducing the hazards

of radioactive contamination of the environment from testing.

In considering agreements that bear on such

vital matters as a restriction on nuclear weapon’s development, each country may assume

as a cautious working hypothesis that the participants would cheat if they were sufficiently

confident that they would not be caught. Verification is a process that is undertaken to confirm treaty compliance and, therefore, the ability of the United States to monitor Soviet

activity is central to the value of any such

treaty.

Verification is most often viewed as a process that improves confidence in a treaty. The

converse, however, is also true. Establishing

a treaty generally improves our ability to monitor Soviet activity. In this way, monitoring

VERIFICATION

and treaties have a mutually beneficial relationship. The United States monitors Soviet

weapons developments whether or not a treaty

exists, because the information is important

for our national security. Treaties make monitoring easier and more accurate, because they

include provisions explicitly intended to aid

verification. Additionally, treaties create paths

of communication that can be used to resolve,

clarify, or correct ambiguous situations. In this

sense, treaties have national security value

that extends beyond their direct purpose.

Verification of treaties is a complex process.

The question of whether a treaty is “verifiable”

cannot be answered in any absolute or technical sense. It can only be answered relatively

by referring to values that are influenced by

a wide range of political and philosophical viewpoints. Consequently, verification involves not

only technical considerations, but also judgements as to how these technical considerations

translate into the policy world.

In the past several years, Congress has been

asked to consider proposals for treaties that

prohibit testing above various thresholds.

Each proposal has sparked controversy within

both the technical and policy communities. For

any given treaty, some within both communities will claim it is “verifiable,” whereas others

will assert that the Soviet Union would be able

to cheat and hence that stricter verification

provisions are needed to ensure our national

security. This chapter is intended to provide

a framework for understanding how to weigh

the risks and benefits of such treaties.

23

76-584 0 - 88 : QL 3 - 2
24

THE DEFINITION AND VALUE JUDGMENT OF

“VERIFICATION”

In the case of test ban treaties, measures are

taken to ensure that the advantages of the

treaty cannot be undermined by the other

country testing clandestinely. These measures,

assessments of the measures’ capabilities,

evaluations of the risks and benefits, and the

political climate within which all of these judgments are made make up the process referred

to as verification.

“To verify” means to establish truth or accuracy. Realistically, in the arena of arms control, verification can never be perfect or absolute: it necessarily involves uncertainty and

this is often described in probabilistic terms.

Because the process of verification involves determining acceptable levels of uncertainty, it

is political as well as scientific. The degree of

verification needed is based on one’s perception of the benefits of the treaty compared with

one’s perception of the disadvantages and the

likelihood of violations. Consequently, the level

of verification required will always be different

for people with different perspectives.

In U.S.-Soviet agreements, the concern about

verification is exacerbated by societal asymmetries whereby monitoring compliance is usually achieved more easily in the United States

than in the Soviet Union. These asymmetries

may cause the United States to insist on stricter verification procedures than the Soviets

would judge are needed. This difference makes

negotiations difficult, and can create the impression that the United States is obstructing negotiations.

A country considering cheating would have

to evaluate the risks and costs of being caught

against the benefits of succeeding. A country

concerned about preventing cheating has to

guess the other country’s values for making

this decision and then evaluate them against

their own estimations of the advantages of the

treaty compared with the risk of violation. If

the countries lack insight into each other’s

value systems and decision processes, this uncertainty will result in the perception that a

high degree of verification is needed. As a result, the degree of verification needed to satisfy

the concerned country may be higher than

what is really needed to discourage cheating.

To illustrate this argument, it is useful to

consider the analogy of a treaty restricting

each party to one side of a river. If the river

freezes over, one or both countries may consider crossing to the other side. If the water

is deep and there is nothing worth having on

the other side, then the ice does not have to

be very thin to discourage a party from crossing. If, on the other hand, the water is shallow

and there is something of great value to be obtained from the other side, than the ice must

be very thin to discourage a party from crossing. The thinness of the ice combined with the

depth of the water is the degree of deterrent

available to dissuade a party from trying to

cross the ice. How thin it has to be to actually

deter depends on each party’s perception of

the risk and the reward. In arms control, crossing the ice represents cheating on a treaty. The

level of verification capability needed to deter

crossing (the thinness of the ice) depends on

each side’s perception of the risks and rewards

of cheating. The attraction of cheating (getting to the other side) would be the belief that

it could result in some sort of advantage that

would lead to a significant improvement to the

country’s national security. The consequence

of being caught (falling through the ice) would

depend on the depth of the water. This would

involve international humiliation, the possible

abrogation of the treaty resulting in the loss

of whatever advantages the treaty had provided, and the potential loss of all other present

and future agreements.
25

ARE TEST BAN TREATIES IN OUR NATIONAL INTEREST?

Test Ban treaties are a seemingly simple approach to arms control, yet their impact is complex and multi-faceted. Determining the advantages of such a treaty depends upon weighing

such questions as:

●

●

●

●

●

●

●

●

●

●

Is testing necessary to develop future

weapon systems? Do we want both the

United States and the Soviet Union to develop new weapon systems?

Is testing necessary to ensure a high degree of reliability of the nuclear stockpile?

Do we want the nuclear arsenals of both

the United States and Soviet Union to be

highly reliable?

Is continued testing necessary to maintain high levels of technical expertise in

the weapons laboratories? Do we want to

continue high levels of expertise in both

the United States and Soviet weapons laboratories, and if so, for what purposes?

Is testing necessary to ensure the safety

of nuclear devices?

Could more conservative design practices

reduce the need for nuclear testing?

Would the effects of a test ban impact the

United States and the Soviet Union differently?

Would a decrease in confidence in nuclear

weapons’ performance increase or decrease

the likelihood of nuclear war?

Would a test ban treaty discourage nuclear proliferation? Could it be extended

to cover other nations?

Would the effects of a treaty be stabilizing or destabilizing?

Overall, do the advantages outweigh the

disadvantages?

Due to the immense uncertainties associated

with nuclear conflict, there are few definitive

answers to these questions. One’s opinion

about the answers is largely dependent on one’s

philosophical position about the role of a nuclear deterrent, and the extent to which arms

control can contribute to national security.

None of these questions, moreover, can be considered in isolation. Disadvantages in one area

must be weighed against advantages in another.

Consequently, all aspects of a new treaty must

be considered together and their cumulative

impact evaluated in terms of a balance with

the Soviet Union. Such a net assessment is difficult because even greater uncertainty is introduced when we try to guess how a given

Box 2-A.—First Interest in Test Ban Treaties

Interest in restricting the testing of nuclear

weapons began with an incident that occurred

over 30 years ago. On February 26, 1954, an

experimental thermonuclear device, named

Bravo, was exploded on the Bikini Atoll in

the Pacific Ocean. The explosion was the

United States’ 46th nuclear explosion. It

produced a yield equivalent to 15 million tons

of TNT, which was over twice what was expected. The radioactive fallout covered an

area larger than anticipated and accidently

contaminated an unfortunate Japanese fishing boat named Lucky Dragon. When the

boat docked at Yaizu Harbor in Japan, twentythree of the crew had radiation sickness resulting from fallout. The captain of the vessel, Aikichi Kuboyana, died of leukemia in

September 1954. In another such accident,

radioactive rain caused by a Soviet hydrogen

bomb test fell on Japan. These incidents focused worldwide attention on the increased

level of nuclear testing and the dangers of

radioactive fallout. Soon after, the first proposal for a test ban was put forth.* The 1954

proposal presented by India’s Prime Minister Jawaharlal Nehru was described as:

. . . some sort of what maybe called “standstill agreement” in respect, at least, of these

actual explosions, even if the arrangements

about the discontinuance of production and

stockpiling must await more substantial

agreements among those principally concerned.

Since that time over 1,600 nuclear explosions have occurred and at least four more

countries (United Kingdom, France, People’s

Republic of China, and India) have successfully tested nuclear devices.

*See Bmce A. Bolt, Nuclear Explosions and Earthquakes, W.H.

Freeman and Company, 1976.
26

treaty would affect the Soviet Union. Finally,

the total net assessment of the effects of a

treaty on our national security must be weighed

against the alternative: no treaty.

The first formal round of negotiations on a

comprehensive test ban treaty began on October 31,1958 when the United States, the Soviet Union, and the United Kingdom opened,

in Geneva, the Conference on the Discontinuance of Nuclear Weapon Tests. Since then, interest in a test ban treaty has weathered three

decades of debate with a level of intensity that

has fluctuated with the political climate.1 During this time, three partial nuclear test limitation treaties were signed. Nuclear explosions

compliant with these restrictions are now conducted only underground, at specific test sites,

and at yield levels no greater than 150 kilotons (kt). These three treaties are:

1.1963 Limited Nuclear Test Ban Treaty

(LTBT). Bans nuclear explosions in the

atmosphere, outer space, and under water.

This treaty was signed August 5, 1963.

Ratification was advised and consented

to by the United States Senate on September 24, 1963 and the treaty has been

in effect since October 10, 1963.

2. Threshold Test Ban Treaty (TTBT). Restricts the testing of underground nuclear

weapons by the United States and Soviet

Union to yields no greater than 150 kt.

This treaty was signed July 3,1974. It was

submitted to the United States Senate for

advice and consent to ratification on July

29, 1976 and again on January 13, 1987.

It remains unratified, but both nations

consider themselves obligated to adhere

to it.

3. Peaceful Nuclear Explosions Treaty (PNE).

This treaty is a complement to the TTBT.

It restricts individual peaceful nuclear explosions by the United States and Soviet

‘For an overview of the history of test ban negotiations, the

reader is referred to G. Allen Greb, “Comprehensive Test Ban

Negotiations 1958-1986: An Overview, ” in IVuclear Weapon

Z’ests: Prohibition or Lizm”tation?, edited by Jozef Goldblat and

David Cox, SIPRI, CIIPS, Oxford University Press, London,

1987.

Union to yields no greater than 150 kt,

and aggregate yields to no greater than

1,500 kt. This treaty was signed May 28,

1976. It was submitted to the United

States Senate for advice and consent to

ratification on July 29, 1976 and again on

January 13, 1987. It remains unratified,

but both nations consider themselves obligated to adhere to it.

Although these treaties have fallen far short

of banning nuclear testing, they have had important environmental and arms control impacts. Since 1963, no signatory country compliant with these treaties has tested nuclear

weapons in the atmosphere, in outer space, or

under water, thus eliminating a major environmental hazard. And from an arms control perspective, testing of warheads over 150 kt has

been prohibited since 1974.

While these treaties have had important

positive impacts, figure 2-1 illustrates that, in

fact, they have not resulted in any decline in

the amount of testing. The development of new

types of warheads and bombs has not been

limited by restricting testing, and so advocates

of test ban treaties continue to push for more

restrictive agreements.

A Comprehensive Test Ban Treaty was the

declared goal of the past six U.S. Administrations, but remained elusive. The Reagan Administration, however, has viewed limitations

on nuclear testing as not in the national security interests of the United States, both at

present and in the foreseeable future. The

stated policy of the Reagan Administration is

as follows:

A Comprehensive Test Ban (CTB) remains

a long-term objective of the United States.

As long as the United States and our friends

and allies must rely upon nuclear weapons to

deter aggression, however, some nuclear testing will continue to be required. We believe

such a ban must be viewed in the context of

a time when we do not need to depend on nuclear deterrence to ensure international security and stability and when we have achieved

broad, deep, and verifiable arms reductions,

substantially improved verification capabil-
27

Figure 2-1.— Nuclear Testing, July 16, 1945-December 31, 1987

USA LTBT TTBT

100

90

80

70

60

50

40

30

20

10

0

i

1

I

I

I

I

I

I

I

1945

USSR

50

40

30

20

10

0 L

1945

1950 1955 1960 1965 1970 1975 1980 1985

I

I I

I

: I

-

i !

I

!

I — I n

1950 1955 1960 I 1965 1970

I

1975 1980 1985

I

1990

1990

France

I

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o

1945 1950 1955 1960 1970

I

1975 1980 1985 1990

UK ! I

10 I

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1945 1950 1955 1960 I 1965 1970

I

1975 1980 1985 1990

I

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China I

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1945 1950 1955 1960 1965 1970 1975 1980 1985 1990

India i

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1945 1950 1955 1960 1965 1970 1975 1980 1985 1990

Atmospheric Underground

nuclear explosions nuclear explosions

SOURCE: Data from the Swedish Defense Research Institute,
28

ities, expanded confidence building measures, Recently, a series of events have heightened

and greater balance in conventional forces.2

worldwide interest in test ban treaties and a

Despite this declared United States position, number of proposals have been brought before

Congress to further limit the testing of nuclear public interest in a test ban remains strong. weapons (see box 2-B). To evaluate these pro2U.S. Department of State, Bureau of Public Affairs, “U.S. posals requires an understanding of the desired

Policy Regarding Limitations on Nuclear Testing, ” Special Re- and available levels of verification needed to

port No. 150, Washington, DC, August 1986, monitor compliance.

Box 2-B.—Recent Interest in Nuclear Test Limitations

August 6, 1985 to February 26, 1987: The Soviet Union observes a 19-month unilateral test moratorium. Upon ending its moratorium, the Soviet Union declares that testing would be stopped again

as soon as a U.S. testing halt was announced.

February 26, 1986: House Joint Resolution 3 (with 207 cosponsors) passes the House with a vote

of 268 to 148, requesting President Reagan to resume negotiations with the Soviet Union towards

a comprehensive test ban treaty and to submit to the Senate for ratification the Threshold Test

Ban Treaty (TTBT) and the Peaceful Nuclear Explosions Treaty (PNE). The wording of the House

Resolution is nearly identical to a similar proposal which previously passed the Senate by a vote

of 77 to 22 as an amendment to the 1985 Department of Defense (DoD) Authorization Bill.

May 28, 1986: The Natural Resources Defense Council, a private environmental group, signs an

agreement with the Soviet Academy of Sciences for the establishment of three independent seismic

monitoring stations near the principal nuclear test sites of each country. The agreement specifies

that the stations are to be jointly manned by American and Soviet scientists and the data is to

be made openly available.

Summer 1986: UN Conference on Disarmament agrees to a global exchange by satellite of sophisticated seismic data.

August 7, 1986: The Five Continent Peace Initiative formed by the leaders of six nonaligned countries (India, Sweden, Argentina, Greece, Mexico, and Tanzania) urges a fully verifiable suspension

of nuclear testing and offers assistance in monitoring the ban.

August 8, 1986: House of Representatives votes 234 to 155 in favor of an amendment to the Defense

Authorization Bill that would delete funding in calendar year 1987 for all nuclear tests with yields

larger than 1 kt, provided that the Soviet Union does not test above 1 kt and that the Soviet Union

accepts a U.S. monitoring program. The House Amendment is dropped prior to the Reykjavik summit when the Administration agrees to submit the TTBT and PNET to the Senate for advice and

consent.

May 19, 1987: House of Representatives votes 234 to 187 in favor of an amendment to the fiscal

year 1988 DoD Authorization Bill to delete funding for all nuclear tests with yields larger than

1 kt during fiscal year 1988 provided that the Soviet Union does the same and, if reciprocal (incountry) monitoring programs are agreed on and implemented.

July 1987: At the expert talks on nuclear testing, Soviets propose calibration of test sites to reduce

the uncertainty in yield estimates. The proposal invites U.S. scientists to the Soviet nuclear testing

site to measure Soviet test yields using both the CORRTEX system and seismic methods. In return, Soviet scientists would measure a U.S. test at the Nevada test site using both methods.

November 9, 1987: Formal opening of negotiations in Geneva on nuclear test limitations.

January 1988: Teams of U.S. and Soviet scientists visit each other’s test site to prepare for joint

calibration experiments to reduce uncertainty in yield estimation.
29

EVALUATING THE CAPABILITY OF A

VERIFICATION NETWORK

Making a decision on whether verification

is adequate requires an understanding of the

capability of the verification system, the significance of the potential violation, and a decision as to what is an acceptable level of risk.

Developing the basis for the decision is difficult because it involves two different communities: those who can assess the system’s capabilities (a technical question) generally do

not form the same community as those who

are officially responsible for assessing the overall risks and benefits (a policy question).

For policymakers to weigh the benefits of

the treaty against the risks posed by the possibility of unilateral noncompliance, a clear understanding of the capabilities of the monitoring system is necessary. In the frozen river

example, this understanding would result from

measurements of how thin the ice is and a technical interpretation of how much weight the

ice can bear. The decision as to what constitutes an acceptable level of risk is a policy decision because it is based on an assessment of

the overall benefits of the treaty weighed

against the risk. In the frozen river example,

this assessment would represent the decision

as to how thin the ice would need to be to deter crossing, how deep the river is, and how

significant a crossing would be.

The burden on the policy-making community, therefore, is to understand technical

descriptions of the verification system’s capability and incorporate this knowledge into their

risk-benefit decision. As we shall see, the difficulty is that monitoring capabilities are not

certain, but rather they can only be described

in probabilistic terms. For example: What are

the chances that a clandestine nuclear test

above a certain yield could go undetected?

What are the chances that a detected seismic

event of a certain magnitude could have been

a nuclear explosion rather than an earthquake?

If an underground nuclear explosion is recorded, how certain can we be that the yield

of the explosion was below a specific threshold? The answers to these questions can be obscured by the manner in which they are portrayed. In particular:

• differences between verification systems

can be made to look superficially either

large or small,

● opportunities for Soviet cheating can be

misrepresented, and

● the decision of what defines adequate verification can be made through an arbitrary

process.

The next three sections illustrate the issues

that arise in assessing a verification capability

—and the misrepresentations that are possible–by considering a question that aroused

much Congressional interest in early 1987:

What is our ability to measure seismically the

yields of Soviet explosions near the 150 kiloton

limit of the Threshold Test Ban Treaty? The first

section presents the statistical representations

that are used to describe yield estimation. This

includes the meaning of uncertainty and confidence levels, along with a comparative discussion that enables the reader to understand

what changes in the uncertainty represent. The

next section examines how these uncertainties

translate into opportunities for Soviet cheating. And finally, the third section illustrates

how the policy decision of what constitutes adequate yield estimation capability has changed

in apparent response to variations in the attractiveness of particular monitoring systems.

Uncertainty and Confidence Levels

In determining the verifiability of the 1974

Threshold Test Ban Treaty, policymakers

wanted to know the capabilities of a seismic

monitoring system for estimating whether Soviet tests are within the treaty’s limits. The

description of such capabilities is accomplished

through the use of statistics. While the statistical calculations are relatively straightforward, difficulties arise in correctly appreciating what the numbers mean. To illustrate how
30

such presentations can be misleading, we will

first use an example from a common and comparatively well-understood event:3

At the end of the 1971 baseball season, the

San Francisco Giants were playing the Los

Angeles Dodgers in a televised game. In the

first inning, Willie Mays, approaching the end

of his illustrious career, hit a home run. Now,

one expects that hitting a home run in the

first inning should be a rather unusual occurrence because the pitcher is at his strongest

and the batter has not had time to get used

to the pitcher. In any case, Willie Mays hit

a home run and it triggered what every baseball fan would recognize as a typical baseball

statistician’s response. The calculations were

made and it was discovered that, of the 646

home runs Mays had hit, 122 of them had

been hit in the first inning: 19 percent! In the

most unlikely one-ninth of the innings, Willie Mays had hit nearly one-fifth of his home

runs. This realization captured the interest

of the reporting community and was discussed extensively in the media. In response

to the publicity, the Giants’ publicity director explained it by saying that”. . . Willie was

always surprising pitchers in the first inning

by going for the long ball before they were

really warmed up. ” The power of statistical

analysis was able to draw out the hidden

truths about Willie Mays’ performance.

Although the data and calculations were correct, the interpretation could not have been

more wrong. Throughout Mays’ career, he had

almost always batted third in the Giants’

lineup (occasionally, he batted fourth). That

meant he almost always batted in the first inning. Because he averaged about four at-bats

per game, approximately one-quarter of his atbats came in the first inning. Therefore, he only

managed to hit 19 percent of his home runs

during the first inning which comprised 25 percent of the time that he was at bat. Of the millions of people who must have heard and read

about the item, not one pointed out the misinterpretation of the statistic. This included not

just casual observers, but also experienced

professionals who spend their careers interpreting just that kind of information.

Whis example is paraphrased from David L. Goodstein, States

of Matter (Englewood Cliffs, NJ: PrenticeHall) 1975.

The point here is that the interpretation of

numbers is tricky and statistical presentations

can often be misleading. The real challenge is

not in calculating the numbers, but in correctly

interpreting what different numbers mean. In

an area with as many technical considerations

and political influences as arms control verification, one has to be particularly careful that

different numbers represent truly significant

differences and not just arbitrary distinctions.

As with every real-world measurement, estimating the size of a nuclear explosion results

in variation, or scatter, among the estimates.

The use of different instruments at different

locations, interpretations of the measurements

by different people in slightly different ways,

and unknown variations in signals being observed result in slightly different estimates.

Similarly, if one were to measure the daily temperature outside using a number of thermometers located in several areas, there would be

slight differences in the temperature depending on the particular thermometer, its location

(surrounded by buildings and streets, or in a

park), how each scale was read, etc.

In seismology, errors come from the instrumentation, from the interpretation of the data,

from our incomplete knowledge about how well

an explosion transmits its energy into seismic

waves (the coupling), and from our limited understanding of how efficiently seismic waves

travel along specific paths (the path bias).

Some of these errors are random-they vary

unpredictably from one measurement to the

next. Other errors are not random, but are systematic and are the same from one measurement to the next.

In our example of measuring temperature,

a systematic error would be introduced if each

reading were made using an improper zero on

the thermometer. With such a systematic error, the measurements would continue to be

distributed randomly, but the distribution

would be shifted by the difference between the

true and incorrect zero. The distance from the

incorrect value to the actual value would represent the size of the systematic error. In seismology, an example of a systematic error re-
31

suits from the failure to allow correctly for the

difference in seismic transmission between the

United States and Soviet test sites. The bias

term added to the calculation corrects for this

effect, although there still remains an uncertainty associated with the bias.

The distinction between random and systematic errors, however, is not a clear boundary.

In many cases, random errors turn out to be

systematic errors once the reason for the error is understood. However, if the systematic

errors are not understood, or if there are lots

of systematic errors all operating in different

ways, then the systematic errors are often approximated as random error. In such cases, the

random uncertainties are inflated to encompass the uncertainties in estimating the systematic effect. In monitoring the yields of

Soviet testing near 150 kt, most of the uncertainty is associated with estimation of systematic error because the test site has not been

calibrated. In describing the capability to

measure Soviet tests, the estimate of the random error has been inflated to account for the

uncertainties in the systematic error.

We will see in the next section that while systematic errors might be exploited if they happen to be to one country’s advantage, random

errors do not provide opportunities for cheating. Furthermore, the uncertainty in the estimates of the systematic errors can often be significantly reduced by negotiating into the

treaty such provisions as the calibration of

each test site with explosions of known yield.

For this reason, calibration is important and

should certainly be part of any future agreement. But, before we discuss how random and

systematic errors affect monitoring, the method

of statistically describing the capabilities needs

to be explained. For this, it is useful to return

to our temperature example.

While collecting measurements of the daily

temperature by using many thermometers in

many locations, we would find that some of

the measurements were high and some were

low, but most of them were somewhere in the

middle. If all of the measurements were plotted, they would cluster around one number

with roughly equal scatter distributed to either side. It would be most likely that the best

actual value for the daily temperature would

be near the central number and it would be increasingly less likely that the actual value

would be off towards either end of the scatter

distribution.

In seismology, that central number, measured using several techniques from many seismometers located in different locations, is referred to as the “central value yield. ” No

matter how accurately we could measure the

yields of explosions known to be 150 kt, we

would still expect—due to the normal random

scatter of measurements and presuming there

were no systematic error—that roughly half

the explosions would be recorded as being below 150 kt and roughly half would be recorded

as being above 150 kt. The width of the distribution above and below depends on the capability of the measuring system and can be described using a “factor of uncertainty. ”

Unless otherwise stated, the factor of uncertainty for a given measurement is defined as

that number which, when multiplied by or

divided into an observed yield, bounds the

range which has a 95 percent chance of including the actual (but unknown) value of the yield.

There is only a 5 percent chance that the measurement would be off by more than this factor. For example, a “factor of 2“ uncertainty

would mean that the measured central value

yield, when multiplied and divided by two,

would define a range within which the true

yield exists 95 percent of the time.

Naturally, the more confident one wants to

be that the true value lies within a given range,

the larger that range will have to be. The 95

percent range is used by convention, but there

is no real reason why this should be the confidence level of choice for comparing monitoring systems. Using a different confidence level

than 95 percent to define the factor of uncertainty would cause the factor to have a different value.

As an example, imagine that all Soviet explosions are detonated with an actual yield of

150 kt and that these explosions are measured
32

Figure 2-2.–Measurements of 150 kt With Factor-of-2

Uncertainty

Measurements of 150 kt

w/ factor-of-2 uncertainty

True yield = 150 kt

0.010 . /

0.005 “

0 75 150 225 300 375

Yield estimate, in kilotons

Measurements of a 150 kt explosion using a factor-of-2 uncertainty would be expected to have this distribution. The

probability that the actual yield lies in a particular range is

given by the area under the curve. Ninety-five percent of the

area lies between 75 and 300 kt.

by a monitoring system described as having

a factor of 2 uncertainty. It would then follow

that 95 percent of the yield estimates will be

between 75 and 300 kt. Therefore, if the Soviets

conduct 100 tests all at 150 kt, we would expect that about 5 of them would be measured

with central yields either above 300 kt or below 75 kt. This is graphically illustrated in figure 2-2, where the probability that the actual

yield lies in a particular range is given by the

area under the curve over that range.

Although the range from 75 to 300 contains

95 percent of the measurements, we can see

that for the distribution assumed (the normal

distribution) most of the measurements are,

in fact, much closer to the actual yield. For example, figure 2-3 showing the 50 percent confidence level for the same distribution illustrates that over half of the measurements will

fall between 118 and 190 kt.

On the other hand, if one wanted to specify

a range in which 99 percent of the measureFigure 2-3.—Fifty-Percent Confidence Level for

Measurements of 150 kt With Factor-of-2 Uncertainty

Measurements of 150 kt

-

W/ factor-of-2 uncertainty

0 75 150 225 300 375

Yield estimate, in kilotons

Same distribution as figure 2-2 with 50 percent of the area

marked. Over 50 percent of the measurements would be expected to fail within 118 and 190 kt.

ments fall, the range would have to be extended

to 60 to 372 kt. In real yield estimation, however, there is no meaningful distinction between 95 percent confidence and 99 percent

confidence. The normal distribution is used as

a convenience that roughly represents reality

near the center of the distribution. The tails

of the distribution are almost certainly not

close approximations of reality. The general

point can still be made: namely that, it takes

ever greater increases in range for slight improvements in the confidence level. Table 2-1

illustrates this for the case of a normal distribution by showing how the yield range varies

for given factors of uncertainty at various confidence levels.4

From the table, it is clear that a quoted range

of values is highly dependent not only on the

factor of uncertainty of the monitoring system,

but also on the chosen confidence level. For

example, the same quoted uncertainty range

4Table 2-1 can be read as follows: “If an explosion is measured at 150 kt using a system with a factor of [1.5] uncertainty,

the [70 percent] percent confidence ranges from [121 to 186] kt. ”

Table 2.1.–Confidence Internals for an Explosion Measured at 150 kt

CONFIDENCE

LEVEL

99%

95%

90%

80%

70%

50%

–––––––– UNCERTAINTY ––––––––– 1

Factor of 2 Factor of 1.5 Factor of 1.3

61-372 88-255 106-211

75-300 100-225 115-195

84-269 106-212 120-187

96-236 116-195 126-179

104-216 121-186 130-173

118-191 130-173 138-164

SOURCE: Office of Technology Assessment, 1988.
33

that can be achieved using a factor of 1.3 monitoring system at the 95 percent confidence

level can be achieved using a factor of 1.5 monitoring system at the 80 percent confidence

level, or even using a factor of 2 monitoring

system at the 50 percent confidence level.

In selecting an appropriate confidence level

to use for quoting uncertainty, the purpose of

the comparison must also be recognized. In the

case of monitoring compliance with a threshold test ban treaty, one is concerned that intentional cheating could be missed or that unacceptable false alarms could occur. It must

be remembered, therefore, that the uncertainty

describes the likelihood that the actual value

will not fall either above or below the range.

For example, the 95 percent confidence level

means that given repeated trials there is only

a 5 percent chance the true yield was either

above or below the range of the measured yield.

In other words, there is a 2.5 percent chance

that it could have been above the range and

a 2.5 percent chance that it could have been

below the range. The 2.5 percent below the

range, however, is not a concern if it is below

the threshold. The concern is only the 2.5 percent chance that it had a yield above the threshold. For the purposes of monitoring a threshold, this would only be the half of the 5 percent

above the threshold, or in other words 2.5 percent. Consequently, the 95 percent confidence

level really corresponds to a 97.5 percent confidence level for monitoring violations of a

threshold. Following the same argument, the

50 percent confidence level really corresponds

to a threshold monitoring at the 75 percent confidence level, and so on.

From the previous discussions, it is obvious

that while it maybe convenient to chose a particular confidence level to compare monitoring systems (such as the 95 percent confidence

level), it should only be done with great caution. In particular, it should be kept in mind

that:

● the choice of any particular confidence

level is arbitrary, in the sense of being only

a convenience to allow comparison of the

accuracies of different yield estimation

methods;

●

●

as seen from table 1-1, differences can be

made to look small or large depending on

which particular confidence level is chosen; and

although very high confidence levels have

large ranges of uncertainty, it is with decreasing likelihood that the actual value

will be at the extremes of those ranges of

uncertainty.

These considerations are important to ensure

that common mistakes are not made, namely

that:

●

●

the range of uncertainty is not equated

with a range in which cheating can occur,

and

the range of uncertainty chosen for comparative reasons does not evolve into the

range of uncertainty that is used to determine what constitutes adequate verification.

The Relationship Between Uncertainty

and Cheating Opportunities

The main reason for designing monitoring

systems with low uncertainty is to reduce the

opportunities for cheating. The relationship between uncertainty and opportunities for cheating, however, is not always straightforward.

Even if the uncertainty of a particular monitoring system is large, this does not mean that

the opportunities for cheating are correspondingly large.

As mentioned before, the uncertainty is created by two types of error: random and systematic. Although we try to estimate the systematic error (such as path bias) as accurately

as possible, there is a chance that our estimates

could be slightly too high or too low. For example, if our estimate of the bias5 is too low,

we would overestimate the yields of Soviet explosions, and Soviet testing near 150 kt would

appear as a series of tests distributed around

a yield value that was above 150 kt. If our estimate of the bias were too high, Soviet testing

5% chapter 7, “Estirnating the Yields of Nuclear Explosions, ”

for an explanation of bias.
34

near 150 kt would appear as a series of tests

distributed around a yield below 150 kt. The

case where we systematically underestimate

Soviet yields and they presume this underestimation is occurring is the only case that provides opportunity for unrecognized cheating.

If this were happening, it could happen only

to the extent that the systematic effect has

been underestimated.

As chapter 7 discusses, the systematic part

of the uncertainty can be significantly reduced

by restricting testing to specific calibrated test

sites. If such calibration were an integral part

of any future treaty, the concern over systematic errors of this kind should be minimal. The

majority of the error that would remain is random. A country considering violating a treaty

could not take advantage of the random error

because it would be unable to predict how the

random error would act.

In early 1987, both the Senate Foreign Relations Committee and the Senate Armed Services Committee held hearings on verification

capabilities in their consideration of advice and

consent to ratification of the 1974 Threshold

Test Ban Treaty and the 1976 Peaceful Nuclear Explosions Treaty. Members of these

Committees wanted to know whether seismic

methods could adequately measure the size of

Soviet underground nuclear tests or whether

more intrusive methods were required. The testimony was often confusing due to the various means of representing statistical uncertainties. For the time being, we will analyze

only the use of statistics and take as given the

underlying information. However, that acceptance is also controversial and is discussed

separately in the chapter on yield estimation

(chapter 7).

The Department of Defense presented the

capabilities of seismic monitoring to the Senate Committee on Foreign Relations on January 13, 1987 and the Senate Committee on

Armed Services on February 26, 1987 in the

following manner:6

‘Testimony of Hon. Robert B. Barker, Assistant to the Secretary of Defense (Atomic Energy) and leader of formal negotiations on Nuclear Test Limitations.

The seismic methods that we currently

must rely onto estimate yields of Soviet nuclear detonations are assessed to have about

a factor-of-two uncertainty for nuclear tests,

and an even greater uncertainty level for Soviet peaceful nuclear explosions.

This uncertainty was then explained as

follows:

This uncertainty factor means, for example, that a Soviet test for which we estimate

a yield of 150 kilotons may have, with 95 percent probability, an actual yield as high as

300 kilotons-twice the legal limit-or as low

as 75 kilotons. 7

These statements are misleading in that they

create the impression that there is a high probability, in fact, almost a certainty, that the

Soviets could test at twice the treaty’s limit

but we would measure the explosions as being within the 150 kt limit. They imply that

a factor of 2 uncertainty means that there is

a high probability that an explosion measured

at 150 kt could, in actuality, have been 300 kt.

Yet as we have seen in the discussion of uncertainty, given a factor of two uncertainty,

the likelihood of an explosion with a yield of

300 kt actually being measured (with 95 percent probability) as 150 kt or below is less than

1 chance in 40.

The chances decrease even further if more

than a single explosion is attempted. For example, the chance of two explosions at 300 kt

both being recorded as 150 kt or less is about

1 in 1,600; and the chance of three explosions

at 300 kt or greater being recorded as 150 kt

or less would be roughly 1 in 64,000. Thus, it

is highly unlikely that explosions could be

repeatedly conducted at 300 kt and systematically recorded as being 150 kt or less.

So far we have been looking only at the likelihood that a test will appear as 150 kt or less.

‘This statement is nearly identical to the wording in the U.S.

Department of State, “Veri~g Nuclear Test Limitations: Possible U.S.-Soviet Cooperation, ” Special Report No. 152, Aug.

14, 1986, which states “A factor of two uncertainty means, for

example, that a Soviet test for which we derive a ‘central yield’

value of 150 kt may have, with a 95 percent probability, a yield

as high as 300 kt or as low as 75 kt. ”
35

From a practical point of view, it must be recognized that the test would not need to look

like 150 kt or less; it would only have to appear as though it were within the error of a

150 kt measurement in order to avoid credible

assertions of non-compliance. Some could misinterpret this as meaning that a test well above

the threshold might have enough uncertainty

associated with it so that its estimate might

appear to be within the expected uncertainty

of a test at 150 kt. They might then conclude

that the opportunity to test well above the

threshold cannot be denied to the Soviets.

Such a one-sided assessment of the uncertainty is extremely misleading because it assumes all of the errors are systematic and can

be manipulated to the evader’s advantage. A

country considering violating the threshold

would also have to consider that even if part

of the systematic uncertainty could be controlled by the evader, the random part of the

uncertainty could just as likely work to its disadvantage. This point was briefly recognized

in the following exchange during a Senate Committee on Foreign Relations hearing:

“

. . . knowing these probabilities, if you

really started to cheat, as a matter of fact you

would take the risk of being out at the far tail.

That would really show up fast. If you set out

to do a 300 kt, you could show up on our seismographs as 450, right?”

Senator Daniel P. Moynihan

“The problem is, if you fired an explosion

at 300 or 350 . . . it could very well look 450

or 500 and the evader has to take that into

consideration in his judgment. ”

Dr. Milo Nordyke,

Director of Verification

Lawrence Livermore

National Laboratory

Also, this analysis assumes that only one

method of yield estimation will be used. Other

methods of yield estimation are also available

and their errors have been shown to be only

partially correlated. The evader would have to

take into account that even if the uncertainty

is known and can be manipulated for one

method of yield estimation, other methods

might not behave in the same manner. Such

considerations would severely diminish the appeal of any such opportunity.

In conclusion, it can be seen that although

the statistical descriptions of the capabilities

of various methods of yield estimation have

been debated extensively, the differences they

represent are often insignificant. There is both

systematic and random uncertainty in the

measurements of Soviet yields. The systematic error would provide only a limited opportunity for cheating, and then only if it was in

the advantage of the cheater. Even in such a

case, only the portion of the error that is systematic can be exploited for cheating. Furthermore, much of the systematic error would be

removed through such treaty provisions as

calibrating the test site. Once the systematic

error had been nearly eliminated, the remaining uncertainty would be random. The random

uncertainty does not provide opportunity for

cheating. In fact, if a country were considering undertaking a testing series above the

threshold, it would have to realize that the random uncertainty would work against it. With

each additional test, there would be a lesser

chance that it would be recorded within the

limit and a greater chance that at least one of

the tests would appear to be unambiguously

outside the limit.

What Constitutes Adequate

Verification?

After the accuracies and uncertainties of

various verification systems have been understood, a decision must be made as to what constitutes an acceptable level of uncertainty. In

1974, when the Threshold Test Ban Treaty was

first negotiated, a factor of 2 uncertainty was

considered to be the capability of seismic methods. At that time, a factor of 2 uncertainty was

also determined to constitute adequate verification.8 Presently, the level of accuracy claimed

80riginally, the factor of 2 uncertainty was established for

the 90 percent confidence level, whereas today it refers to the

95 percent confidence level. It should be noted that a factor

of 2 at the 90 percent confidence level corresponds to about a

factor of 2.5 at the 95 percent confidence level. Thus the accepted level of uncertainty in 1974 was really about a factor

of 2.5 using the present confidence level. The insistence on a

(continued on next page)
36

for the on-site CORRTEX method is a factor

of 1.3.9 This level of 1.3 has subsequently been

defined as the new acceptable level of uncertainty, although many believe it was defined

as such only because it corresponds to the capabilities of this newly proposed system.

It appears that the determination of adequate compliance is a subjective process that

has been influenced by the capabilities of specific monitoring systems. A decision as to what

constitutes adequate verification should not

be determined by the political attractiveness

● of any particular monitoring system, but rather

it should represent a fair assessment of the protection required against non-compliance. In the

frozen river analogy, this would be a fair assessment of how thin the ice must be to deter someone from crossing. Monitoring capability cer-

{continued from previous page)

higher confidence level occurred simply because it was more

convenient to use the 95 percent confidence level which corresponds to 2 standard deviations.

‘See appendix, Hydrodynamic Methods of Yield Estimation.

tainly influences our decision as to whether a

treaty is worthwhile, but it should not influence the standards we set to make that decision. Also, the capability of a monitoring system is just one aspect to be considered, along

with other important issues such as negotiability and intrusiveness.

What constitutes adequate verification may

also vary for different treaty threshold levels.

For example, a factor of 2 uncertainty for monitoring a 100 kt threshold would mean that 95

percent of the measurements at the threshold

limit would be expected to fall within 50 and

200 kt (a total range of 150 kt), while a factor

of 2 uncertainty for monitoring a 1 kt threshold would mean that 95 percent of the measurements at the threshold limit would be expected to fall within% and 2 kt (a total range

of 1.5 kt). A range of uncertainty of 1.5 kt may

not provide the same opportunities or incentives for cheating as a range of uncertainty of

150 kt. Consequently, at lower treaty thresholds,

the significance of a given yield uncertainty

will almost certainly diminish.

THE QUESTION OF DETERMINING COMPLIANCE

In addition to understanding the accuracy

and uncertainty of the verification system, and

deciding on an acceptable level of uncertainty,

a decision will also have to be made as to what

would constitute compliance and non-compliance. Violations of the treaty must be distin-

. guished from errors in the measurements (both

systematic and random) and errors in the test.

This is of particular concern in light of findings by the administration that:

Soviet nuclear testing activities for a number of tests constitute a likely violation of legal obligations under the Threshold Test Ban

Treaty .’”

To examine the context in which this must

be viewed, we can once again return to the frozen river analogy and imagine a situation

‘“’’ The President’s Unclassified Report on Soviet Noncompliance with Arms Control Agreements,” transmitted to the

Congress Mar. 10, 1987.

where we come by and see marks on the ice.

We must then determine whether the marks

indicate that someone successfully crossed the

ice. This could be misleading because all that

we are doing is looking in isolation at the probability that a certain mark could have been

made by someone crossing the ice. Thus the

likelihood that a mark was made by a person

becomes the likelihood that someone crossed

the ice. This, however, is only part of the issue. If we knew for example that the ice were

so thin that there was only a 1 in 10 chance

it could have been successfully crossed, that

the water was deep, and that there was no reason to get to the other side, these factors might

weigh in our determination of whether a mark

was mane-made or not. (Why would a person

have taken such risks to gain no value?) On

the other hand, if we knew the ice were thick

and could be crossed with high confidence, that

the water was shallow, and that real value was

to be obtained by crossing, then we might
37

make a different judgment as to whether the

marks were man-made because there would

really be understandable motivation. Thus the

question of compliance is also dependent on

a judgment reflecting one’s perception of the

advantages that could be obtained through a

violation.

In the case of test ban treaties, there are also

“gray areas” due to the associated error of the

measurements. For example, it must be assumed that a country will test up to the limit

of the treaty, and therefore, some of the estimates would be expected to fall above 150 kt

simply due to random error.11

Assuming that the errors are known and that

apparent violations of the treaty due to such

errors are recognized, there may also be other

violations that cause concern but do not negate the benefits of the treaty. These include

accidental violations, technical violations, and

violations of the “spirit” of the treaty.

Accidental violations are violations of the

treaty that may occur unintentionally due to

the inexact nature of a nuclear explosion. It

is possible that the explosion of a device with

a yield that was intended to be within the limit

of the treaty would produce an unexpectedly

higher yield instead. This possibility was recognized during negotiations of the TTBT. The

transmittal documents which accompanied the

TTBT and the PNE Treaty when they were

submitted to the Senate for advice and consent to ratification on July 29, 1976 included

the following understanding recognized by

both the United States and Soviet Union:

Both Parties will make every effort to comply fully with all the provisions of the TTB

Treaty. However, there are technical uncertainties associated with predicting the precise yields of nuclear weapon tests. These uncertainties may result in slight, unintended

breaches of the 150 kt threshold. Therefore,

the two sides have discussed the problem and

agreed that: (1) One or two slight, unintended

“For example, if the Soviet Union tested 20 devices at 150

kt and we estimated the yields using a system that was described

as having a factor of 2 uncertainty, the probability of measuring at least one of them as being 225 kt or greater is 92 percent.

breaches per year would not be considered a

violation of the Treaty; (2) such breaches

would be a cause for concern, however, and,

at the request of either Party, would be subject for consultations.

Technical violations are violations of the

treaty that do not result in any sort of strategic advantage. An example would be a technical violation of the 1963 Limited Test Ban

Treaty (LTBT) which prohibits any explosion

that:

. . . causes radioactive debris to be present

outside the territorial limits of the State under whose jurisdiction or control such explosion is conducted.12

This prohibition includes the venting of

radioactive debris from underground explosions. Both the United States and the Soviet

Union have accused each other of releasing

radioactive material across borders and of violations of the 1963 LTBT. These violations are

“technical” if the treaty is viewed as an arms

control measure. However, they are material

violations if the treaty is viewed as an environmental protection measure.

Violations of the “spirit” of the treaty are

also of concern. These include, for example, actions which are contrary to the treaty’s preamble. The treaty’s preamble declares the intentions and provides a context for the treaty.

Such declarations, however, are nonbinding.

Another area concerns treaties that have

been signed but never ratified. Both the 1974

TTBT and the 1976 PNE Treaty remain unratified, although they were signed over 10

years ago. Because neither the United States

nor the Soviet Union have indicated an intention not to ratify the treaties, both parties are

obligated under international law (Article 18,

the 1969 Vienna Convention on the Law of

Treaties) to refrain from acts which would defeat their objectives and purposes.

All of these types of violations contribute

to the gray area of compliance versus noncompliance and illustrate why determining com-

‘

2 Article I,b.
38

pliance is a political as well as a technical decision. In the case of monitoring underground

nuclear tests, the actual measured yield that

would constitute clear evidence of a violation

would always be higher than the yield limit

of the treaty. Perhaps an analogy for uncertainties in yield estimates and Soviet compliance under the TTBT is in monitoring a speed

limit of 55 mph. Under the present 150 kt limit,

an observed yield of 160 kt is like comparing

58.7 mph to 55 mph. The police do not give

tickets when their radar shows a speed of 58.7

mph because most speedometers are not that

accurate or well calibrated, and because curves

and other factors can lead to small uncertainties in radar estimates of speed. Similarly, although a 160 kt measurement maybe regarded

by some as a legal lack of compliance, such a

number can well arise from uncertainties in

seismic estimates. At radar measurements

over 65 mph the police do not question that

the 55 mph limit has been exceeded, and the

speeder gets a ticket. With this standard, it

would take a calculated yield of about 180 to

190 kt to conclude that a violation had likely

taken place. As mentioned before, however,

this argument does not mean that the Soviets

could test up to 180 to 190 with confidence,

because the uncertainty could just as likely

work against them. A 180 to 190 kt test might

produce an observed yield well over 200 kt just

as likely as it might produce a yield within the

expected error range of a treaty compliant test.

It must be recognized, however, that the calculated yield for declaring a treaty violation

will always be higher than the limit of the

treaty. Consequently, one or two small breaches

of the treaty could occur within the expected

uncertainty of the measurements. A country

intent on cheating might try to take advantage of this by risking one or two tests within

the limits of the uncertainty range. Even if detected, a rare violation slightly above the permitted threshold could be explained away as

an accidental violation due to an incorrect

prediction of the precise yield of the nuclear

test. This should be kept in mind when choosing a threshold so that small violations of the

limit (whether apparent or real) do not fall in

a range that is perceived to be particularly sensitive.

What is done about violations is an additional problem. In domestic law there are various kinds of violations. Traffic tickets, misdemeanors, felonies, capital crime–all are

different levels. Similarly, in monitoring compliance, there are some things that amount to

traffic tickets and some that amount to felonies. We must decide in which cases violations

or noncompliance are at the heart of a treaty

and in which cases they area marginal problem.
Chapter 3

The Role of Seismology
CONTENTS

Page

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

The Creation of Seismic Waves, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

Types of Seismic Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

Teleseismic Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Body Waves... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Surface Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

Regional Waves. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... 45

Recording Seismic Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

Seismic Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

Figures

Figure No.

3-1.

3-2.

3-3.

3-4.

3-5.

3-6.

3-7.

3-8.

3-9.

3-1o.

3-11.

3-12.

3-13.

Page

Seismic Waves Propagating Through the Earth . . . . . . . . . . . . . . . . . . 42

Body Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

Seismic Radiation Patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

Reflected and Refracted Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

Surface Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

Seismograph Recording of P, S, and Surface Waves From a Distant

Earthquake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

Regional and Teleseismic Signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

Pn Wave . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

Pg Wave . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

Lg Wave . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

Seismic Instrumentation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

Distribution of RSTN Stations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

Beamforming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
Chapter 3

The Role of Seismology

Seismology provides a technical means for monitoring

underground nuclear testing

INTRODUCTION

Verifying a ban on nuclear testing requires

global monitoring systems capable of detecting explosions in the atmosphere, underwater,

and below ground. Tests in the atmosphere and

under water can be readily detected with high

degrees of confidence. The atmosphere is monitored effectively with satellites containing sensors that can detect the visible and nearinfrared light emitted by a nuclear explosion.

The oceans can also be monitored very effectively because water transmits acoustic waves

efficiently. Underwater explosions would be

detected by the acoustic sensors already in

place as part of anti-submarine warfare systems. The most uncertain part of the global

verification system is the monitoring of underground nuclear explosions that might be conducted within the Soviet Union. The main technical tools for monitoring underground nuclear

explosions come from the field of seismology,

which is the study of earthquakes and related

phenomena.

THE CREATION OF SEISMIC WAVES

A nuclear explosion releases its energy in less

than 1/1,000,000 of a second (a microsecond).

The explosion initially produces a gas bubble

containing vaporized rock and explosive material. Within a microsecond, the temperature

within the bubble reaches about 1 million

degrees and the pressure increases to several

million atmospheres. A spherical shock wave

expands outward from the bubble into the surrounding rock, crushing the rock as it travels.

As the hot gases expand, rock is vaporized near

the point of the explosion and a cavity is

created.

ing medium. Eventually, the shock wave

weakens to the point where the rock is no

longer crushed, but is merely compressed and

then returns (after a few oscillations) to its

original state. These cycles of compression and

relaxation become seismic waves that travel

through the Earth and are recorded by seismometers. By looking at seismic records (seismograms) and knowing the general properties

of the travel paths of various waves, seismologists are able to calculate the distance to the

seismic event and what type of motion caused

the wave.

While the cavity is forming, the shockwave

continues to travel outward into the surroundTYPES OF SEISMIC WAVES

As with earthquakes, seismic waves result- eling along the Earth’s surface (surface waves).

ing from an explosion radiate outward in all The body and surface waves that can be

directions (figure 3-l). These waves travel long recorded at a considerable distance (over 2,000

distances either by passing deep through the km) from the earthquake or explosion that cre

body of the Earth (body waves), or else by trav- ated them are referred to by seismologists as

41
42

Figure 3-1 .—Seismic Waves Propagating Through

Sourc e=.

An earthquake or underground explosion generates seismic

waves that propagate through the Earth.

SOURCE: Office of Technology Assessment, 1988.

teleseismic waves. Teleseismic body and surface waves are important to monitoring because to be recorded they do not usually require a network of seismometers near the

source. This allows for seismometers to be located outside the region that is being monitored.

Teleseismic waves contrast with what seismologists call regional waves. Regional waves

are seismic waves that travel at relatively high

frequencies within the Earth’s crust or outer

layers and typically are observed only at distances of less than 2,000 km. Regional waves

are therefore recorded at seismic stations that,

for application to monitoring Soviet nuclear

explosions, would have to be located within the

territory of the Soviet Union. Such stations

are called in-country or internal stations.

When seismic waves reach a seismic station,

the motion of the ground that they cause is

recorded by seismometers. Plots of the wave

forms are called seismograms. Because the

different waves travel at different speeds and

along different routes, they arrive at seismic

stations at different times. The farther away

the seismic station is from the source of the

waves, in general, the more dispersed in time

the different arrivals will be. By studying seismograms, seismologists are able to recognize

the various types of waves produced by events

such as earthquakes and nuclear explosions.

TELESEISMIC WAVES

The various types of teleseismic waves are

differentiated by both the paths along which

they travel and the type of motion that enables them to propagate. The two main subdivisions (body waves and surface waves) are distinguished by the areas through which they

travel.

Body Waves

The waves traveling through the body of the

Earth are of two main types: compressional

waves (also called P waves) and shear waves

(also called S waves). The designations P and

S are abbreviations originally referring to

primary and secondary arrivals; compressional

waves travel faster than shear waves and, consequently, arrive first at a recording station.

Apart from their different speeds, P waves and

S waves also have different characteristics.

P waves travel in a manner similar to sound

waves, that is, by molecules “bumping” into

each other resulting in compression and dilation of the material in which they propagate.

A cycle of compression moves through the

Earth followed by expansion. If one imagined

a particle within the wave, its motion would

be one of shaking back and forth in the direction of propagation in response to the alternating compressions and expansions as the

wave trains move through. The particle motion is in the direction of travel and the wave

can propagate through both solids and liquids.

A sudden push or pull in the direction of wave

propagation will create P waves (figure 3-2a).

In contrast, S waves propagate by molecules

trying to “slide” past each other much as happens when one shakes a rope from side to side

and the disturbance passes down the rope. The
43

Figure 3-2.—Body Waves

P wave I Compressions -1 Undisturbed medium

– Dilatations

S wave

I Amplitud e

I

1

—-------- 

Wavelength

Waves traveling through the body of the Earth are of two main types: compressional waves (P waves) and shear waves (S waves).

SOURCE B A Bolt, Nuclear Explosions and Earthquakes, W H Freeman & Co , 1976

wave motion is at right angles to the direction

of travel. Any particle affected by the wave

would experience a shearing motion as the

wave passed through. Because liquids have little resistance to shearing motion, S waves can

only pass through solid materials. (Liquids can

be compressed, which is why P waves can

travel through both solids and liquids.) A sudden shearing motion of a solid will result in

S waves that will travel at right angles to the

direction of propagation (figure 3-2 b).

In a P wave, particle motion is longitudinal

in the direction of propagation, thus there can

be no polarization of a P wave. S waves,

however, being transverse, are polarized and

can be distinguished as either horizontal or vertical. Particle motion in an SH wave is horizontal. In an SV wave, particles oscillate in a vertical plane perpendicular to the path.

An underground explosion creates a uniform

pressure outward in all directions. Therefore,

explosions should be a source of nearly pure

P waves. In practice, however, some S waves

are also observed. These waves are usually due

to asymmetries of the cavity or the pressure

within the cavity created by the explosion,

structural inconsistencies in the surrounding

rock, or the presence of pre-existing stresses

in the host rock that are released by the explosion. An earthquake, on the other hand, is

generally thought of in terms of blocks of the

Earth’s crust breaking or slipping past one

another along a fault region. Because of the

shearing motion, an earthquake creates mostly

S waves. P waves are also generated from an

earthquake, but only in a four-lobed pattern

that reflects the opposite areas of compression

and expansion caused by the shearing motion

(figure 3-3). As discussed in chapter 5, this

difference in source geometry can be used as

a means of distinguishing the seismic signals

caused by explosions from the seismic signals

generated by earthquakes.
44

Figure 3.3.—Seismic Radiation Patterns

Explosion Earthquake

The pattern of seismic body waves generated

by a given source becomes complicated when

the waves interact with the structures of the

Earth. In general, when a wave hits a boundary within the Earth, such as where two different rock types meet, both reflected and transmitted P and S waves are generated at several

different angles. (figure 3-4)

Because of all these possibilities, P waves

and S waves break into many types as they

travel through the Earth. At a depth of 30 to

60 kilometers in the Earth, the velocity with

which sound passes through the Earth increases markedly. This discontinuity, called

the Mohorovicic Discontinuity (Moho), serves

as a wave guide to trap seismic energy in the

upper crust of the Earth.

Surface Waves

At the Earth’s surface, two additional seismic wave types are found. These surface waves,

called Rayleigh waves and Love waves,1 are

produced by constructive interference of body

wave energy in the upper layers of the crust.

The particle motion of Rayleigh waves is

somewhat analogous to that of ripples spreading over the surface of a lake. The analogy is

not exact, however, because unlike a water

wave, the orbital motion of a particle in a Ray-

‘These waves are named after the mathematicians who first

developed the theory to describe their motion (Lord Rayleigh

and A.E.H. Love).

Figure 3-4.– Reflected and Refracted Waves

Reflected

Incident s

s

Reflected

Medium 1

Medium2 

Refracted S

When a wave hits a boundary within the Earth both reflected

and refracted waves can be generated.

leigh wave is retrograde to the path of wave

propagation (figure 3-5a). The energy of the

Rayleigh wave is trapped near the ground surface, so as depth increases, the rock particle

displacements decrease until, ultimately, no

motion occurs.

The second type of surface wave which

travels around the Earth is called a Love wave.

Love waves have a horizontal motion that is

shear, or transverse, to the direction of propagation (figure 3-5b). It is built up of trapped

SH waves and has no vertical motion.

While Rayleigh waves can be detected by

seismometers sensitive either to vertical or

horizontal motion of the Earth’s surface, Love

waves are only detected by seismometers sensing horizontal motions.

Surface waves of either type, with a period

of 10 seconds, travel with a velocity of about

3 kilometers per second. This corresponds to

a wavelength of about 60 kilometers. Because

of this long wavelength, surface waves are

much less likely to be affected by small-scale

variations in Earth structure than short period

body waves. This makes the use of surface

waves attractive for measuring yields of Soviet tests. Body waves (P waves and S waves)

are faster than surface waves and therefore arrive first at a recording station. This is fortunate because the surface waves have larger am-
45

Figure 3-5.-Surface Waves

Rayleigh wave

Love wave

Waves traveling along the surface of the Earth are of two main types: Rayleigh waves and Love waves.

SOURCE: B.A Bolt, Nuclear Explosions and Earthquakes, W.H. Freeman & Co., 1976.

plitudes than the body waves. If they all unless the records were filtered to take advanarrived simultaneously, the smaller amplitude tage of the different frequency content of the

P waves would be hidden by the surface waves two types of waves (figure 3-6).

REGIONAL WAVES

In contrast with the teleseismic waves described above, regional waves are usually observed only at distances of less than 2,000 km.

In general they have larger amplitudes and

higher frequency content then waves from the

same source recorded at teleseismic distances.

Depending on their propagation characteristics, such regional waves are denoted by Pn

,

Pg, Sn, and Lg. A comparison of teleseismic

and regional seismic waves is illustrated in figure 3-7. Note the different time scales. The amplitude scales are also different.

At regional distances, Pn is usually the first

wave to arrive at any given station. It is a wave

that goes down through the crust of the Earth,

then travels mostly horizontally near the top

of the upper mantle (in contrast to the usual

body, which goes deeply within the mantle),

and finally travels upward through the crust,

where the receiver is located. This path is

shown in the upper part of figure 3-8. In the

lower part are shown some of the multiple

bounces that also contribute to the Pn wave.

The Moho marks the boundary between the

crust and the upper mantle.

Pg is a wave that comes in later than Pn, and

travels the path between source and receiver

wholly within the Earth’s crust. As shown in
46

Figure 3-6.—Seismograph Recording of P, S, and Surface Waves From a Distant Earthquake

P

Surface s waves

A

Horizontal ground motion

SOURCE: Modified from F. Press and R. Seiver, Earth, W.H. Freeman & Co., San Francisco, 1974.

figure 3-9, Pg is thought to be guided by a

boundary layer within the crust. It does not

propagate to as great a distance as Pn.

Sn is a shear wave that travels a path very

similar to that of Pn

, but arrives later. Sn arrives later because it is composed of shear

waves, which propagate slower than the P

waves that make up Pn.

For purposes of explosion monitoring, L

can be the most important of the regional

waves because it is typically the largest wave

observed on a seismogram at regional distances. Lg is a type of guided shear wave that

has most of its energy trapped in the Earth’s

crust. In fact, this wave can be so strong that

(contrary to what is implied by its inclusion

in the class of “regional waves”) L for a large

explosion can be observed even at teleseismic

distances, i.e. well in excess of 2,000 km. The

observation of Lg out to teleseismic distances,

however, can only occur across continents. Beneath an ocean, where the crust is much thinner, Lg fails to propagate even short distances.

Figure 3-10 illustrates how the Lg wave

propagates. At the top is shown a seismic

source within the Earth’s crust. Energy departing downward is partially reflected in the

crust and partially transmitted down into the

mantle. However, for waves that travel in the

more horizontal direction, as shown in the middle part of the figure, energy cannot get into

the mantle and is wholly reflected back into

the crust. The type of reflection occurring here

is the total internal reflection that is similar

to that occurring within the prisms of a pair

of binoculars. For a fixed source and receiver,

there may be many reflection paths, all totally
47

Figure 3-7.—Regional and Teleseismic Signals

Regional signal

Pn Pg

S n

1 1 I

o 50 100

Time (seconds)

I I

o 200 400

Time (seconds)

Upper seismogram is of a “regional distance” from the source. Signals here are associated with waves that propagate in the

crust and upper most mantle. Lower seismogram is of a “teleseismic distance. ” This wave has propagated through the deep

interior of the Earth.

SOURCE” Modified from Defense Advanced Research Projects Agency.

Figure 3-8.—Pn Wave Figure 3-10.—Lg Wave

Source Receiver

Source Crust

(slow; low attenuation)

Moho

\

Pn - wave

Figure 3-9.-Pg Wave

Source Receiver

Lower crust

Moho

Mantle

(faster; higher attenuation)

Total reflection

Wave guide. Many “trapped” rays in continental

Receiver

All totally reflected

Lg - wave

crust.

Moho

Pg - wave
48

reflected and thus trapped within the crust.

This is illustrated in the bottom of the figure.

The Lg wave is composed of the entire family

of trapped waves, with the crest acting as a

wave guide. For regions in which the crust is

composed of material that transmits seismic

waves efficiently, Lg may be recorded at distances of thousands of kilometers.

For most of the last 30 years, regional waves

have been thought of mainly in the context of

monitoring small explosions which may not be

detected teleseismically. In recent years, however, Lg has come to be recognized as useful

for estimating yields for large explosions, independent of the more conventional body wave

and surface wave measurements (see ch. 7).

RECORDING SEISMIC WAVES

Seismic waves are measured at observatories around the world by recording the ground

motion. Most observatories have “triaxial”

seismometers, meaning that they record

ground motion in three directions at right angles to each other. Typically they are oriented

north-south, east-west, and vertical. By having all three components, seismologists can

reconstruct the complete three-dimensional

ground motion from the seismograms.

The principle by which the seismometers

work can bethought of as a heavy mass freely

supported by a spring from a frame fixed to

the Earth. When an earthquake or explosion

occurs, seismic waves traveling through the

Earth reach the seismometer. The frame is

shaken in response to the motion of the wave.

Although the frame is displaced by the ground

motion, the heavy mass tends to remain stationary because of its inertia. The displacement

of the grounded frame relative to the stationary mass is therefore a measure of the ground

motion. This movement is then electronically

magnified so that displacements as small as

0.00000001 centimeters (the same order as

atomic spacings) can be detected.

If the Earth were perfectly still, recording

small earthquakes and underground explosions would be easy. However, processes such

as the winds, ocean waves, tides, man’s activity, and seismic waves generated by earthquakes in other regions continually cause motions of the Earth. All of this motion is sensed

by seismometers and recorded. Although seismic instruments are sensitive enough to detect seismic waves generated by even the

smallest explosion, it will be seen in the following chapters that naturally occurring background noise levels are the limiting factor in

detecting small earthquakes and explosions.

To reduce the background noise caused by

wind and other surface effects, seismometers

have been designed to fit into torpedo-shaped

casings and placed in narrow boreholes at a

depth of about 100 meters. These types of stations are currently being used as part of the

Regional Seismic Test Network (RSTN). The

RSTN is a prototype system designed to evaluate the utility of in-country stations that could

be placed within the Soviet Union to monitor

a low-yield or comprehensive test ban treaty.

A typical high quality installation contains

three primary seismometers mounted in mutually perpendicular orientations along with

three back-up seismometers (figure 3-11).

The seismometer installation is protected

against tampering both electronically and

through the inherent ability of the instrument

to sense disturbances in the ground. The borehole package sends data to a surface station

that contains transmitting and receiving

equipment and in some cases to an antenna

for satellite communications. Also contained

in surface stations that might be used for monitoring are power sources, environmental con-
49

Figure 3-11 .—Seismic Instrumentation

Photo credits: Sandia National Laboratories

Seismic station downhole package containing

seismometers, authentication circuits, and

processing electronics

Three seismometers mounted

in mutually perpendicular

orientations

Seismometer
50

Photo credit: Sand/a National Laboratories

An example of what an internal seismic station

might look like.

trol apparatus, and tamper-protection equipment. The distribution of six of these stations

in North America is designed to simulate the

distribution of 10 internal stations within the

Soviet Union (figure 3-12).

Figure 3-12.— Distribution of RSTN Stations

Six Regional Seismic Test Network (RSTN) stations are distributed throughout North American to simulate the distribution of 10 internal stations within the Soviet Union.

SOURCE: Modified from Sandia National Laboratories.

SEISMIC ARRAYS

The use of seismic arrays has been proposed

as an alternative and/or supplement to single

three-component seismic stations. A seismic

array is a cluster of seismometers distributed

over a fairly small area, usually on the order

of a few kilometers. The signals from the various instruments are then combined to improve

the detection and identification of any small

or weak signals that maybe recorded. Seismic

arrays can function like phased array radar

receivers, sensitive to waves from a particular direction while excluding waves from other

directions. By doing so, arrays can pull small

signals out from the surrounding background

noise.

Figure 3-13.—Beamforming

To test the application of arrays for monitoring regional seismic events, the United

States and Norway installed the Norwegian

Regional Seismic Array (NORESS) in 1984.

NORESS is located north of Oslo and consists

of 25 individual sensors arranged in 4 concentric rings with a maximum diameter of 3 km.

Beamforming is a process of shifting and adding recorded

waveforms to emphasize signals and reduce noise. The original waveforms (a) can be shifted to remove propagation delays (b). The signal waveforms are now aligned but the noise

is not. The average of the shifted waveforms in (b) would be

the beam.

SOURCE: Energy & Technology Review, Lawrence Livermore National Laboratory, August 1986, p. 13.
51

To imagine how an array works, consider an

example where a seismic wave is coming from

a known location and with a known speed. The

time it takes for the wave to travel to each sensor in the array can be predicted from the

known direction and speed. In contrast, the

seismic background noise will vary randomly

from sensor to sensor. The recorded signals

from each sensor in the array can then be

shifted in time to allow for propagation across

the array and combined (figure 3-13). The sensors are close enough so that the signal from

the coherent source is nearly the same from

each seismometer and remains unchanged. The

background noise varies more rapidly and cancels out when the records are added together.

Thus, it is possible to suppress the noise relative to the signal and make the signal easier

to detect. This process of shifting and adding

the recorded waveforms is called beamforming. The combination of the shifted waveforms

is called the beam.

Arrays do, however, have some drawbacks

compared to three-component single stations:

1.

2.

3.

A

they are more expensive because they require more hardware and higher installation costs;

they require access to a larger area; and

they have more sensors that, in turn, generate more data that must be transmitted, stored and processed.

comparison of the advantages and disadvantages of arrays versus three-component

single stations is presented in chapter 4.
Chapter 4

Detecting Seismic Events
CONTENTS

Page

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

The Meaning of ’’Detection” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

Locating Seismic Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

Defining Monitoring Capability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

Seismic Magnitude . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

Converting Magnitude to Explosive Yield. . . . . . . . . . . . . . . . . . . . . . . . . . 60

Seismic Monitoring in Probabilistic Terms . . . . . . . . . . . . . . . . . . . . . . . . . 60

Limitations to Seismic Monitoring Capability . . . . . . . . . . . . . . . . . . . . . . . . 61

Seismic Noise.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

Reduction of Signals at Source or Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . 61

Seismic Instrumentation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

Seismic Magnitude Estimation Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

Seismic Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

Existing Networks and Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

Planned Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

Hypothetical Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

Seismic Monitoring Capability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

Calculating Seismic Monitoring Capability . . . . . . . . . . . . . . . . . . . . . . . . . 68

Global Detection Capability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

Detection Capability Within the U.S.S.R. Using No Internal Stations . . 69

Detection Capability Within the U.S.S.R. Using Internal Stations . . . . . 69

Considerations in Choosing Monitoring Thresholds . . . . . . . . . . . . . . . . . . 70

Use of High-Frequency Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

Arrays v. Three-Component Stations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

Box

Box Page

4-A. Locating A Seismic Event. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

Figures

Figure No. Page

4-1. Seismic Signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

4-2. Seismic Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

4-3. Effect of Noise on Event Magnitude Computation. . . . . . . . . . . . ., . . . 63

4-4. Contributing Seismograph Stations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

4-5. Worldwide Standardized Network Station Distribution . . . . . . . . . . . . . 66

4-6. Detection Capability of a Seismic Station . . . . . . . . . . . . . . . . . . . . . . . . 68

4-7. Example of Calculated Detection Capability . . . . . . . . . . . . . . . . . . . . . . 71
Chapter 4

Detecting Seismic Events

The first requirement for a seismic monitoring network is to detect and locate

seismic events that could have been caused by an underground nuclear

explosion.

INTRODUCTION

The first requirement for a seismic monitoring network is that it be capable of detecting

seismic events. If the Earth were perfectly

quiet, this would be easy. Modern seismometers are highly sophisticated and can detect

remarkably small motions. However, processes

such as the winds, ocean waves, and even rush

hour traffic continually shake the Earth. All

of this ground movement is sensed by seismometers and creates a background from which

signals must be recognized. Seismic networks,

consisting of groups of instruments, are designed to detect events like earthquakes and

distinguish them from normal background

noise. The extent to which a seismic network

is capable of detecting events, referred to as

the network’s detection threshold, is dependent on many factors. Of particular importance

are the types of seismic stations used, the number and distribution of the stations, and the

amount of background noise at the station locations. This chapter reviews these factors and

discusses the capability of networks to detect

seismic events within the Soviet Union.

THE MEANING OF “DETECTION”

In practical terms, detecting a seismic event

means more than observing a signal above the

noise level at one station. There must be enough

observations (generally from more than one

station) to estimate the location of the event

that created the detected signal. Measurements of the seismic waves’ amplitudes and

arrival times must be combined according to

standard analytical techniques to give the location and origin time of the event that generated the signal. The event could be any of several natural or man-made phenomena, such as

earthquakes, nuclear or chemical explosions,

meteorite impacts, volcanic eruptions, or rock

bursts.

The seismic signals from these events travel

from their source to individual seismic recording stations along different pathways through

anon-uniform Earth. Consequently, no two signals recorded at separate stations will be identical. Even if they came from the same source,

the amplitudes, shape, and transit times of the

signals would vary according to the path they

took through the Earth. This fact has an important impact on the results of the calculations used to determine magnitude and location.

The results will never have perfect precision.

They will be based on averages and will have

associated with them statements of what the

possible errors in the most probable solution

might be. Origin times and locations of seismic events, the parameters that make up a “detection, ” are always based on some averaging

of the data from individual stations. To determine the origin time, and location of a seismic

event, the data from several stations must be

brought together at a single place to carry out

the required analysis. Seismic stations that

routinely send their data to a central location

for analysis are said to form a network.

The energy radiating from an underground

nuclear explosion expands outward. While

most of the explosive energy is dissipated by

crushing and melting the surrounding rock, a

55

76-584 0 - 88 : QL 3 - 3
56

small fraction is transformed into seismic

waves. These seismic waves propagate outward and, in so doing, encounter various boundaries and rock layers both at the surface and

deep within the Earth. The boundaries cause

a separation of the seismic waves into a variety of wave types, some of which travel deep

through the interior of the Earth (body waves)

and some of which travel along the surface (surface waves). See chapter 3 for a full discussion

of wave types and travel paths.

Seismic signals are detected when they are

sufficiently above the background noise in

some frequency band. Figure 4-1 shows seismograms with standard filters designed to enhance the detection of distant events. In this

case, the signal can be seen because it is larger

than the noise at high frequency. When data

are in digital form (as they are in the latest

generation of seismic instrumentation), filters

are applied in various frequency bands before

detection processing.

Relatively sophisticated techniques can now

be used to detect a seismic signal in the presence of noise. In those cases where three perpendicular components of ground motion are

recorded using a vertical and two horizontal

component seismometers, use can also be made

of the known particle motion of P waves to

differentiate a P wave signal from background

noise. Another technique which can enhance

the probability of detecting a signal requires

a number of closely spaced seismic sensors

known as an array. The data recorded by these

sensors can be summed together in a manner

which takes account of the expected signal

propagation time across the array. The array

enhances signals from great distance that propFigure 4-1.—Seismic Signals

Signal

Small seismic signals in the presence of seismic noise at four different stations.

SOURCE: U.S. Geological Survey.
57

agate vertically through the array and can reject noise that travels horizontally. Therefore,

the array summation process tends to enhance

the signal and to reduce the noise.

For many years, most seismic verification

efforts in the United States concentrated on

the use of teleseismic signals. Teleseismic signals are seismic waves which travel to distances greater than 2,000 km and go deep

through the interior of the Earth. Teleseismic

waves are used because all seismometers monitoring Soviet testing are located outside the

U.S.S.R. and generally at distances which are

teleseismic to the Soviet test sites. The possibility of establishing U.S. seismic stations

within the U. S. S. R., close to Soviet nuclear

testing areas, was discussed seriously as part

of negotiations for a Comprehensive Test Ban

Treaty. Because techniques for monitoring explosions with regional signals were not as well

understood as techniques for monitoring with

teleseismic data, research efforts to improve

regional monitoring greatly increased during

the late 1970s. Regional distances are defined

to be less than 2,000 kilometers and waves

propagating to this distance travel almost entirely through the Earth’s outer crustal layers.

LOCATING SEISMIC EVENTS

The procedure for estimating the location of

a seismic event, using seismic data, involves

determining four numbers: the latitude and longitude of the event location, the event depth, and

the event origin time. Determination of these

four numbers requires at least four separate

measurements from the observed seismic signals. These values are usually taken as the arrival time of the P wave at four or more different seismic stations. In some cases, however,

determination of the numbers can be accomplished with only two stations by using the

arrival times of two separate seismic waves

at each station and by using a measure of the

direction of the arriving signal at each station.

A relatively poor estimate of location can also

be obtained using data from only a single array.

The event location process is an iterative one

in which one compares calculated arrival times

(based on empirical travel-time curves) with

the observed arrival times. The differences between calculated and observed arrival times

are minimized to the extent possible for each

station in the process of determining the location of the seismic event.

All seismically determined event locations

have some error associated with them. The error results from differences between the expected travel time and the actual travel time

of the waves being measured and from imprecision in the actual measurements. Generally,

these errors are smallest for the P wave (the

first signal to arrive), and this is why the capability to detect P waves is emphasized. Location errors are computed as part of the location estimation process, and they are usually

represented as an ellipse within which the

event is expected to be. Generally, this computation is made in such a way that there is

a 95 percent probability that the seismic event

occurred within the area of the error ellipse.

Thus, location capability estimates are usually

given by specifying the size of this error ellipse.

Similarly, there is always an uncertainty associated with the measured depth of the event

beneath the Earth’s surface.
58

Box 4-A.—Locating A Seismic Event

Estimating the location of a seismic event can be compared to deducing the distance to a lightning bolt by timing the interval between the arrival of the flash and the arrival of the sound of

the thunder. As an example, consider the use of the P wave and S wave, where the flash is the firstarriving P wave and the thunder is the slower traveling S wave. The time interval between the

arrival of the P wave and the arrival of the S wave (which travels at about half the speed of the

P wave), increases with distance. By measuring the time between these two arrivals and knowing

the different speeds the two waves travel, the distance from the event to the seismometer could

be determined (figure A). Knowing the distance from several stations allows the location to be pinpointed (figure B).

2000 4000 6000 6000 10,000

Distance from earthquake (km)

The time required for P, S, and surface waves to travel a given distance can be represented by curves on a graph of travel time against

distance over the surface. To locate an earthquake the time interval

observed at a given station is matched against the travel-time curves

for P and S waves until the distance is found at which the separation

between the curves agrees with the observed P-S time difference.

Knowing the distance from the three stations, A, B, and C, one can

locate the epicenter as in figure B.

Knowing the distance, say XA, of an earthquake from a given

station, as by the method Figure A, one can say only that the

earthquake lies on a circle of radius A, centered on the station. If,

however, one also knows the distances from two additional

stations B and C, the three circles centered on the three stations,

with radii XA

, XB, Xc, intersect uniquely at the point Q, the

location of the seismic event.

SOURCE: This example is taken from Frank Press and Raymond Seiver, Earth (San Francisco, CA: W. H. Freeman and Company, San Francisco, CA, 1974).

DEFINING MONITORING CAPABILITY

Seismic Magnitude of these tasks is generally described as a funcA seismic monitoring system must be able tion of a measure-called the seismic magnitude

to detect the occurrence of an explosion, to of an event.

locate the explosion, to identify the explosion, Seismic magnitude was first developed as

and to estimate the yield of the explosion. The a means for describing the strength of an earthcapability of a seismic network to perform each quake by measuring the motion recorded on
59

a seismometer. To make sure that the measurement was uniform, a standard method was

needed. The original calculation procedure was

developed in the 1930s by Charles F. Richter,

who defined local magnitude, ML, as the logarithm (to the base 10) of the maximum amplitude (in micrometers) of seismic waves observed

on a Wood-Andersen torsion seismograph at

a distance of 100 kilometers (60 miles) from

the earthquake.

Subsequently, the definition of seismic magnitude has been extended, so that the measurement can be made using different types of

seismic waves and at any distance. For body

waves, the equation for seismic magnitude mb

is:

mb = log (A/T) + B(d,h),

where A is the maximurn vertical displacement

of the ground during the first few seconds of

the P wave, and T is the period of the P wave.

The B term is a correction term used to compensate for variations in the distance (d) between the seismic event and the recording station and the depth (h) of the seismic event. For

a seismic event at the surface of the Earth,

h=0. The B correction term has been determined as a function of d and h by observing

seismic signals from a large number of earthquakes.

For surface waves, seismic magnitude Ms

can be calculated by a similar equation:

M s = log (A/T) + b log d + C,

where b and c are numbers determined from

experience. A number of formulas, involving

slightly different values of b and c for MS,

have been proposed.

The terms in both the body wave and surface wave magnitude equations that are used

to compensate for distance reflect an important physical phenomenon associated with

seismic wave propagate. This phenomenon, referred to as attenuation, can be simply stated

as follows: the greater the distance any particular seismic wave travels, the smaller the

wave amplitude generally becomes. Attenuation of wave amplitude occurs for a number

of reasons including:

1. the spreading of the wave front over a

greater area, thereby reducing the energy

at any one point on the wavefront;

2. the dissipation of energy through natural absorption processes; and

3. energy redirection through diffraction,

refraction, reflection and scattering of the

wave at various boundaries and layers

within the Earth.

As a consequence, a correction term is needed

to obtain the same magnitude measurement

for a given seismic event from data taken at

any seismic station. The correction term increases the amplitude measurement to compensate exactly for the amplitude decrease

caused by the different attenuation factors.

Therefore, if the amplitude of the seismic

wave is to be used to estimate the size of the

seismic event (whether it is an explosion or a

naturally occurring earthquake), a good understanding of how amplitude decreases with distance is needed for both body and surface

waves. The distance-dependent numbers in the

body and surface wave equations represent

average corrections which have been developed

from many observations. In general, these corrections will not be exact for any one particular path from a particular seismic event to a

particular sensor. It is important, therefore,

either to calibrate the site to receiver path or

to compute the magnitude of the event using

seismic signals recorded at a number of wellcalibrated stations. If multiple stations are

used, the event magnitude is calculated by

averaging the individual station magnitudes

for an event. From this procedure, an average

body wave magnitude (rob) and an average

surface wave magnitude (Ms are determined

for the event.

Obviously, the distance the wave has traveled must be known to determine the attenuation in amplitude and correct for it. Therefore

the seismic event must be located before its

magnitude can be determined.
60

Converting Magnitude

to Explosive Yield

The detection and identification capabilities

of seismic networks are described most conveniently in terms of seismic magnitudes, typically mb

. This measure is used because mb is

directly related to seismic signal strength.

When interpreting capabilities in terms of explosive yield, however, an additional step is

required to translate mb to kilotons. The same

magnitude value can correspond to yields that

range over a factor of about 10. Variations in

the magnitude-yield relationship are caused by

variations in the structure of the Earth in the

vicinity of the test site (low signal attenuation

versus high signal attenuation areas), the material in which the explosion is emplaced (hard,

water-saturated rock versus dry, porous materials), and the way in which the explosion is

emplaced (tamped versus detonated in a large

cavity designed to muffle the signal).

For example, if an explosion is “well-coupled,”

that is, if the energy is well transmitted from

the explosion to the surrounding rock, an mb

of 4.0 corresponds to an explosion of about 1

kt. This relationship is true only for explosions

in hard rock and may vary considerably depending on how well the seismic waves are

transmitted through the area’s geology. In

areas that are geologically old and stable, seismic waves are transmitted more efficiently. An

mb of 4.0 produced by a well-coupled explosion in an area of good transmission might correspond to an explosion much smaller than a

kt. In areas that are geologically young and

active, seismic waves are not transmitted as

efficiently and an mb of 4.0 may correspond

to a well-coupled explosion larger than 1 kt.

Even greater changes in the relationship between mb and yield can occur if the explosion

is intentionally “de-coupled” from the surrounding rock in a deliberate attempt to muffle the seismic signal. As we will see in chapter 6, decoupling can be accomplished at low

yields under some situations by detonating the

blast in a large underground cavity. Through

such evasion methods, the same 1 kt explosion

that produced a magnitude mb of 4.0 when

“well-coupled” might be muffled down to a

seismic signal of around mb 2.() at low frequencies. Lesser reductions can be accomplished by detonating the explosion in dry porous material.

Because the yield that corresponds to a specific mb depends so much on the scenario that

is being discussed, seismologists generally use

seismic magnitude to describe monitoring capabilities. In translating seismic magnitudes

to yields, the reader must consider the context

in which the comparison is made. In particular,

it should be considered whether the explosion

is being recorded in an area of good transmission and whether the explosion is well-coupled.

Unless specifically stated, this report translates seismic magnitudes to yields corresponding to “tamped” conditions, that is, a wellcoupled explosion in hard rock. Situations

where decoupling is feasible, and the effects

of such decoupling, are discussed in chapter 6.

Seismic Monitoring in Probabilistic

Terms

Whether seismic measurements are made by

hand or by computer, some error is involved.

Even greater additional errors arise from the

imperfect estimates of how well seismic waves

travel through different parts of the Earth and

how well seismic energy is coupled to the Earth

during the explosion. All of these errors result

in some uncertainty in the final determined parameters. This is true whether these parameters are event magnitude, location, identification characteristics, or yield. In all cases,

however, it is possible to estimate a confidence

factor in probability terms for the determined

parameter. It is important to realize, therefore,

that while the numbers are not presented with

100 percent certainty, estimates of the uncertainty are known. In general, this uncertainty

is greatest for the small events and decreases

for the larger events. A discussion of the uncertainty and what it means in terms of national security concerns is presented in chapter 2.
61

LIMITATIONS TO SEISMIC

The strength of a seismic signal diminishes

with distance. In general, the closer the seismic station is to the source, the stronger the

signal will be. Hence, a principal element of

monitoring strategy is to get close to areas of

concern. It follows that the more high quality

stations distributed throughout a given area,

the greater the capability will be to detect small

events.

Seismic Noise

As noted previously, if the Earth were perfectly still, detecting even the smallest seismic event would be easy. However, the Earth’s

surface is in constant motion. This motion is

the result of many different energy sources.

Major storms over ocean areas and the resultant wave action on continental shores cause

significant noise in the 2- to 8-second band.

Wind noise and noise from atmospheric pressure fronts are particularly prominent on

horizontal-component seismic recordings. These

more or less continuous motions of the Earth

are referred to as seismic noise or microseisms

(figure 4-2). For purposes of siting a seismic

station, it is highly desirable to find an area

that has a low background level of seismic

noise. Generally, the lower the background

seismic noise at any station, the smaller will

be the seismic signal which can be detected at

that station.

Cultural activities can also generate seismic

noise that appears in the frequency range used

to monitor nuclear explosions. Generally, this

man-made noise has frequencies higher than

1 Hz. Heavy machinery, motors, pumping stations, and mills can all generate observable

seismic noise. However, careful siting of seismic stations can minimize the problem of most

man-made seismic noise. From a monitoring

point of view, it is important that noise surveys be made prior to the final selection of sites

for seismic stations. If such sites are negotiated within other countries, provisions should

be made for relocating the sites should seismic noise conditions change.

MONITORING CAPABILITY

Seismic signals and noise are concentrated

in various frequency bands. Only the noise

within the frequency bands in which seismic

signals are observed is a problem. Even strong

noise can be eliminated by filtering as long as

the noise is outside the detection bands of interest.

The possibility also exists that a seismic signal from one event can be masked by the seismic signal from another event. While this does

indeed happen, it is only a problem for monitoring at yields around 1 kt or less without internal stations. For events of interest in the

U.S.S.R. above a magnitude of about mb 4.0,

there are a sufficient number of stations detecting the event so that masking of the signal at a couple of stations generally poses no

serious problem. For events much below mb

4.0, a number of stations at regional distances

(distances less than about 2,000 km) would

have to be used to avoid the masking problem.

Such stations would be available if the United

States obtains access to data from seismic stations placed within the U.S.S.R.

negotiated agreement between

States and the Soviet Union.

Reduction of Signals

Source or Sensor

as part of a

the United

at

Poor coupling to the geologic media, either

at the explosion source or at the seismic sensor, will act to reduce the amplitude of the seismic signal received. If the explosion is in dense

hard rock or in water saturated rock, the source

coupling will be good. If the explosion is in alluvium, dry porous rock, or within a cavity,

the coupling will not be as good. Decoupling

an explosion by detonation within a cavity is

an important evasion scenario which will be

discussed in chapter 6.

At the seismic sensor, signal reduction can

occur if the sensor is not placed on hard rock.

In particular, if there is a layer of soil upon

which the sensor sits, the signal-to-noise ratio

at the sensor can be far less than if the sensor
62

Figure 4-2.—Seismic Noise

-

Background seismic noise at three different stations.

SOURCE: U.S. Geological Survey.

were placed on or within hard rock. Sensors

placed in boreholes in hard rock provide superior coupling to the Earth and also provide

a more stable environment for the instrument

packages, with a concomitant reduction in

noise.

Seismic Instrumentation

Until recently, the instrumentation that was

available for detecting, digitizing, and recording seismic signals did not have the capability

to record all the signal frequencies of interest

with sufficient range. Further, the mechanical and electronic components comprising seismic recording systems generate internal noise,

which is recorded along with true ground noise

and seismic event signals, and this internal

noise was the limiting factor in recording seismic signals in the high frequency range. Specifically, the internal noise of the older designs

of high frequency seismic detectors was higher
63

than ground noise at frequencies above about

5 Hz. Thus, while ground noise is now known

to decrease with increasing frequency, the system noise remained constant or increased with

increasing frequency in the older systems.

Therefore, trade-offs were made, and the entire frequency range of the signal was generally not recorded. Specifically, in the high frequency range data was generally not recorded

above 10 Hz and even then was highly contaminated in the 5-10 Hz range by internal system noise. Consequently, small seismic events,

particularly small explosions, with expected

maximum signal energy in the high frequency

range above 5 Hz were not detected because

their high frequency signals were below the internal system noise levels. Most existing seismic stations are of this type and so are limited

for nuclear test monitoring.

Today, broadband systems capable of recording the entire frequency range with a large

dynamic range and with low internal noise are

available. However, the best high performance

systems are not widely distributed. To establish confident detection-identification capabilities using high frequency seismic signals at

low event magnitudes, it will be necessary to

expand the number of high performance stations and to place them in diverse geologic environments in order to simulate the requirements

of in-country monitoring.

Seismic Magnitude Estimation

Problems

As discussed earlier, the estimate of an

event’s seismic magnitude is made by combining the estimates obtained from many single

stations in an averaging procedure to reduce

random errors. This procedure works well for

an event which is neither too small nor too

large.

For a small event, however, the averaging

procedure can result in a network magnitude

value which is biased high. This follows from

the fact that for a small event, the signals will

be small. At those stations where signals fall

below the noise, the small signal amplitudes

will not be seen. Consequently, only higher amplitude values from other stations are available for use in computing the network average. With the low values missing, this network

average is biased high unless a statistical correction is made.

Figure 4-3 illustrates this effect. All six stations (A through F) record the same magnitude 4 event. Stations A, B, and C record the

Figure 4-3.—Effect of Noise on Event Magnitude Computation

4.8

4.2

4.5

3.2

STA-A STA-B STA-C

STA-D

STA-E

I

seismic

noise

level

Average event magnitude = 4.0 (6 stations) STA-F

Computed average magnitude = 4.5 (3 stations)

SOURCE: Modified from Air Force Technical Applications Center.
64

event below average. Stations D, E, and F record the event above average. Normally, the stations would all average out to a magnitude 4.0.

For a small event, however, the stations that

record low (A, B, &C) do not record the signal

because it is below the noise level. Only the

stations that record higher (D, E, & F) show

the event. Without the values from the low stations, the calculation of the average is made

using only the stations that record high. The

resulting calculation biases the average to the

values of the higher stations, giving a false

average magnitude value of (in this example)

4.5 for a 4.0 seismic event.

In the past, computed magnitudes of small

events were systematically biased high in this

manner. As a result, for most of the last 25

years, the U.S. capability to detect seismic

events within the U.S.S.R. has, in fact, been

significantly better than the estimates of this

capability. Not until the late 1970s was it demonstrated that the small events being detected

were 0.2 to 0.4 magnitude units smaller than

previously thought. In terms of yield, this

means that the networks were actually capable of detecting events down to half as large

as previously thought possible. Within the last

few years, analysis procedures have been employed to correct for most of this bias using

a procedure called maximum likelihood estimation.

For large events, a similar bias problem used

to exist occasionally, but for a different reason. Old seismometers could not record very

large signals without clipping the signal. Larger

amplitude signals were either not available or

were under estimated. The resulting bias of

large events, however, did not affect estimates

of detection capability and only became a problem in the determination of the size of very

large events.

SEISMIC NETWORKS

Existing Networks and Arrays

Although many thousands of seismic stations exist around the world, the actual number of stations which routinely report data to

national and international data centers is a few

thousand. For example, in figure 4-4 the 3,500

stations are shown that routinely report data

to the National Earthquake Information Center (NEIC), a center in Colorado operated by

the United States Geological Survey. Some of

these stations report much more often than

do others. The instrumentation at these stations is diverse and the quality of the data varied. While these stations are very useful for

seismic signal detection, they are less useful

for purposes of magnitude estimation and for

research requiring stations evenly distributed

around the world.

For purposes of treaty monitoring operations and research, a well distributed network

with a common set of instrumentation at all

stations is most useful. To obtain such a standard network, the United States funded the development and deployment of the Worldwide

Standardized Seismograph Network (WWSSN)

in the early 1960s (figure 4-5). The WWSSN

is maintained by the United States Geological Survey (USGS). The quality and performance of the WWSSN is generally very good,

but the recording system is limited in dynamic

range and resolution because of the use of what

is now obsolete analog equipment and also because of high internal noise in the amplifying

equipment. (For example, the data are currently recorded only on photographic paper

records.)

Beginning in the early 1970s, digital recording seismic stations were developed by the

United States and other countries. The data

from these stations can be easily processed by

digital computers to enhance the signal-tonoise for signal detection and to analyze the

data for seismic source determination and for

research purposes. These stations are included

in such networks as:

. The Regional Seismic Test Network

(RSTN). These are high quality stations
●

●

● ✎

●

●

●

✎

.

/’-7’-.

o

●

●

●

●

z

● I

●

●

● 9

●

●

.

●

. “
66

● ’
67

●

●

●

designed by the Department of Energy

and now operated by the USGS. They were

intended to be prototypes of in-country

stations. There are five RSTN stations distributed over North America at interstation distances that represent monitoring in the Soviet Union with 10 internal

stations.

The NORESS seismic array in Norway.

This seismic array is funded by the Defense Advanced Research Projects Agency

(DARPA) and the Department of Energy

(DOE). The prototype array is located in

southern Norway, an area thought to be

geologically similar to the western part

of the Soviet Union.

The recently installed China Digital Seismic Network (CDSN), which is a cooperative program between the People’s Republic of China and the USGS.

The Atomic Energy Detection System

(AEDS) seismic network. This network is

operated by the Air Force Technical Applications Center (AFTAC). The purpose

of the AEDS network is to monitor treaty

compliance of the Soviet testing program.

Consequently, the stations are located so

as to provide coverage primarily of the

U.S.S.R. The capabilities of the AEDS

network are described in a classified annex

to this report. The USGS and the AEDS

stations are the main sources of routine

information on Soviet testing.

In addition to these networks, there is also

a jointly operated NRDC-Soviet Academy test

site monitoring network in the United States

and the Soviet Union. The network consists

of three stations in each country around the

Kazakh and Nevada test sites at distances of

about zOO kilometers from the boundaries of

each test site. These stations are supplying

high-quality seismic signal data, in the high

and intermediate frequency range from 0.1 Hz

to about 80 Hz. The stations are designed to

be modern prototypes of the in-country seismic stations required for monitoring a low

threshold test ban treaty and they are not

limited by system noise in the high frequency

range. Plans call for the addition of five more

such stations distributed across the Soviet

Union and for several more to be similarly distributed across the United States.

Planned Networks

There are a number of planned new networks

that will provide increased capability to detect,

locate, and characterize seismic events around

the world. These networks are being developed

by the United States and other countries.

Hypothetical Networks

Existing unclassified networks external to

the U.S.S.R. have an excellent capability for

monitoring events with seismic magnitudes

greater than 4.0 within the U.S.S.R. However,

for explosions less than a few kt, the possibility exists that the seismic signals from such

explosions could be reduced through an evasion method. To demonstrate a capability to

defeat credible evasion scenarios that could be

applied to explosions with yields less than a

few kt, seismic stations within the Soviet

Union would be necessary.

Obviously, there are a number of requirements for such internal stations and their data.

Among these are the following: the data must

be provided in an uninterrupted manner; the

data must be of high quality; the seismic noise

at the stations should be low; the operating

parameters of the stations and the characteristics of the data should be completely known

at all times; the data should be available to

the United States within a reasonable time

frame; the stations should be located for effective monitoring of the U. S. S. R.; and any interruption or tampering with the operation of

the station should be detectable by the United

States. Obviously, these requirements can

most easily be achieved by deploying U.S. designed and built seismic stations within the

U.S.S.R. at sites chosen by the United States.

The number of stations required within the

U.S.S.R. is a function of a number of factors

including: the threshold level down to which

monitoring is desired, the seismic noise at the

stations, the signal-to-noise enhancement ca-
68

pability of the stations, the signal propagation posed distributions of internal stations capacharacteristics within the U. S. S. R., and the ble of detecting seismic events down to varipossibilities for various evasion scenarios ous thresholds. The number of internal stations

thought to be effective within different areas proposed for the various distributions ranges

of the U.S.S.R. Many seismologists have pro- between 10 and 50.

SEISMIC MONITORING CAPABILITY

Calculating Seismic Monitoring

Capability

Calculating the detection capability of existing seismic stations is straightforward. For

hypothetical stations, however, the detection

capability must be estimated by adopting a

number of assumptions. Because there exists

a range of possible assumptions which can be

argued to have validity, there are also a range

of possible capabilities for a network of hypothetical internal stations.

For existing stations, the average detection

capability is easily determined by observing

the number of seismic events detected as a

function of log amplitude (figure 4-6). The station can be expected to detect all events within

a given region down to some magnitude level.

A cumulative plot of the detections, such as

illustrated in figure 4-6, will show that a straight

line can fit these values down to this magnitude threshold level. This threshold marks the

Figure 4-6.—Detection Capability of a Seismic Station

1,000

o 3

Log amplitude

SOURCE: Modified from Air Force Technical Applications Center

point where the station fails to detect all

events. The 90-percent detection threshold (or

any other threshold) can be determined from

this plot. If the event magnitude rather than

the observed amplitude is used, it is important

that all magnitudes used in such plots be corrected for low-magnitude bias as previously

discussed. By examining all stations of a network in this manner, the station detection parameters can be determined and used to compute overall network performance for the given

region.

For hypothetical internal stations, no detection statistics are generally available for computing the cumulative detection curve as a

function of magnitude. Therefore, the detection capability must be estimated by assuming the following factors: the seismic noise at

the station, the propagation and attenuation

characteristics of the region through which the

signals will travel, the efficiency of the seismic source, and the signal-to-noise ratio required for the signal to be detected. All the

above factors must be evaluated in the frequency range assumed to be best for detecting a signal. The result of all these factors,

when considered together, is to provide an estimate of the probability that a signal from a

source of a given size and at a given location

will be detected at a certain station. Individual station detection probabilities are then

combined to determine the overall probability

that four or more stations will detect the event.

Translating this capability to situations where

evasion might take place requires additional

considerations (see ch. 6).

Global Detection Capability

There are many seismic stations that exist

around the world from which data can be ob-
69

tained. While no attempt has ever been made

to determine the global detection capability

of all these stations, a rough estimate could

be made by reviewing the various reporting

bulletins and lists. However, the current global

detection capability does not really matter because it will soon change as various planned

networks become installed. Consequently, an

accurate assessment of global capability is best

addressed by discussing planned networks.

For regions external to the U. S. S. R., and particularly for regions of the Southern Hemisphere, the greatest detection and location capability will reside not with the AEDS, but

with a number of existing and planned seismic networks which are unclassified. This is

a logical consequence of the AEDS being targeted primarily at events within the U.S.S.R.

In particular, national networks such as those

of Australia, China, the United States, Italy,

and Canada will provide significant global capability.

Given all the national and global data sources,

a cautious estimate of the global detection capability by the year 1991 (assuming 90 percent confidence of four or more stations detecting an event using only open unclassified

stations) is mb 4.2. For many regions, such as

the Northern Hemisphere, the detection capability will be, of course, much better. Therefore, by 1991, any explosion with a magnitude

corresponding to 1-2 kt well-coupled that is detonated anywhere on Earth will have a high

probability of being detected and located by

networks external to the U.S.S.R. Opportunities to evade the seismic network outside the

Soviet Union are limited. Because evasion

scenarios require large amounts of clandestine

work, they are most feasible within the borders

of a closed country such as the Soviet Union.

Consequently, monitoring networks are designed to target principally the Soviet Union.

Detection Capability Within the

U.S.S.R. Using No Internal Stations

Given that a large range of possible networks

exists, a few type examples are useful to convey a sense of what can be accomplished. For

example, the capability of a hypothetical network consisting of a dozen or so seismic arrays that are all outside the borders of the

Soviet Union can be calculated. A cautious estimate is that if such a network were operated

as a high-quality system, it would have 90 percent probability of detecting at four or more

stations all seismic events within the Soviet

Union with a magnitude at least as low as 3.5.

This corresponds to an explosion having a yield

below 1 kt unless the explosion is decoupled.

The hypothetical detection threshold of mb

3.5 is considered cautious because it is known

that a greater detection capability might exist at least for parts of the U.S.S.R. For example, the single large NORSAR array in Norway has the potential to achieve detection

thresholds equivalent to an event of mb 2.5 or

lower (corresponding to a well-coupled explosion between 0.1 and 0.01 kt) overlarge regions

of the Soviet Union.1

Also, fewer stations (fewer than the four

needed above) may detect much smaller events,

and this can provide useful information. However, detection by one or a few stations may

not be adequate to, with high confidence, locate

or identify events. Also, reductions of the detection threshold must be accompanied by a

comparable capability to locate the events (for

focusing other intelligence resources) and to

separate nuclear explosions from earthquakes

and legitimate industrial explosions.

Detection Capability Within the

U.S.S.R. Using Internal Stations

Seismic stations located internalto a country for the purpose of monitoring have a number of important advantages: improved detection capability, improved location capability,

and improved identification capability.

The potential instantaneous detection thresholds for the large

NORSAR array (42 seismometers spread over an area of about

3,000 km’), as described in “Teleseismic Detection at High Frequencies Using NORSAR Data” by F. Ringdal in IVORSAR

Semiannual ?’ecfmical Summary, Apr. I-Sept. 30, 1984, are:

West of Ural Mountains-mb 2.0-2.5 (possibly better)

Caspian Area–rob 2.0-2.5

Semipalatinsk-mb 2.5-3.0

Siberia–mb 2.5-3.5
70

Although much debate is associated with the

predicted detection capabilities of internal stations, improved detection capability alone is

probably not of the greatest significance at this

time because the current detection capability

is already very good. The improvement that

internal stations will provide to identification

capability (differentiating explosions from natural events) is by far the most important reason for requiring internal stations and should

be considered the basic requirement for internal stations. The problem of detecting and

identifying seismic events in the face of various evasion scenarios will be discussed in the

next two chapters.

Based on cautious assumptions for a network of 30 internal arrays or about 50 threecomponent internal stations, it appears likely

that a detection threshold of mb 2.5 (90 percent probability of detection at four or more

stations) could be reached. This corresponds

to a well-coupled explosion of 0.1-0.01 kt, or

a fully decoupled nuclear explosion with a yield

of about 1 kt. Based on more optimistic assumptions about the conditions to be encountered and prospective improvements in data

processing capability, this same network could

have a detection capability as low as mb 2.0.

Detection capability contours for one such proposed 30-array internal network are shown in

figure 4-7.

Considerations in Choosing

Monitoring Thresholds

Depending on the number of internal stations, detection capabilities could either increase or decrease. The point, however, is that

very low detection thresholds, down to magnitude 2.0, can be achieved. In fact, almost any

desired signal detection level can theoretically

be obtained by deploying a sufficient number

of internal stations; although there maybe disagreements over the number and types of stations needed to achieve a given threshold. The

disagreements could be resolved as part of a

learning process if the internal network is built

up in stages.

Another consideration is that all detection

estimates used in this report are based on a

90-percent probability of four or more stations

detecting an event. While this maybe a prudent estimation procedure from the monitoring point of view, an evader who did not wish

to be caught might adopt a considerably more

cautious point of view. (See chapter 2 for a discussion of the relationship between uncertainty

and cheating opportunities.) Such concerns

might be increased by the realization that for

many seismic events, there will beat least one

station that will receive the signal from the

event with a large signal-to-noise ratio. The signal will be so large with respect to the noise

at this station that the validity of the signal

will be obvious and will cause a search for other

associated signals from neighboring seismic

stations. The possible occurrence of such a situation would be of concern to a country contemplating a clandestine test.

Throughout all of this discussion it must be

kept in mind that an improvement in detection capability does not necessarily correspond

directly to an improvement in our monitoring

capability. Although a reduced detection threshold must be accompanied by a reduced identification threshold; occurrences such as industrial explosions might ultimately limit the

identification threshold. For example, the estimate of detection capability for internal stations which is given above, mb 2.0 to 2.5, corresponds to a decoupled explosion of about 1

kt. A decoupled 1 kt explosion produces the

same mb signal as a 1/70 kt (15) ton chemical

explosion. At such small magnitude levels,

there are hundreds of chemical explosions detonated in any given month in the U.S.S.R. and

in the United States.

Use of High-Frequency Data

It has long been known to seismologists that

seismic signals of moderate to high frequencies can indeed be detected at large distances

from the source under favorable geological conditions. Such is the case in the eastern United

States, and it is generally thought that this

is also true of most of the Soviet Union. In con-
71
72

trast, tectonic regions such as the western

United States and the southern fringe of the

U.S.S.R. are generally characterized by stronger

attenuation of high frequencies, which are therefore lost beyond relatively short distances.

The design and development of a nuclear

monitoring strategy based on high-frequency

seismic signals calls for:

● A determination of Earth structure within

and around the region to be monitored,

and an evaluation of its signal transmission characteristics at high frequencies.

● A methodology for identifying and selecting sites with low ground noise, and equipping such sites with high performance sensors and recording systems, so as to achieve

the largest possible signal-to-noise ratio.

● A reliable understanding of the high-frequency radiation of natural (earthquakes)

and man-made (explosions) seismic sources.

Although empirical evidence based on direct observations is sufficient in principle,

a predictive capability, based on theory,

is required to assess properly new and untested monitoring conditions.

Such requirements parallel in every respect

the usual constraints placed on standard monitoring systems. However, direct experimentation pertinent to high-frequency monitoring

has been rather limited so far, and the relevant

data available today are neither abundant nor

diverse. Consequently, an assessment of whether

these requirements can be met relies of necessity on some degree of extrapolation from our

present experience, based on theoretical arguments and models. This situation leaves room

for debate and even controversy.2

Recently, it has been argued that the capability to detect and identify low-yield nuclear

explosions could be greatly improved by using

‘High quality seismic data is now becoming available from

the NRDC-Soviet Academy of Sciences stations in the Soviet

Union and the United States, with more widely distributed stations to be added in 1988 in both countries. This data may help

reduce the necessity for extrapolation and decrease the uncertainties that foster the debate.

high-frequency (30 -40 Hz) seismic data.3The

major points of the argument are:

●

●

●

that natural seismic ground noise levels

are very low at high frequencies, and that

large seasonal fluctuations are not anticipated;

that careful station selection could make

it possible to emplace seismic sensors in

particularly quiet sites; and

that present seismic recording technology

allows high-fidelity recording; by suppression of system-generated noise to levels

below ground noise even at high frequencies and at quiet sites.

Advocates of high-frequency monitoring explain the efficiency of high-frequency wave

propagation observed in the North American

shield in terms of a simple model for attenuation of seismic waves in stable continental regions and argue that the model applies as well

to stable continental Eurasia. Finally, they arretical models of earthquakes and underground

explosions provide adequate predictive estimates of the relative production of high-frequency energy by various seismic sources and

justify their choice by comparison with limited

observations.

Based on these arguments and a systematic

modeling procedure, some seismologists reason that the most favorable signal-to-noise ratio for detection of low yield (i.e. small magnitude) events in stable continental areas will be

found at moderate to high frequencies (about

30 Hz). They further infer that a well-designed

network of 25 internal and 15 external highquality stations using high frequencies would

yield multi-station, high signal-to-noise detection of fully decoupled 1-kt explosions from any

of the potential decoupling sites within the

U. S. S. R., and result in a monitoring capability at the l-kt level.

On the other hand, the inference that such

significant benefits would necessarily accrue

‘For example, J. F. Evemden, C. B. Archarnbeau, and E. Cranswick, “An Evaluation of seismic Decoupling and Underground

Nuclear Test Monitoring Using High-Frequency Seismic Data, ”

Reviews of Geophysics, vol. 24, No. 2, 1986, pp. 143-215.
73

by relying on high-frequency recordings has

been strongly questioned in the seismological

community. Indeed many scientists feel that

the case is currently unproven. Major points

of disagreement include:

● The concern that the theoretical seismic

source models used so far in the analysis

described above are too simple. Studies

aimed at constructing more realistic models

indicate that the high-frequency waves

generated by seismic sources are strongly

affected by complexities of source behavior that the simple models do not take

into account. On the other hand, advocates of high frequency seismic monitoring believe that the models they have used

have successfully predicted a number of

characteristics of seismic sources that

were subsequently verified and that none

of the many well-documented observations of seismic wave characteristics from

large events are in conflict with their theoretical model predictions. Thus they argue that the model predictions, for somewhat smaller events at somewhat higher

frequencies than are ordinarily studied,

are reasonable extrapolations.

● The concern that it may be difficult to

identify candidate station sites where the

high-frequency noise is sufficiently low to

permit actual realization of the desired

benefits. Experience to date is limited, and

one does not really know whether a given

site is suitable until it has been occupied

and studied for at least a year. On the

other hand, advocates of high frequency

monitoring feel that suitable low-noise

sites are not at all rare and can be rather

easily found in most, if not all, geologic

environments within the continents. They

argue that stations selected so far have

had adequately low high-frequency noise

characteristics and the selection process

was neither difficult nor time consuming.

They conjecture that doubts are based on

misidentification of high-frequency internal seismic recording system noise as

ground noise, and that once high-performance systems with low system noise become more wide-spread, this concern will

disappear.

● The concern that observations which can

be employed to test directly the validity

of the proposed use of high frequencies are

as yet quite scant, and their interpretation is not free of ambiguities. For example, the characterization of source spectra and the propagation and attenuation

of high-frequency waves remain issues

which are not resolved unequivocally by

observations, and yet are critical to the

formulation of a high-frequency monitoring strategy. Similarly, available data

often exhibit an optimal signal-to-noise ratio at frequencies near 10 Hz, in apparent

disagreement with the arguments enunciated earlier. In response to these concerns, proponents argue that the NRDCSoviet observations of signal-to-noise ratios greater than 1 and out to frequencies

above 20 Hz provides evidence for the

potential of high frequency monitoring.

While proponents of high frequency monitoring agree that the observations of the

largest signal to noise ratios for signals

from seismic events often occur near 10

Hz, this is not in disagreement with the

predictions or technical arguments advanced for high-frequency monitoring.

They argue that these observations are obtained from seismic receivers with system

noise that is greater than ground noise at

frequencies above 10 Hz, and that many

of the observations are from industrial explosions that are ripple-fired and so are

expected to have lower high-frequency

content than a small nuclear test.

● The concern stated earlier that the motivation for the proposed high-frequency

monitoring approach uses contested theoretical arguments and models to extrapolate from our present experience and thus

attempt to guide further steps towards a

significantly improved capability. In the

present case, models are used to extrapolate both toward high frequencies and

toward small yields, and just how far one
74

may extrapolate safely remains a matter

of debate. The proponents of high frequency monitoring agree that the data relating to high frequency monitoring is

limited with respect to the geologic regions to which it pertains. Furthermore,

it is clear that experience in the systematic detection and identification of very

small seismic events using high-frequency

data is absent and that, as a consequence,

it has been necessary to extrapolate from

experience with larger seismic events

where lower frequency data is used. Proponents believe, however, that what limited

data is available does support the most

critically important predictions; these being the apparent availability of low-noise

sites and the efficiency of high-frequency

wave propagation to large distances.

These controversial aspects notwithstanding, there is general agreement among seismologists that good signal-to-noise ratios persist

to higher frequencies than those used routinely

today for nuclear monitoring. In particular,

data in the 10-20 Hz band show clear signals

which are undoubtedly not used optimally.

Given the fact that recording of even higher

frequencies is demonstrably feasible in some

situations, and given the potential advantages

for low-yield monitoring, the augmentation of

our experience with such data, the concomitant continued development of appropriate

analysis techniques to deal with them, and the

validation of the models used in their interpretation are goals to be pursued aggressively.

Not until a sufficient body of well-documented

observations of this nature has been collected

can we expect to achieve a broad consensus

about the performance of high-frequency monitoring systems.

Arrays v. Three-Component Stations

Both small-aperture, vertical-component arrays such as NORESS, and three-component,

single-site stations such as the RSTN station

have been considered for use as internal stations. In choosing which to use, it should be

realized that many combinations of arrays and

single stations will provide the same capability. For example, for any array network, there

is a single station network with comparable

capability; but the network of single stations

probably requires about twice as many sites.

Although a single array has advantages over

a single three-component station (see chapter

3), for monitoring purposes it is preferable to

have a large number of station sites with threecomponent stations rather than to have a small

number of sites with arrays. This is true because it permits better accommodation to details of regional geology, and better protection

against noise sources temporarily reducing capability of the network as a whole. However,

if the number of sites is limited by negotiated

agreement, but the instrumentation can include either arrays or three-component stations, then arrays are preferable. This is true

both because of the inherent redundancy of arrays and their somewhat better signal-to-noise

enhancement capability over single three-component stations.
Chapter 5

Identifying Seismic Events
CONTENTS

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Basis for Seismic Identification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Earthquakes . . . . . . . . . .

Chemical Explosions . . .

Rockbursts . . . . . . . . . . .

Methods of Identification

Location . . . . . . . . . . . . .

Depth . . . . . . . . . . . . . . .

Ms:mb . . . . . . . . . . . . . . .

Other Simple Methods .

Spectral Methods. . . . . .

High-Frequency Signals

Identification Capability .

Identification Capability

Stations . . . . . . . . . . . .

Identification Capability

Box

5-A. Theory and

5-B. Progress in

Figure No.

5-i.

5-2.

5-3.

5-4.

5-5.

5-6.

5-7.

5-8.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . ....!!. . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Within the U.S.S.R. Using No Internal

.Within the U.S.S.R Using Internal

Boxes

Stations::

77

78

78

84

85

85

85

85

86

88

88

89

90

91

91

Observation. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Seismic Monitoring . . . . . . . . . . . . . . . . . . . . . .

Figures

. . . . . . . . . . 82

. . . . . . . . . . 92

Seismicity of the World, 1971 -1986 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Seismicity of the U.S.S.R. and the Surrounding Areas, 1971-1986 . . . .

Cross-Sectional View of Seismicity Along the Kurile-Kamchatka

Coast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Earthquakes v. Explosions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Ms v. mb for a Suite of Earthquakes and Explosions.. . . . . . . . . . . . . .

Msv. mb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

The mb:MsDiscriminant for Populations of NTS Explosions and

Western U.S. Earthquakes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

The VFM Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

79

80

81

83

86

86

87

89

Table No. Page

5-l. Approximate Numbers of Earthquakes Each Year, Above Different

Magnitude Levels . . . . . . ........ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
Chapter 5

Identifying Seismic Events

Once a seismic event has been detected, the next step is to determine whether

it was created by an underground nuclear explosion.

INTRODUCTION

Once the signals from a seismic event have

been detected, and the event located, the next

step in seismic monitoring is that of identification. Was the event definitely, or possibly,

an underground nuclear explosion, or can the

signals be identified unambiguously as having another cause? As discussed in the previous chapter, seismic signals are generated by

natural earthquakes and by natural rockbursts

in mines, as well as by chemical and nuclear

explosions. The identification problem in seismic monitoring, called the discrimination prob -

lem, is to distinguish underground nuclear explosions from other seismic sources.

In the case of a located seismic event for

which signals are large, that is, larger than

could be ascribed to a chemical explosion or

a rockburst, the only candidates for the source

of the signals are an earthquake or a nuclear

explosion. Physical differences between earthquakes and nuclear explosions cause their seismic signals to differ, and these differences can

be used to identify the events.

Identification becomes more complicated,

however, for small events. When identifying

small events (comparable in size to an explosion of less than 10 kilotons), the identification procedures encounter four types of difficulty that do not arise for larger events:

1.

2.

3.

4.

the quality of available signals is typically

lower;

the number of natural events that must

be discriminated against is larger;

the possibilities now include chemical explosions and rockbursts;

the event, if nuclear, is of a size where attempts may be made to muffle or hide the

seismic signal.

Each of these difficulties becomes more severe

at lower yields.

The last difficulty listed above brings up the

subject of evasion which is discussed in the

next chapter. For the purposes of understanding this chapter, however, the reader should

recognize that identification capabilities must

always be considered against the feasibility of

various evasion scenarios. The successful use

of a muffling or decoupling evasion technique

could cause a 1-10 kt decoupled nuclear explosion to produce seismic signals comparable to

either a chemical explosion of 10-100 tons respectively, or an earthquake ranging from

magnitude 2-3 respectively. The similarity in

size and, in some cases, the properties of the

signals for these three types of events pose the

most serious monitoring challenges. The monitoring system must be able to demonstrate

a capability to identify with high confidence

seismic events whose signals might be intentionally reduced or hidden through credible

evasion techniques. It is the need to demonstrate such a capability, as signals become

smaller, that ultimately sets the threshold for

seismic identification.

The present chapter describes the different

kinds of seismic sources and the basis for solving the identification problem. It then describes

capabilities of current seismic networks, and

how these capabilities might be improved by

the addition of new seismic stations, including stations within the U.S.S.R.

77
78

BASIS FOR SEISMIC IDENTIFICATION

The seismic signals from a nuclear explosion

must be distinguished from seismic signals created by other events, particularly earthquakes,

chemical explosions, and rockbursts. Most

seismic signals from located events are caused

by earthquakes, although chemical explosions

are also present in large numbers. Significant

rockbursts and other sources of seismic signals are rare. A monitoring network, however,

will encounter seismic signals created by all

of these sources and must be able to identify

them. Consequently, the burden on any monitoring network will be to demonstrate a capability to detect and identify with high confidence a clandestine nuclear test against the

background of large numbers of earthquakes

and industrial explosions and infrequent rockbursts. This section reviews basic properties

of these other seismic sources and discusses

the physical basis for discriminating them from

nuclear explosions.

Earthquakes

Earthquake activity is a global phenomenon,

though most of the larger earthquakes are concentrated inactive tectonic regions along edges

of the Earth’s continental and oceanic plates

(figure 5-l). In the U. S. S. R., most large earthquakes are located in a few active regions along

the southern and eastern borders of the country (figure 5-2). For the many earthquakes

occurring along the Kurile Islands region on

the Pacific border of the Soviet Union, the shallow earthquakes generally occur on the ocean

side of the islands, while the deeper earthquakes occur beneath the islands and toward

the U.S.S.R. landmass (figure 5-3). Elsewhere,

earthquakes can occur as deep as 700 kilometers in the Earth’s mantle.

In figure 5-2, large areas of the U.S.S.R. are

shown for which there is no significant earthquake activity. Given presently available information, such areas are referred to as aseismic; although some activity may occur in these

regions at magnitudes below mb 3.0.

As table 1 illustrates, there are about 7,500

earthquakes that occur with mb 4.0 or above

each year, and about 7 percent of these occur

in the Soviet Union. However, approximately

two-thirds of the Soviet seismicity occurs in

oceanic areas, mainly off the Kurile-Kamchatka

coast. Seismicity in oceanic areas does not raise

an identification problem because acoustic sensors provide excellent identification capability for nuclear explosions in water. Consequently, it is earthquakes on Soviet land areas

from which nuclear explosions need to be differentiated. For example, at the magnitude

level of mb 4.0 and above, where there are

7,500 earthquakes each year, approximately

183 occur on Soviet land areas and would need

to be identified. The number of such earthquakes that occur each year above various

magnitudes is shown in the third column of

table 1. The smaller the magnitude, the more

earthquakes there are.

Earthquakes and their associated seismograms have been studied on a quantitative basis for about 100 years. Since 1959, seismologists have engaged in a substantial research

effort to discriminate between earthquakes and

explosions by analyzing seismic signals. Thousands of studies have been conducted and

reports written. As will be discussed in the following section, this identification problem is

now considered to be solved for events above

a certain magnitude.

Table 5-1.—Approximate Numbers of Earthquakes

Each Year, Above Different Magnitude Levels

m b Globally a

Soviet Unionb Soviet land areasc

5.0 950 70 23

4.5 2,700 200 67

4.0 7,500 550 183

21,000 1,500 500

3.0 59,000 4,200 1,400

2.5 170,000 12,000 4,000

aBased On F. Ringdal’s global study of a 10-year period, for which he finds the

statistical fit log N = 7.47-O.9m . F. Ringdal, “Studyof Magnitudes, Seismicity,

and Earthquake DetectabilityJ sing a Global Network,” in The Vela Program,

Ann U. Kerr, (cd.), Defense Advanced Research Projects Agency, 1985.

bBa9ed on 7 percent of global earthquakes with numbers rounded uP slightly

cBa9ed on removing two-thirds of Soviet Union earthquakes. Two-thirds of the

earthquakes in the Soviet Union occur in oceanic areas, e.g., off the KurileKamchatka coast and, therefore, do not present an identification problem.
79

b

●

●

●
80

●

●
81

Figure 5-3.—Cross-Sectional View of Seismicity Along the Kurile-Kamchatka Coast

Kurile Islands Kurile-Kamchatka Trench

/ Pacific Ocean

Two-thirds of Soviet seismicity at magnitudes greater than mb 3.5 is off the Kurile-Kamchatka coast. This sectional view shows

that almost all these events are deep, below land or shallow, below ocean. Neither case is a candidate explosion location.

SOURCE: L.R. Sykes, J.F. Evernden, I.L. Cifuentes, American Institute of Physics, Conference Proceed/r@ 704, pp. 85-133, 1983.

From these studies, a number of identification methods have evolved. These methods all

have as their basis a few fundamental differences between nuclear explosions and earthquakes. These differences are in:

●

●

●

location,

geometrical differences between the point

source of an explosion and the much larger

rupture surface of an earthquake, and

the relative efficiencies of seismic wave

generation at different wavelengths.

With regard to the first difference, many

earthquakes occur deeper than 10 km, whereas

the deepest underground nuclear explosions

to date have been around 2.5 km and routine

weapons tests under the 150 kt threshold appear all to be conducted at depths less than

1 km. Almost no holes, for any purpose, have

been drilled to more than 10 km deep. The exceptions are few, well known, and of a scale

that would be difficult to hide. Because nuclear

explosions are restricted to shallow depths (less

than 2.5 km), discrimination between many

earthquakes and explosions on the basis of

depth is possible in principle. In practice, the

uncertainty in depth determination makes the

division less clear. To compensate for the possibility of very deep emplacement of an explosion, the depth must be determined to be below 15 km with high confidence before the

event is to be identified unequivocality as an

earthquake. Events with shallower depths that

show large uncertainties in depth determination might be considered as unidentified on the

basis of depth.
82

With regard to source geometry, the fundamental differences are due to how the seismic

signals are generated (see box 5-A). As discussed in chapter 3, an underground nuclear

explosion is a highly concentrated source of

compressional seismic waves (P waves), sent

out with approximately the same strength in

all directions. This type of signal occurs because the explosion forces apply a fairly uniform pressure to the walls of the cavity created by the explosion. The simple model of a

nuclear explosion is a spherically symmetric

source of P waves only. This contrasts with

an earthquake, which is generated as a result

of massive rock failure that typically produces

Box 5-A.—Theory and Observation

One of the points on which early efforts at

discrimination stumbled was the claim that

explosions, because they are concentrated

pressure sources, would generate insignificant SH waves and Love waves. (These waves

entail types of shearing motion, in which the

ground moves in a horizontal direction.) But

it was soon found, in the early 1960s, that

some explosions generated quite strong SH

and Love waves, so this discriminant came

to be seen as unreliable.

A salutary further effect was that discriminants based purely on theoretical predictions came to carry little weight until they

had passed stringent tests with actual data

from explosion and earthquake sources. Occasionally, some useful discriminants are discovered empirically and a theoretical understanding is not immediately available. A

recent example of this is the observation that

regional phases Pn, Pg

, and Lg for small

earthquakes in the western U.S. typically exhibit more high-frequency energy (at 6-8 Hz)

than do small NTS nuclear explosions of comparable seismic magnitude.* However, in the

symbiotic relation between theory and observation, there is generally a framework of understanding in which progress in one field

guides workers to new results in the other.

*J-R. Murphy and T.J. “A Discrimination Analysis of

Short-Period Regional Seismic Data Recorded at Tonto Forest

Observatory, ” BuLletin of the Seismological Society of America, vol. 72, No. 4, August 1982, pp. 1351-1366.

a net shearing motion. It is common here to

think in terms of two blocks of material (within

the crust, for shallow events) on opposite sides

of a fault. The stress release of an earthquake

is expressed in part by the two blocks moving

rapidly (on a time scale of seconds) with respect

to each other, sliding in frictional contact over

the plane of the fault as a result of the spontaneous process of stress release of rock—

stress that has accumulated over time through

geologic processes. Because of this shearing

motion, an earthquake radiates predominantly

transverse motions–i.e., S waves, from all

parts of the fault that rupture. Though P waves

from an earthquake are generated at about 20

percent of the S-wave level, they have a fourlobed radiation pattern of alternating compressions and rarefactions in the radiated first motions, rather than the relatively uniform pattern of P-wave compressions radiated in all

directions from an explosion source. These

idealized radiation patterns,. P waves from an

earthquake and an explosion, are shown schematically at the top of figure 5-4. These two

types of source geometries result in the following differences:

1.

2.

Energy partitioning. The energy of a seismic event is partitioned into compressional, shear, and surface waves. Earthquakes tend to emit more energy in the

form of shear waves and surface waves

than explosions with comparable compressional waves (figure 5-4). At lower magnitudes, however, differences of this type

may be difficult to distinguish if the surface waves from either source become too

small to detect.

Dimensions of the source. Earthquakes

tend to have larger source dimensions

than explosions because they involve a

larger volume of rock. The bigger the

source volume, the longer the wavelength

of seismic waves setup by the source. As

a result, earthquakes usually emit seismic

waves with longer wavelengths than explosions. Instead of describing the signal

in terms of wavelength, an equivalent

description can be given in terms of seismic wave frequency. Explosions, which
83

Figure 5-4.—Earthquakes v. Explosions

Earthquake 

p +

Short period I Long period

(P waves) (surface waves)

I

I

Earthquake

I

I

I

+

I

Explosion

— .

I

I

5 seconds I 5 minutes

I

(Upper) Different radiation patterns for earthquakes and explosions. Earthquakes involve shear motion along a fault plane.

Explosions are compressional sources of energy and radiate P waves in all directions.

(Lower) Recorded signals (i.e., seismograms) of earthquakes and explosions generally have different characteristic features.

Note the much stronger P-wave: surface-wave ratio, for explosions as compared to the earthquake.

SOURCE: F. Ringdal, “Seismological Verification of Comprehensive Test Ban Treaty,” paper presented in “Workshop on Seismological Verification of a Comprehensive

Test Ban Treaty,” June 4-7, 1985, Oslo, Norway.

are small, intense sources, send out stronger been observed and the distinction may

signals at high frequencies; and earth- diminish for small events. The difference

quakes generally have more low-frequency in frequency content of the respective sig-

(long-wavelength) energy than explosions nals, particularly at high frequencies, is

(figure 5-4). However, exceptions have a topic of active research.
84

These basic differences in seismic signal are

understood on theoretical grounds; and this

theoretical framework guides the search for

new empirical methods of identification. As

new types of seismic data become available (for

example, data from within the Soviet Union),

it maybe expected that methods of identification will be found that work at smaller magnitude levels. At low magnitudes, however, we

have not only the problem of distinguishing

nuclear explosions from earthquakes, but also

of distinguishing nuclear explosions from the

large chemical explosions that are commonly

used for industrial purposes such as mining.

This is a much more difficult problem because

in theory the two source types should produce

similar signals.

Chemical Explosions

Chemical explosions are used routinely in the

mining and construction industries, and they

occur also in military programs and on nuclear

test sites. In general, from the seismic monitoring perspective, a chemical explosion is a

small spherical source of energy very similar

to a nuclear explosion. The magnitudes caused

by chemical explosions are generally below mb

4.0, although events with magnitudes up to

mb 4.5 occasionally occur. The fact that large

numbers of chemical explosions in the yield

range from 0.001 to 0.01 kt are detected and

located seismically is a testament to the capability of local and regional seismic networks

to work with signals below mb 3.0.

There is little summary information available on the number and location of chemical

explosions in the U. S. S. R., though it appears

that useful summaries could be prepared from

currently available seismic data. In the United

States, chemical explosions around 0.2 kt are

common at about 20 mines. At each of these

special locations, tens of such explosions may

occur each year. Presuming that similar operations are mounted in the U. S. S. R., this activity is clearly a challenge when monitoring nuclear explosions with yields below about 1 kt.

It is also a challenge when considering the possibility that a nuclear testing nation may seek

to muffle a larger nuclear explosion (say, around

5 kt), so that its seismic signals resemble those

of a much smaller (say, around 0.1 kt) chemical explosion.

The problem of identifying industrial explosions can be partially constrained in three

ways:

1.

2.

In the United States, almost all chemical

explosions detonated with yields 0.1 kt or

greater for industrial purposes are in fact

a series of more than a hundred small explosions spaced a few meters apart and

fired at shallow depth with smalltime delays between individual detonations. One

effect of such “ripple-firing” is to generate seismic signals rather like that of a

small earthquake, in that both these types

of sources (ripple-fired chemical explosions, and small earthquakes) occur over

a large area and thus lose some of the characteristics of a highly concentrated source

such as a nuclear explosion. If individual

salvos are large enough, however, they

might be able to mask the signals from

a sub-kiloton nuclear explosion. Recent research on this problem has shown the importance of acquiring seismic data at high

frequencies (30-50 Hz); and indeed, such

data suggest the existence of a distinctive

signature for ripple-fired explosions.1 23

Many of the rare chemical explosions

above about 0.5 kt that are not ripple fired

can be expected to result in substantial

ground deformation (e.g., cratering).4 Such

surface effects, together with absence of

a radiochemical signal, would indicate a

chemical rather than a nuclear explosion

source. The basis for this discriminant is

IA.T. Smith and R.D. Grose, “High-Frequency Observations

of Signals and Noise Near RSON: Implications for Discrimination of Ripple-fired Mining Blasts,” LLNL UCID-20945, 1987.

2A.T. Smith, “Seismic Site Selection at High Frequencies: A

Case Study, ” LLNL UCID-21047, 1987.

9D.R. Baumgardt, “Spectral Evidence for Source Differences

between Earthquakes and Mining Explosions in Norway,” Seismologi”cal Researcb Letters, vol. 38, January-March 1987, p. 17.

4

1t is also relevant that chemical explosions in the 0.1 -0.5

kt range are commonly observed to be quite efficient in generating seismic waves. For example, 0.5 kt chemical explosions

are known to have m~ around 4.5 for shield regions.
85

3.

that chemical explosions are typically

very shallow.

To the extent that remote methods of

monitoring chemical explosions are deemed

inadequate (for example, in a low yield

threshold or comprehensive test ban regime), solutions could besought by requiring such constraints as prior announcement

of certain types of chemical explosions, inspections, shot-time monitoring, and incountry radiochemical monitoring. Note

that in the United States (and so perhaps

also in the U. S. S. R.) chemical explosions

above 0.1 kt occur routinely at only a

limited number of sites.

Rockbursts

In underground mining involving tunneling

activities, the rock face in the deeper tunnels

may occasionally rupture suddenly into the

tunnel. This is referred to as a rockburst and

results from the difference between the low

pressure existing within the tunnel and the

great pressure that exists within the surrounding rock. To prevent such mine rockbursts,

bracing structures are used in the deeper

tunnels.

In terms of magnitude, rockbursts are all

small. They occur over very restricted regions

of the Earth and generally have a seismic magnitude of less than 4.0 mb

.

The source mechanisms of rockbursts are

very similar to those of small earthquakes. In

particular, the direction of the first seismic motion from a rockburst will have a pattern similar to that for earthquakes. Therefore, for the

seismic identification problem, rockbursts can

be considered small earthquakes which occur

at very shallow depths.

METHODS OF IDENTIFICATION

Over the years, a number of identification

methods have been shown to be fairly robust.

Some of these methods perform the identification process by identifying certain earthquakes as being earthquakes (but not identifying explosions as being explosions). Other

identification methods identify certain earthquakes as being earthquakes and certain explosions as being explosions. The identification

process is therefore a winnowing process.

Location

The principal identification method is based

on the location of a detected seismic source.

If the epicenter (the point on the Earth’s surface above the location) is determined to be in

an oceanic area, but no hydroacoustic signals

were recorded, then the event is identified as

an earthquake. Large numbers of seismic events

can routinely be identified in this way, because

so much of the Earth’s seismic activity occurs

beneath the ocean. If the location is determined

to be land, then in certain cases the event can

still be identified as an earthquake on the basis of location alone, e.g. if the site is clearly

not suitable for nuclear explosions (such as

near population centers) or if there is no evidence of human activity in the area.

Depth

With the exception of epicenter locations,

seismic source depth is the most useful discriminant for identifying large numbers of

earthquakes. A seismic event can be identified

with high confidence as an earthquake if its

depth is determined to be below 15 kilometers.

The procedure for determining source depth

is part of finding the event location using the

arrival time of four or more P-wave signals.

Also, certain seismic signals, caused by energy

that has traveled upward from the source,

reflected off the Earth’s surface above the

source region, and then traveled down into the

Earth, are similar to the P wave out to great

distances. A depth estimate can be obtained

by measuring the time-difference between the

first arriving P-wave energy and the arrival

time of these reflected signals. The analysis
86

of broadband data through wave-form modeling is particularly useful for detecting these

reflected signals. Empirical methods can also

be used, based on comparison with previously

interpreted seismic events in the same general

region as the event under study.

In principle, an advantage of depth as an

identification method is that it is not dependent on magnitude: it will work for small events

as well as large ones, provided the basic data

are of adequate signal quality and the signals

are detected at a sufficient number of stations.

It will not alone, however, distinguish between

chemical and nuclear explosions unless the nuclear explosion is relatively large and deep; or

between underground nuclear explosions and

earthquakes unless the earthquakes are sufficiently deep.

Ms:mb

Underground nuclear explosions generate

signals which tend to have surface wave magnitude (Ms

) and body wave magnitudes (rob)

that differ from those of earthquake signals.

This is basically a result of explosions emitting more energy in the form of body waves

(high-frequency seismic radiation), and earthquakes emitting more energy in the form of

surface waves (low frequency seismic radiation). The phenomenon is often apparent in the

original seismograms (figure 5-4), and examples are shown in terms of Ms

:mb diagrams in

figures 5-5, 5-6, and 5-7. These diagrams can

be thought of as separating the population of

explosions from that of earthquakes. For any

event which is clearly in one population or

another, the event is identified. For any event

which is between the two populations (as occasionally happens for explosions at the Nevada Test Site), this method does not provide

reliable identification. The method, therefore,

has the potential of identifying certain earthquakes as being earthquakes and certain explosions as being explosions.

To use this identification method, both mb

and MS values are required for the event. This

is no problem for the larger events, but for

smaller events (below mb 4.5) it can be very

difficult to detect low-frequency surface waves

Figure 5-5.—Ms v. mb for a Suite of

Earthquakes and Explosions

7.0

6.0

4.0

2.0 4.0 5.0 6.0

Ms

SOURCE: P.D. Marshall and P.W. Basham, Geophysical Journal of the Royal As.

tronomical Society, vol. 28, pp. 431-458, 1972.

Figure 5-6.—MS v. mb

9

8

7

6

o = Earthquakes

o

0

0 0 0

. 0 . o 0 0

o

A

I

A

A

5 6 7 8

mb

SOURCE: L.R. Sykes, J.F. Evernden, I.L. Cifuentes, American Institute of Physics, Conference Proceeding 104, pp. 85-133, 1983. (All of the events

(both earthquakes and explosions) were corrected for bias.]

using external stations alone.5 This difficulty

is present particularly for explosions, with the

$rn ~Pi~ studies of redon~ data that evaluate the Ms:mb

discriminant, it has been found that between 10 percent and

20 percent of the surface waves of small events are masked by

signals caused by other events. This, however, has little net

effect on the discrimin ation of events for which more than one

station is close enough to receive surface waves.
87

Figure 5-7.—The mb:Ms Discriminant for Populations of NTS Explosions and Western U.S. Earthquakes

6

2

o

x “x

- +

.

1

1 I I I 1

3 4 5 6 7 8

M b

X Explosion

SOURCE: Modified from S.R. Taylor, M.D. Denny, and E.S. Vergino, Regional mb

:Ms Discrimination of NTS Explosions and Western United States Earthquakes A Progress

Report, Lawrence Livermore National Laboratory -

January- 1986. “ -

result that the Ms:mb method works intrinsically better for identifying small earthquakes

than it does for identifying small explosions.

Internal stations would provide important additional capability for obtaining Ms values for

events down to mb 3.0 and perhaps below.

It is known that Ms:mb diagrams, with their

separate populations as shown in figures 5-5,

5-6, and 5-7, tend in detail to exhibit somewhat

different properties for sources occurring in

different geophysical provinces. That is, the

best identification capabilities are obtained,

for a particular event of interest, if there is already a data base and an associated Ms

:mb

diagram tailored for the general region of the

Earth in which that event occurred.

The Ms:mb method has proven to be the

most robust identification technique available

for shallow events. For events below mb 4.0,

the separation between the two Ms

:mb populations has been found to decrease in some

studies. In the opinion of many seismologists,

however, a useful separation (at low magnitudes) may be possible if good quality data for

such small events can be found. Again, in the

context of monitoring for small underground

explosions in the U. S. S. R., obtaining such data

would require in-country stations and empirical confirmation.

76-584 0 - 88 : QL 3 - 4
88

Other Simple Methods

Several other methods, with applications in

particular circumstances, are used to provide

identification. Several are still in the research

stage, and are having an impact on the design

of new instruments and the development of

new procedures in data analysis. Some of these

methods, based on the whole spectrum of frequencies contained in a seismic signal, are described in the following section. Other methods have been used for decades, and are still

occasionally important for use in identifying

certain problem events. In order of decreasing importance, the remaining three simple

methods can be listed as follows:

1. The use of “first motion.” By this term is

meant an identification method based on

differences in the direction of the initial

motion of P waves. As illustrated in figure 5-4, an explosion is expected to create

initial compressive motions in all directions away from the source, whereas an

earthquake typically creates initial compressive motions in some directions and

rarefactional motions in others. In a compressive wave, the ground first moves

away from the source. In a rarefaction, the

ground first moves toward the source.

From a good seismometer recording, it is

often possible to observe the direction of

this initial motion (for example, from observation of whether the ground first

moves up or down at the seismometer).

This identification method can be powerful if the signal-to-noise ratio is large. But

for small events in the presence of seismic noise (as discussed in chapter 4) it can

be difficult or impossible to determine the

direction of the first motion. Because it

is never clear that rarefactional motions

may not exist in directions for which network coverage is poor, the method at best

identifies an earthquake as an earthquake,

but cannot unequivocally identify an explosion as an explosion.

2. The observation of S waves. Because of the

compressive nature of explosion sources,

explosions typically generate less shear

wave energy than do most earthquakes.

3.

Therefore, the observation of significant

shear wave energy is indicative that the

event is probably an earthquake.

Complexity. Many explosion-generated

body wave signals tend to be relatively

simple, consisting of just a few cycles.

Many earthquake-generated body wave

signals tend to be relatively complex, consisting of a long series (known as the coda)

following the initial few P-wave cycles.

The concept of complexity was developed

in an attempt to quantify this difference

in signal duration. Complexity is the comparison of amplitudes of the initial part

of the short-period signal with those of the

succeeding coda. There are cases, however,

where an explosion signal is complex at

some stations and an earthquake signal

is simple. Therefore, the complexity

method is regarded as not as reliable as

Ms:mb. In practice, whenever the complexity method works, other identification

methods also work very well.

Spectral Methods

The basis of the success of Ms:mb diagrams

is, in part, the fact that the ratio of low frequency waves to high frequency waves is typically different for earthquakes and explosions.

Thus, Ms is a measure of signal strength at

around a frequency of 0.05 Hz, and mb is a

measure of signal strength at around a frequency of 1 Hz. As a way of exploiting such

differences, Ms

:mb diagrams are quite crude

compared to methods that use a more complete

characterization of the frequency content of

seismic signals. The analysis of earthquake and

explosion signals across their entire spectrum

of frequencies is an important component of

the current research and development effort

in seismic monitoring.

Because Ms:mb diagrams work well when

surface waves are large enough to be measured,

the main contribution required of more sophisticated analysis is to make better use of the

information contained in the P-wave signals

and other large amplitude, high frequency signals. This offers the prospect of improved identification capabilities just where they are most
89

needed, namely for smaller events. Thus, instead of boiling down the P-wave signals at

all stations simply to a network mb value, one

can measure the amplitude at a variety of frequencies and seek a discriminant which works

on systematic differences in the way earthquake and explosion signals vary with frequency. One such procedure is the variable

frequency-magnitude (VFM) method, in which

the short-period body wave signal strength is

measured from seismograms filtered to pass

energy at different frequencies, say, fl and f2.

6

Discrimination based on a comparison of mb(f1)

and mb(f2) in many cases shows a clear separation of earthquake and explosion populations

(figure 5-8).7 89

An analogy for the VFM discriminant would

be a person’s ability to tell by ear the difference between a choir with loud sopranos and

weak contraltos, and a choir with loud contraltos and weak sopranos. Ms

:mb is like comparing basses and contraltos. For small events,

VFM is complementary to Ms:mb in several

ways: VFM is improved by interference of the

main P wave with waves reflected from the surface above the explosion, while Ms

:mb is

degraded; VFM is more effective for hard rock

explosions, while Ms

:mb will preferentially discriminate explosions in low-velocity materials;

and VFM is insensitive to fault orientation.

Further, the Ms

:mb method requires averaging over observations from a network of stations surrounding the event, while the VFM

method may be applied at a single station at

any distance and direction from the source. It

is found in practice, however, that the VFM

method is most reliable when several high performance stations are used. (Obviously if sev-

‘W.B. Archambeau, D.G. Harkrider and D.V. Hehnberger, U.S.

Arms Control Agency Report, “Study of Multiple Seismic

Events,” California Institute of Technology, ACDA/ST-220,

1974.

7J.F. Evernden, “Spectral Characteristics of the P-codas of

Eurasian Earthquakes and Explosions,” Bulletin of the Seismological Society of America, vol. 67, 1977, pp. 1153-1171.

‘J.M. Savino, C.B. Archambeau, and J.F. Masse, Discri~”-

nation Results From a Ten Station Network, DARPA Report

SSS-CR-79-4566, S-CUBED, La Jolla, CA, 1980.

‘J.L. Stevens and S.M. Day, “The Physical Basis for m~:M,

and Variable Frequency Magnitude Methods for Earthquake/Explosion Discrimination, ’ Journal of Geophysical Research, vol. 90, March 1985, pp. 3009-3020.

eral stations provide VFM data independently

identifying an event, as an explosion for example, then assigning a numerical probability

that the event observed was indeed an explosion can be accomplished with more reliability and confidence.) VFM is more sensitive to

noise and regional attenuation differences than

Ms:mb, but the main point is that simultaneous use of both methods should allow improved

discrimination of small events.

High-Frequency Signals

The use of high-frequency signals in seismic

monitoring is strongly linked to the question

of what can be learned from seismic stations

in the U.S.S.R. whose data is made available

to the United States. Such “in-country” or “internal” stations were considered in CTBT negotiations in the late 1950s and early 1960s,

and the technical community concerned with

monitoring in those years was well aware that

seismic signals up to several tens of Hz could

propagate from explosions out to distances of

Figure 5-8.—The VFM Method

8.0 I I I

 Earthquakes

 Presumed explosions o

7.0

t

00 80

mb [2.25 Hz]

The body-wave magnitude is determined at two different frequencies. Earthquakes and explosions fall into separate

populations because explosions are relatively efficient at

generating higher frequencies.

SOURCE: J.L. Stevens and S.M. Day, Journal of Geophysical Research, voI 90,

pp. 3009-3020, 1985.
90

several hundred kilometers. The seismic waves

most prominent at these so-called “regional

distances” (by convention, this means distances less than about 2,000 km) are known

as Pg,Pn,Sn, and L (see ch. 3 for a discus -

sion of these regional waves). Pg propagates

wholly within the crust; Pn and Sn travel at

the top of the mantle just below the base of

the crust; and Lg, guided largely by the crustal layer, is often the strongest signal and is

sometimes observed across continents even at

distances of several thousand kilometers.

However, this early interest in regional

waves lessened once the Limited Test Ban

Treaty of 1963 was signed and it was recognized that subsequent programs of large underground nuclear explosions could be monitored teleseismically (i.e., at distances beyond

2,000 km), so that internal stations were not

needed. At teleseismic distances, seismic body

wave signals are usually simpler and have indeed proved adequate for most purposes of current seismic monitoring, even under treaty restrictions on underground nuclear testing that

have developed since 1963. Thus, it is in the

context of considering further restraints on

testing, such as a comprehensive or low-yield

test ban, that the seismic monitoring community needs to evaluate high-frequency regional

seismic signals. The basis for the role of highfrequency monitoring, in such a hypothetical

new testing regime, is that the greatest challenge will come from decoupled nuclear explosions of a few kilotons, and for them the most

favorable signal-to-noise ratios may beat high

frequency. Internal stations, of high quality

and at quiet sites, will be essential to address

the monitoring issues associated with such

events.

It is clear that a dedicated program to make

optimal use of high-frequency seismic data

from internal stations should lead to significant improvements in detection capability.10

The daily recording of many very small seismic events, together with the possibilities for

evasion at low-yields, focuses attention on the

discrimination problem for events in the magnitude range mb 2.0 to mb 4.0.

For an explosion that is well-coupled to the

ground, an mb = 2.0 can be caused by only

a few thousandths of a kiloton, that is, a few

tons of TNT equivalent. At this level, a monitoring program would confront perhaps 1,000

to 10,000 chemical explosions per year. Given

the difficulty of discriminating between chemical and nuclear explosions, it is clear that below some level events will still be detected but

identification will not be possible with high confidence. However, recognizing that relatively

few chemical explosions occur at the upper end

of this mb range (2.0 - 4.0), where many chemical explosions can be identified by characteristics of their signals over a broad frequency

band (see the earlier discussion of ripple-fired

chemical explosions), high frequency data

recorded within the U.S.S.R. can clearly contribute substantially to an improved monitoring capability. It is difficult to reach precise

but general conclusions on what future yield

levels could be monitored, because much will

depend on the degree of effort put into new

seismic networks and data analysis. Conclusions would depend too on judgments about

the level of effort that might be put into clandestine nuclear testing. The discussion of evasion scenarios is taken up in the next chapter,

and high-frequency seismic data are clearly

useful for defeating some attempts to hide

small nuclear explosions.

IOFor a more complete discussion of the use of high-frequency

data for detection, see ch. 4, “Detecting Seismic Events. ”

IDENTIFICATION CAPABILITY

By applying one or more of the methods dis- set of different identification methods, taken

cussed above, many seismic events can be iden- as a whole and applied in a systematic fashtified with high confidence. Note, however, that ion, that must be assessed when giving sumno one method is completely reliable. It is the maries on capability.
91

Identification Capability Within the

U.S.S.R. Using No Internal Stations

The identification capability of any network

of seismic stations will always be poorer than

the detection capability for that network. In

general, a rough estimate of the 90 percent

identification threshold will be about 0.5 mb

above the 90 percent detection threshold for

magnitudes above mb 3.5. Below a magnitude

of about 3.5 mb, this difference may be larer

than 0.5 mb.

In the previous chapter, a network consisting of a dozen or so arrays all outside the Soviet Union was discussed as a type example

to convey a sense of what capabilities can be

achieved. A cautious calculation of the detection capability for such a network was that it

would have a 90 percent probability of detecting at four or more stations all seismic events

within the Soviet Union with a magnitude

(rob) of 3.5 or greater.” Therefore, for seismic

events in the U. S. S. R., a cautious estimate of

the identification capability of such a network

external to the Soviet Union is that the 90 identification threshold will be mb 4.0. From table 5-1, it can be seen that approximately 180

earthquakes occur at magnitude 4.0 or above

each year on Soviet land areas. This would

mean that approximately 18 of these earthquakes would not be identified with high confidence by routine seismic means alone. This,

however, does not translate into an opportunity to cheat, because from the cheater’s perspective there is only a one-in-ten chance that

any given event above this magnitude will not

be identified through seismic means.

Events larger than this will be identified seismically with even higher confidence. A larger

percentage of the events that are smaller will

not be identified, and the number of events will

also increase with decreasing seismic magnitude.

“See ch. 4, “Detecting Seismic Events. ”

Identification Capability Within the

U.S.S.R. Using Internal Stations

With a number of internal stations, it is

clearly possible to attain a much improved detection and location capability within the Soviet Union. From estimates described in the

previous chapter, there is consensus that detection capability (90 percent probability of detection at four or more stations) of mb 2.() -2.5

can be realized, although the debate is not yet

resolved on the number of internal stations

that would be required, nor on whether the

value is closer to mb 2.0 or mb 2.5.

A detection capability down to somewhere

in the range mb 2.0-2.5, however, cannot be

easily translated into a statement about identification capability. Estimation of the capability of hypothetical networks, using regional

seismic waves, is difficult because data on noise

levels and signal propagation efficiency are not

usually available and assumptions must be

made that turn out to have a strong influence

on conclusions. Although there is now general

agreement on the detection capability of

hypothetical networks, there are some significant differences in opinion on the identification capability such networks provide.

Discussion of identification capability for

small events (mb in the range 2.5- 4.0), using

internal stations, is one of the main areas of

technical debate in seismic monitoring. Internal stations will significantly improve the capability to identify many small earthquakes

as earthquakes, because such stations extend

the discriminants related to the MS:mb

method down to lower magnitudes. (Internal

stations will also permit more accurate determination of epicenters and source depths, thus

supplying for small events the most basic information upon which most sources are identified.) But certain types of shallow earthquakes, below some observable magnitude

level, are recognized as difficult if not impossible to identify. Also, preliminary use of high

frequencies in the United States, to discriminate the explosion signals of small nuclear

tests in Nevada from signals of small earthquakes in the western United States, has had
92

some discouraging results. While some say

that this is indicative merely of difficulties

associated with high frequency seismic wave

propagation in the western United States (and

that high frequencies propagate with higher

signal-to-noise ratios in the U. S. S. R.) or that

the instrumentation used in the study prevented data of sufficiently high quality from

being brought to bear on the problem, others

claim that these preliminary results are valid

and are indicative of a fundamental lack of capability in the seismic method, at very small

magnitudes.

In the absence of access to extensive Soviet

data (particularly, explosion data), some guidance in what might be possible is given by

detailed studies of experience with U.S. explosions. Here, it is recognized that use of more

than one discriminant results in improvement

over use of the single best discriminant (which

is usually Ms:mb). In one multi-discriminant

experiment for U.S. explosions and earthquakes

which drew heavily on data such as that presented in the last diagram of figure 5-5 (in

which mb for some earthquakes is less than

3.0), seismic discrimination was achieved for

all events that had both mb and MS values.12 13

The discriminants tested were Ms:mb, combined with a list that included relative excitation of short-period SH waves; relative signal

amplitudes for Pn

, Pg, Lg, and the largest part

of the P wave; generation of higher mode surface waves; long-period surface wave energy

density; relative amplitudes of crustal Love

and Rayleigh waves; excitation of Sn; spectral

ratios of Pn, Pg and L ; various other spectral

methods; and a dept discriminant (see box 

5-B). Not included, was what in practice in the

U.S.S.R. would be a key but not definitive discriminant, namely an interpretation of the epicenter location.

From this body of experience, there appears

to be agreement that, with internal stations

‘*M.D. Denny, S.R. Taylor, and E.S. Vergino, “Investigations

of m~ and MS Formulas for the Western U.S. and Their Impact

on the m~:M, Discriminant, ” UCRL-95103 LLNL, August

1986.

19R.E. Glaser, S.R. Taylor, M.D. Denny, et.al., “Regional Discrirninants of NTS Explosions and Western U.S. Earthquakes;

Multivariate Discriminants, ” UCID-20930 LLNL, November

1986.

Box 5-B.—Progress in Seismic Monitoring

Progress in seismic monitoring has been

characterized by research results that, when

first offered, seemed optimistic but which in

several key areas have withstood detailed

subsequent study and thus have become accepted. Occasionally there have been setbacks,

as noted elsewhere in describing explosioninduced S-waves, and signal complexity.

The current situation is still one of active

research, in that spectral discriminants have

been proposed that (if corroborated by empirical data, which is now lacking) would permit monitoring down to fractions of a kiloton. A key problem in estimating what future

capability is possible is that current research

suggestions often entail data analysis significantly more sophisticated than that required

for conventional discriminants. Database

management would also have to be improved.

The requirement, for operational purposes,

that a discriminant be simple to apply, is thus

in conflict with the requirement that the maximum amount of information be extracted

from seismic signals.

that detect down tomb 2.0-2.5, identification

can be accomplished in the U.S.S.R. down to

at least as low as mb 3.5. This cautious identification threshold is currently set by the uncertainty associated with identifying routine

chemical explosions that occur below this level.

Many experts claim that this identification

threshold is too cautious and that with an internal network, identification could be done

with high confidence down to mb 3.0. At

present, however, this has not been accepted

as a consensus view, partly because some experts are on principle unwilling to extrapolate

from experience with limited U.S. data to a

hypothetical situation that relies on internal

stations in the U.S.S.R. On the other hand, advocates of high-frequency monitoring maintain

that identification can be routine at thresholds

well below mb 3.0. The acceptance of an identification capability below mb 3.0, however,

would probably require practical experience

with data from a monitoring network throughout the Soviet Union.
Chapter 6

Methods of Evading

a Monitoring Network
CONTENTS

Page

Introduction. , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

Evasion Schemes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

Testing Behind the Sun or in Deep Space . . . . . . . . . . . ., . . . . . . . . . . . . . 96

Simulating an Earthquake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

Testing During an Earthquake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

Testing in a Large Underground Cavity-Decoupling . . . . . . . . . . . . . . . . 97

Testing in a Nonspherical Cavity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

Testing in Low-Coupling Materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

Masking a Test With a Large Chemical Explosion. . . . . . . . . . . . . . . . . . . 97

Physics of Cavity Decoupling . . . . . . . . . . . . . . . . . . . ., . . . . . . . . . . . . . . . . . 98

Efficiency of Cavity Decoupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

Limits on Cavity Construction in Salt Deposits . . . . . . . . . . . . . . . . . . . . . 98

Limits on Cavity Construction in Hard Rock . . . . . . . . . . . . . . . . . . . . . . . 99

Cavity Size Requirement for Decoupling . . . . . . . . . . . . . . , . ..........,100

Constraints on Decoupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ........101

Decoupling Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. ...101

Low Frequencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ......101

High Frequencies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. ....102

Decoupling Opportunities in the Soviet Union . . . . . . . . . . . . . . . . . . . . ....103

Partial Decoupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ., ..., ...105

Monitoring Capabilities Considering Evasion . . . . . . . . . . . . . . . . . . . .. ....106

Monitoring Capability Within The U.S.S.R. Using No Internal

Stations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .........107

Monitoring Capabilities Within The U.S.S.R. With Internal Stations ...108

Figures

Figure No. Page

6-l. Minimum Cavity Size Required To Decouple a 5 kt Nuclear

Explosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..............100

6-2. Decoupling Factor as a Function of Frequency. . ..................102

6-3. Salt Deposits in the Soviet Union . . . . . . . . . . . . . . . . . . . . ., . ........104

6-4. Partial Decoupling Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..........105

Table

Table No. Page

6-1. Numbers of Decoupled Explosions of Yield Greater Than 1 ktThat

May Be Possible in Cavities Created by Contained U.S.S.R.

Underground Explosions, 1961-86 . . . . . . . . . . . . . . . . . . . . . . . . . .. ....103
Chapter 6

Methods of Evading a Monitoring Network

Seismic monitoring when combined with treaty constraints and other

monitoring methods must demonstrate a capability to defeat any plausible

scenario for evading the monitoring network

INTRODUCTION

The previous chapters have discussed the capability of various networks to detect and identify seismic events. From this discussion it is

clear that well-coupled nuclear explosions within

the Soviet Union could be detected and identified with high confidence down to yields wellbelow 1 kiloton using a high-quality seismic

network. Yet, in deciding what limits on underground nuclear testing could be verified,

further considerations are necessary. A country attempting to conduct a clandestine test

would presumably use every practical means

to evade the monitoring network by reducing,

masking, and disguising the seismic signal created by the explosion. Consequently, detection

and identification thresholds cannot be directly

translated into monitoring capabilities without considering the various possibilities for

evasion.

As we will see in this chapter, certain evasion scenarios could create serious problems

for a seismic monitoring system under certain

conditions. The need to demonstrate that these

evasion scenarios can be defeated (i.e., the explosions in question identified) with high confidence is what limits our monitoring capability. The problem of evasion must be dealt with

by a combination of seismic methods, treaty

constraints, and other monitoring methods

that reduce the difficulties and uncertainties

of applying seismic monitoring methods to

every conceivable test situation. In short, seismic monitoring needs some help and the obvious approach is to require the structuring of

any treaty or agreement to create a testing

environment that makes it much more likely

that a combination of prohibitions, inspections,

and seismic methods will provide the desired

high levels of verification capability.

EVASION SCHEMES

Over the past three decades, researchers

have conceived a number of theoretical scenarios by which a low-threshold test ban treaty

might be evaded. These include: testing behind

the sun, testing in deep space, detonating a

series of explosions to simulate an earthquake,

testing during or soon after an earthquake,

testing in large underground cavities, testing

in nonspherical cavities, testing in low-coupling

material such as deposits of dry alluvium, and

masking a test with a large, legitimate industrial explosion. While some of these scenarios

warrant genuine attention from a monitoring

perspective, others can be dismissed because

of extreme difficulty of execution or even infeasibility. To determine which are which,

standards of credibility need to be applied.

For an evasion scenario to be credible it must

be technically feasible and it must create a

worthwhile advantage for the country considering cheating. As discussed in chapter 2, a

country considering cheating would have to

evaluate the risks and costs of being caught

against the benefits if not caught. The country concerned about preventing cheating has

to guess the other country’s values for making such decisions. While a slight probability

95
of detection might be sufficient to deter cheating, a much more stringent standard is usually needed to achieve high confidence that any

cheating would be detected. Thus, the degree

of confidence needed to satisfy the concerned

country is often higher than what is needed

in practice to prevent cheating.

Although the majority of proposed evasion

scenarios have been shown to be readily defeated

by a good seismic monitoring network, a few

concepts have evoked serious concern and analysis on the part of seismologists and other scientists. The remainder of this first section protides a brief listing of the various evasion

scenarios. The following three sections discuss

in detail evasion scenarios involving cavity

decoupling and how opportunities for decoupling

could be reduced. The final section assesses

the extent to which the most threatening evasion scenarios limit our capability to monitor

seismically underground nuclear explosions.

Testing Behind the Sun

or in Deep Space

It has been suggested that the Soviet Union

could cheat on all test limitation treaties simply by testing in deep space or behind the sun.

The idea is that one or two space vehicles would

go behind the sun or into deep space. A nuclear device would be detonated and an instrument package would record the testing information and at a later time transmit the data

back to Earth. Some feel that such a testing

scenario is both technically and economically

feasible. Others feel that the technical sophistication, risk, and uncertainty of such a test

exceeds the utility of any information that

could be obtained in such a manner. Such a

test would bean unambiguous violation of several treaties, and hence, discovery would be

costly to the tester. In any case, if clandestine

testing behind the sun or in deep space is demonstrated on technical grounds to be a concern,

the risk could addressed, albeit at considerable

expense, by deploying satellites to orbit around

the sun. Such satellites equipped with detectors for thermal photons could monitor explosions in deep space. Alternatively, the risk

could be addressed politically by negotiating

an agreement to conduct simple inspections

of the rare vehicles that go into deep space.

Simulating an Earthquake

It has been suggested that a series of nuclear explosions could be sequentially detonated over the period of a few seconds to mimic

the seismic signal created by a naturally occurring earthquake. The purpose of such a sequential detonation would be to create a P-wave

amplitude that would indicate an earthquake

when using the Ms:mb discrhninant.

1 This evasion method has been dismissed, however, because it only works if the P-wave amplitude

is measured over just one cycle. If the P-wave

amplitude is measured over several cycles, the

M s: mb discrimin ant will indicate an explosion.

Furthermore, the sequence of waves simulated

by the explosion will only appear as an earthquake over a particular distance range. Consequently, a well-distributed network that

records over a variety of ranges would not be

fooled. In addition, such an evasion attempt

would create large seismic signals that other

discriminants might recognize as being created

by an explosion.

Testing During an Earthquake

The hide-in-earthquake scenario posits that

a small explosive test can be conducted without detection by detonating it shortly after a

nearby naturally occurring earthquake. If the

earthquake is sufficiently large and the explosion is properly timed, the seismic signal of

the explosion will be partially or completely

hidden by the larger seismic signal of the earthquake. This evasion method was at one time

considered a challenge to seismic monitoring

even though the technical difficulties associated with the execution have long been known

to be great. For example, seismologists currently have no reliable techniques for the shortterm prediction of the time, location, and size

of earthquakes, and this limitation is unlikely

to be overcome in the near future.

‘A discussion of M,:m~ and other discriminants is presented

in chapter 5, “Identifying Seismic Events. ”
97

Recent developments in seismic instrumentation and data handling have further reduced

the feasibility of this evasion scenario. New

seismic instrumentation is now capable of filtering so as to pass only high-frequency seismic waves. Because nuclear explosions produce

higher frequency seismic waves than earthquakes, it is often possible to remove the effects of distant earthquakes and see the waves

created by the explosion. For this reason and

the difficulty of detonating an explosion at the

right location and time, the hide-in-earthquake

scenario is no longer considered a credible evasion threat. However, because high-frequency

seismic waves may not always be detectable

at great distances, it maybe necessary to have

seismic stations within the Soviet Union to obviate the hide-in-earthquake scenario at yield

limits as small as a few kilotons.

Testing in a Large Underground

Cavity—Decoupling

If a nuclear explosion is set off in a sufficiently large underground cavity, it will emit

seismic waves that are much smaller than

those from the same size explosion detonated

in a conventional underground test. This scheme,

called cavity decoupling, has been experimentally verified at small yields. It is the consensus

of geologists that significant opportunities exist within the Soviet Union to construct underground cavities suitable for decoupling low

yield explosions. Furthermore, it is the consensus of seismologists that seismic waves can

be muffled by this technique. Consequently,

the technical capability to conduct clandestine

decoupled nuclear tests determines the yield

threshold below which treaty verification by

seismic means alone is no longer possible with

high confidence. The later sections of this chapter discuss cavity decoupling scenarios in detail.

Testing in a Nonspherical Cavity

This evasion scenario suggests that the detonation of an explosion in an nonspherical

cavity could be used to focus the resulting seismic waves away from monitoring stations.

This evasion scenario has been dismissed for

two reasons. First, a nonspherical cavity would

have no better and perhaps worse decoupling

than a spherical cavity of the same volume.2

Second, a monitoring network would have seismic stations in many directions, not just one.

The presence of such stations would increase

the risk of detection by at least one station,

possibly at an enhanced level.

Testing in Low-Coupling Materials

As discussed in the previous chapters, the

proportion of explosive energy converted into

seismic waves depends on the type of rock in

which the explosion occurs. Low-coupling materials such as dry porous alluvium have airfilled pore spaces that absorb much of the explosive energy. This has led to the concern that

a monitoring network could be evaded by detonating an explosion in low-coupling material.

The opportunities for such evasion are thought

to be limited in the Soviet Union because no

great thicknesses of dry alluvium are known

to exist there. In fact, large areas of the Soviet Union are covered with permafrost that

would produce well-coupled seismic signals.

Estimates of the maximum thickness of alluvium in the Soviet Union indicate that it would

only be sufficient to muffle explosions up to

1 or 2 kt. Even if such an opportunity does

exist, alluvium is a risky medium for testing

because it is easily disturbed. An explosion in

alluvium could create a subsidence crater or

other surface expression. Consequently, clandestine testing in low-coupling material is considered feasible only for explosions below 1 or

2 kt.

Masking a Test With a Large

Chemical Explosion

As discussed in chapter 5, chemical explosions are used routinely in the mining and construction industries. In monitoring a low-yield

or comprehensive test ban treaty, there would

be concern that large chemical explosions could

2L.A. Glenn and J.A. Rial, “Blast-Wave Effects on Decoupling

With Axis-Symmetric Cavities, ” Geophysical Journal of the

Royal Astronomical Society, October 1987, pp. 229-239.
98

be used to mask the signals from a nuclear test. decoupled in a large underground cavity and

Unlike an earthquake, such explosions could the reduced seismic signal either masked with

be timed to coincide with a clandestine nuclear the simultaneous detonation of a very large

test. If done in combination with cavity chemical explosion or attributed to a chemidecoupling, this evasion scenario would be a cal explosion. The combination of decoupling

challenge to a monitoring network. For exam- and masking is discussed further in the deple, a nuclear explosion of a few kt could be tailed sections on decoupling scenarios.

PHYSICS OF CAVITY DECOUPLING

An underground nuclear explosion creates

seismic waves with a broad range of frequencies. For purposes of seismic detection and

identification of small events, frequencies from

roughly 1 Hz to perhaps as high as 30 or 50

Hz may be important.

For the lower end of this frequency range,

the amplitude or size of the seismic waves created by an explosion is approximately proportional to the total amount of new cavity volume created by the explosion. A conventional,

or tamped, test is detonated in a hole whose

initial volume is negligible compared to its

post-test volume. Because the initial hole is

small, the rock surrounding the explosion is

driven beyond its elastic limit by the explosion and flows plastically. This flow results in

large displacements of the surrounding rock

mass, and therefore leads to a large cavityvolume increase around the explosion, and efficient generation of seismic waves.

If, on the other hand, the explosion occurs

in a hole of much greater initial volume, the

explosive stresses at the cavity wall will be

smaller. This results in less flow of the rock,

hence less cavity expansion and reduced coupling to seismic waves. If the initial hole is sufficiently large that the stresses in the surrounding rock never exceed the elastic limit, the

seismic couplings minimized. Further increase

of the emplacement hole size will not further

reduce coupling at low frequencies, and the explosion is said to be fullly decoupled.

Cavity construction on a scale required for

explosion decoupling is possible in either salt

of sufficient thickness or in hard rock such as

granite. In either case, the cavity volume required for full decoupling increases in proportion to the explosion yield and decreases as the

strength of the rock increases.

EFFICIENCY OF CAVITY DECOUPLING

Limits on Cavity Construction

in Salt Deposits

Large cavities suitable for decoupled nuclear

testing above 1 kt can be constructed in salt

deposits either by detonating a nuclear explosion of several tens of kilotons, or by solution

mining. For example, a stable, free-standing

cavity was created by the U.S. “Salmon” test,

a 5.3 kt explosion in a salt dome.3 This cavity

‘U.S. Department of Energy, Office of Public Affairs, Nevada

Operations Office, Anncmnced United States Nuclear Tests, July

1945 Through December 1983, NVO-209 (rev.4), January 1984.

was sufficiently large to decouple the subsequent 0.38 kt “Sterling” nuclear test, which

was detonated in the Salmon explosion cavity.4

Nuclear explosions create cavity volume approximately in proportion to their yield. Thus,

applying the yield ratio given in the Salmon/

Sterling experiment, an explosion greater than

14 kt would be required to create a cavity sufficient to fully decouple a 1 kt test; similarly,

an explosion greater than 140 kt would be required to create a cavity sufficiently large to

41bid.
99

fully decouple a 10 kt test. Explosive construction of cavities adequate to decouple shots

above 1 kt would obviously be impossible to

accomplish clandestinely. Past nuclear tests

in the Soviet Union have produced many cavities suitable for decoupling, but the location

and approximate size of most of these is known;

and thus evasion opportunities at these sites

could be limited if activity at them is monitored.

Solution mining on the required scale would

also be difficult to conceal. For example, with

present techniques it would take many months

or perhaps even a year of continuous operation at high circulation rates to solution mine

a cavity adequate to fully decouple a 5 kt explosion. The technology also requires enormous

amounts of water and the disposal of enormous

amounts of brine, further hindering concealment. However, given that such an operation

would be detected and monitored, it might be

difficult to distinguish legitimate and evasionrelated activity. Concealment of the site would

not be necessary if appropriate activity (mining of salt) exists. Consequently, areas of salt

deposits might require treaty provisions dealing with chemical explosions with magnitudes

comparable to decoupled nuclear explosions.

Apart from the significant problems of concealment and resource application, there do not

appear to be constraints preventing the construction, by solution mining, of cavities large

enough to fully decouple explosions up to 10

kt. In fact, the Soviet literature reports solution-mined cavities with volumes up to one million cubic meters. If such a cavity could be constructed with a spherical shape, it would be

60 meters in radius. A spherical cavity with

a 60 meter radius would have sufficient volume to decouple an explosion up to 14 kt, based

on cube root scaling of the U.S. Salmon/Sterling salt dome decoupling experiment. However,

these existing large, solution-mined cavities

are not spherical. They are highly elongated,

irregular in shape, and filled with brine. These

features reduce the size of the explosion that

could be decoupled in the cavity. Furthermore,

the brine in the cavity supports through its

own hydrostatic pressure a considerable portion of the overburden (i.e. the weight of the

overlying rocks). If the cavity was empty, the

overburden pressure would not be supported

and the stability of the cavity would be uncertain.

Both salt domes and bedded salt regions

have to be considered as candidate locations

for construction of decoupling cavities in salt,

although the mining procedure would be more

complex in bedded salt deposits. To create a

cavity in bedded salt, solution mining of soluble layers would have to be combined with explosive mining of insoluble interbeds.

The creep strength of natural rock salt controls the maximum depth at which a stable

cavity can be maintained. Cavity collapse or

major changes in cavity shape occur over time

scales of a few months when the overburden

pressure at cavity depth exceeds the internal

pressure in the cavity by more than about 20

MPa (200 times atmospheric pressure). This

corresponds to a maximum depth of about 1

km for a stable, empty cavity. A brine-filled

or gas-pressurized cavity might be stable to

about 2 km depth. If a cavity is made by an

explosion or by solution mining, the salt will

be weakened. This will be a consideration because for weak salt a larger cavity is needed,

than predicted for strong salt, to fully decouple a given explosion.

Limits on Cavity Construction

in Hard Rock

No cavities have been constructed in hard

rock on the scale of those known in salt. There

is agreement among verification experts that

decoupling cavities with radii up to about 25

meters, suitable for repeated testing up to

about 1 or 2 kt, can probably be constructed

with existing technology. Repeated testing

could likely be detected well enough to get good

locations; and the detection of repeated events

at the same location would be suspicious. There

appear to be no known technological limitations preventing construction of cavities up

to perhaps 45 meters radius, suitable for decoupling explosions up to about 10 kt. How-
ever, the long-term stability under repeated explosive loading is questionable.

In constructing a cavity of radius larger than

about 25 meters, a very extensive network of

long cables would be needed to strengthen and

pre-stress a large region of the rock surrounding the cavity. Such construction would require

an elaborate network of additional tunnels and

shafts in the surrounding rock. The technology is untested on this scale and construction

may be severely complicated in many areas by

the presence of high compressive stresses in

the rock and by joints, fractures, and other rock

inhomogeneities that are present in even the

most uniform granites.

Concealment of such a massive excavation

operation from satellite reconnaissance or

other National Technical Means would be extremely difficult, and thus some plausible cover

operation would probably be necessary. A potential evader would also have to consider the

possible leakage along joints of radioactive

products such as bomb-produced noble gases.

Finally, the evader would also have to be concerned that explosions might result in the

unexpected collapse of the cavity and the formation of a crater on the surface. Such a possibility is not without precedent: in the 1984

“Midas Myth” test in Nevada, 14 people were

hurt and one man killed during the unexpected

formation of a crater above a tiny collapsed

cavern at a depth of 1,400 feet.

Cavity Size Requirement for

Decoupling

The minimum cavity radius required for full

decoupling is proportional to the cube root of

the explosion yield and inversely proportional

to the cube root of the maximum pressure

which the overlying rock can sustain without

blowing out or collapsing. In salt, the maximum sustainable cavity pressure increases approximately in proportion to depth. Therefore,

the minimum cavity size for decoupling is inversely proportional to the cube root of depth

(i.e. smaller cavities will work at deeper depths).

As noted above, however, there is a limit to

how deep a cavity can be maintained in salt,

due to salt’s low creep strength. Thus, there

are two separate issues regarding the depth

of cavities: 1) the deeper the cavity, the smaller

the size required to decouple an explosion of

a given yield, and 2) the deeper the cavity, the

lower the strength of the salt and the more difficult it is to maintain an open cavity. The low

strength of salt eventually limits the depth at

which a cavity can be created. Even the smallest

hole below 1-2 km will squeeze shut.

As discussed earlier, the limiting depth for

stability of a large, empty cavity in salt is

about 1 km. This depth implies that the Salmon/

Sterling cavity (at 0.82 km depth), was very

near the maximum depth for stability of an

empty cavity. Consequently, the Salmon cavity

size approximately sets the lower bound on

Figure 6-1 .—Minimum Cavity Size Required To

Decouple a 5 kt Nuclear Explosion

To fully decouple a 5 kt explosion in salt, a spherical cavity

with a radius of at least 43 meters would be required. The

height of the Statue of Liberty with pedestal (240 ft) is 85°/0

of the required diameter (282 ft).

SOURCE: Office of Technology Assessment, 1988
101

cavity size for full decoupling (at the yield of

the Sterling test). This implies a minimum

cavity radius of at least 25 meters for full

decoupling of a 1 kt test in salt. Cavity requirements are expected to scale as the cube root

of the explosion yield. For example, to fully

decouple a 5 kt explosion in salt, a spherical

cavity with a radius of at least 43 meters (25

times the cube root of 5) would be required (figure 6-l).

In granite, a smaller cavity, perhaps around

20 meters in radius, might be expected to successfully decouple a 1 kt explosion, while a 34

meter cavity would be needed to decouple 5

kt. This number is a rough estimate because

it does not take into account the joints and

fractures that would be present in even the

most uniform granites. The difference between

the salt and granite estimates is due to the

greater strength of granite, which might therefore sustain a somewhat higher pressure. Such

estimates, however, remain uncertain and

some experts doubt that the effective strength

of granites would be greater than salt and believe that a radius comparable to that of salt

would be needed to decouple the same size explosion in granite.

CONSTRAINTS ON DECOUPLING

Decoupling Factors

The reduction of seismic wave amplitudes

achievable by full decoupling is called the

decoupling factor. On theoretical grounds, the

decoupling factor is expected to be smaller at

high frequencies than at low frequencies.5 This

expectation has been confirmed experimentally.

The transition from low-frequency decoupling

to high-frequency decoupling occurs over a

range of frequencies rather than abruptly, and

the transition frequency range depends on

yield. For a 1 kt explosion, seismic waves of

about 6 Hz and below can be assumed to be

controlled by the full low-frequency decoupling

factor, whereas seismic waves above 6 Hz will

exhibit much less decoupling.

Low Frequencies

Several decoupling experiments have been

carried out by the United States. Taken together, these experiments permit us to estimate the low-frequency decoupling factor with

considerable confidence. In the 1966 Salmon/

Sterling experiment, a smaller nuclear explosion was detonated in the cavity created by

‘Donald B. Larson (cd.), Lawrence Livermore National Laboratory, Proc&”ngs of the Department of Energy Sponsored

Cavity Decoupling Workshop, Pajaro Dunes, CaZiforn;a, July

29-31, 1985.

a larger explosion. Analysis of the seismic

waves from these events led to the conclusion

that the low-frequency decoupling factor is approximately 70. That is, a fully decoupled explosion in salt has its low-frequency seismic

amplitude reduced by a factor of 70 compared

to a “tamped,” or “well-coupled,” explosion

of the same yield in salt. The 1985 Diamond

Beech/Mill Yard experiment compared decoupled and tamped nuclear explosions in tuff. In

this case, the observed decoupling factor was

again 70. The 1959 Cowboy series of tests in

dome salt used conventional explosives instead

of nuclear explosives. While initial estimates

of the decoupling factor from Cowboy ranged

from 100 to 150, it was subsequently determined that conventional explosives are significantly less efficient when detonated in a large

cavity than when detonated under tamped conditions. When a correction was made for this

effect, the Cowboy data yielded an estimate

of the full low-frequency decoupling factor of

approximately 70, in close agreement with the

results obtained in the nuclear experiments.

Earlier theoretical estimates that the lowfrequency decoupling factor could be as high

as 200 or greater were based on several simplifying assumptions. Seismologists are now in

agreement that the experimentally determined

decoupling factor of 70 is appropriate at low frequencies.
102

High Frequencies

Roughly speaking, if two explosions excite

low-frequency seismic waves whose amplitudes

differ by a factor F, their high-frequency seismic waves are expected to have amplitudes

whose ratio is approximately the cube root of

F. Thus, seismic theory predicts that the decoupling factor will be much reduced at high

frequencies. The Salmon/Sterling experimental

data corroborate this prediction. Figure 6-2

shows the decoupling factor as a function of

frequency inferred from the Salmon/Sterling

experiment, with both explosions scaled to 1

kt. As already discussed, the low-frequency

decoupling factor averages about 70. However,

the experimentally observed decoupling factor begins to drop at about 6 to 8 Hz, and the

drop is quite sharp above about 10 Hz. At 20

Hz, the decoupling factor is down to approximately 7. This result can reasonably be extrapolated to estimate decoupling for other yields

by scaling the frequency axis in figure 6-2 by

the inverse of the cube root of yield. For example, a 5 kt explosion would be expected to

be decoupled by a factor of approximately 7

at a frequency of about 12 Hz (20 divided by

the cube root of 5). These scaling considerations provide an additional argument in favor

of using high-frequency recordings to extend

monitoring capabilities down to lower yield

levels.

At this time, the exact value of the highfrequency decoupling factor is considered less

certain than the low-frequency factor because

the instruments recording the Salmon explosion lacked sufficient dynamic range to provide reliable data above 20 Hz. High-frequency

data from the Diamond Beech/Mill Yard experiment in tuff also show high-frequency

decoupling factors less than 10, consistent with

the Salmon/Sterling experience in salt. However, interpretation of the Diamond Beech/Mill

Yard data in terms of decoupling is complicated by the facts that the decoupled event

was a factor of 100 smaller than the tamped

event, the decoupling cavity was hemispherical, the measurements were made at short distances, and the events were not co-located.

Data from a better experiment could reduce

the uncertainty in the high-frequency decoupling

factor.

The evidence for reduced decoupling at high

frequencies comes from experiments with spherical (or hemispherical) cavities. However, theoretical calculations show that this conclusion

is not altered even when highly elongated

Figure 6-2.—Decoupling Factor

of Frequency

103

102

10’

10°

as a Function

10° 10’ 102

Frequency, Hz

--- Theoretical _ Average Salmon/Sterling spectral

ratio scaled to 1 kt

● . ● ● 95% Confidence interval

The experimentally observed decoupling factor decreases at

higher frequencies.

SOURCE: Modified from J.R. Murphy, Summary of presentation at the ARPA

Decoupling Conference, Feb. 7, 1979.
103

cavity geometry is considered, that is, it does ated, elongated decoupling cavity may enhance

not appear to be possible to increase the high- high-frequency decoupling in certain preferred

frequency decoupling factor by constructing directions, but will decrease high-frequency

specially shaped, air-filled cavities.6 An evacu- decoupling in other directions.

6Glenn and Rial, op. cit., footnote 2.

DECOUPLING OPPORTUNITIES IN THE SOVIET UNION

Cavity construction for low-yield decoupling

is possible in salt domes, bedded salt, and dry

hard rock. These geologic categories exclude

few areas of the Soviet Union. However, it is

generally agreed that salt domes provide the

most suitable host rock for large, stable cavities. Salt domes are the most suitable because

of the homogeneity of rock salt in domes, the

relative simplicity compared to hard rock of

constructing stable cavities explosively or by

solution mining, and the fact that numerous

large cavities already exist in salt domes in the

Soviet Union. Cavities confined to a single,

homogeneous salt layer in bedded salt, on the

other hand, are limited in size by the layer

thickness. Assuming that the radius of a cavity

in bedded salt should not exceed one-half the

layer thickness, decoupling opportunities are

probably limited to 1 or 2 kt in bedded salt.

Vast regions of the Soviet Union are underlain by salt deposits. The general distribution

of these deposits is indicated by the map in

figure 6-3. However, we probably do not know

the full extent of Soviet salt deposits. Furthermore, although figure 6-3 indicates those areas

where salt domes are prevalent, without access to detailed subsurface geologic data it is

not possible to rule out the presence of domes

in any area of bedded salt in the Soviet Union.

Because the construction of large salt dome

cavities may be difficult to conceal, it is useful to estimate the decoupling opportunity provided by already existing cavities created by

Soviet underground explosions (presumed

tamped). Table 6-1 summarizes this information. At each yield level, the table shows the

number of existing holes large enough for full

decoupling. These numbers refer to cavities

presumed to have remained open following the

largest known Soviet salt dome explosions.

The yields in table 6-1 were estimated by dividing the seismically estimated yields of the

largest Soviet salt dome explosions by the

Salmon/Sterling yield ratio (5.3/0.38 = 14). The

use of this ratio is justifiable because the cavity

volume created by a tamped explosion (in this

case 5.3 kt, Salmon) is proportional to yield

and the largest fully decoupled explosion (in

this case 0.38 kt, Sterling) is proportional to

cavity volume.

Table 6-1 indicates that Soviet decoupling

opportunities at 1 kt and above, using existing explosion-generated cavities, are limited

to three regions: the North Caspian region, the

East Siberian Basin, and a single site in Central Asia. On the basis of table 6-1, there are

Table 6-1.— Numbers of Decoupled Explosions of Yield

Greater Than 1 kt That May Be Possible in Cavities

Created by Contained U.S.S.R. Underground

Explosions, 1961-86a

Yield (kt)

Areas of known salt deposits 123 4 5

b

North Caspian region:

Azgir . . . . . . . . . . . . . . . . . . . . . . . . . . 2 4 2 0

Astrakhan c . . . . . . . . . . . . . . . . . . . . . 1 — — — O

Orenburg. . . . . . . . . . . . . . . . . . . . . ..2 — — — O

Karachaganak. . . . . . . . . . . . . . . . . . . — 5 — — O

Lake Aralsor. . . . . . . . . . . . . . . . . . . . — 1 — — O

Ishimbay . . . . . . . . . . . . . . . . . . . . . . . — — — 1 0

East Siberian Basin:

NW of Lake Baikal . . . . . . ........3 2 — — O

Within a few x 100 km of basin . . . 1 — — 1 0

Bukhara, Central Asia (explosion used

to extinguish fire in oil well) . . . . . . — — — 1 0

Full decoupling in salt: minimum radius (meters) = 25 ● (explosive yield (kt))l/

3

Full decoupling in hard rock” minimum radius (meters) = 20. (explosive yield

(kt))1/3

%btained from yield of known explosion at site divided by yield ratio for

Salmon/Sterling = 5.3/0.38 = 14.

b

(or greater)

cMany cavities capable of being used for full decoupling at yields Of about O 5

kt; some could be connected.
104

. . . . . . . . . . . . . .

.,”.

,.

.

.,
105

no opportunities in existing explosion-gen- mation procedure used in constructing table

erated cavities above 4 kt. It should be noted 6-1 is one that most seismologists would supthat the potential decoupled yields estimated port, within an uncertainty of about 50 perin table 6-1 depend critically on seismic yield cent. This uncertainty translates into 50 perestimates made for the corresponding cavity- cent uncertainty in the decoupled yields in

generating explosions. The seismic yield esti- table 6-1.

PARTIAL DECOUPLING

The size of an explosion that could be decoupled is limited by the maximum size of an airfilled cavity that could reasonably be created

and remain stable. Concern has been expressed,

however, that even if a nuclear device is too

large to be fully decoupled, it could perhaps

be partially decoupled, thus reducing its seismic signal to some extent. It has been further

suggested that by partially decoupling large

explosions, a country might be able to clandestinely test above the 150 kt threshold level

of the Threshold Test Ban Treaty. As figure

6-4 illustrates, however, partial decoupling is

not straightforward.

Figure 6-4 is scaled for the case of a 1 kt explosion and shows how the size of the seismic

signal is affected by partial decoupling. As the

cavity size first increases, the seismic signal

actually gets larger, reaching a maximum for

a cavity about 2 meters in radius. The large

seismic signal is produced at first in a small

cavity because less energy goes into melting

rock and more energy is transmitted into seismic waves. For the case of a 1 kt explosion,

the radius has to exceed about 4 meters before any reduction of the seismic signal occurs,

and must exceed about 6 meters to obtain reduction by more than a factor of 2. After that,

further reduction occurs rapidly. If the relationship for a 1 kt explosion is extrapolated

to larger explosions, the radius (in meters) of

the cavity to begin partial decoupling = 6 *

[size of explosion (kt)]1/3. For a 10 kt explosion,

the size of the cavity required to begin partial

decoupling would be 6 * 101/3 = 13 meter radius; for 150 kt, the radius would have to exceed 32 meters.

Conducting a partially decoupled explosion

also has many risks that the potential evader

would have to consider, including the following:

A. Partially decoupling a nuclear explosion

is uncertain. As seen in figure 6-4, the reduction in seismic signal occurs along a

steep curve. It would be difficult to predict from such a steep curve how much

actual decoupling there will be for a given

explosion. If partial decoupling is not

achieved, an explosion inside a cavity

Figure 6-4.— Partial Decoupling

—

I

.

Effect

I I I I I I I I I ● 1 I I

0.1 1.0 10 100

Initial cavity radius (M) for a 1 kt explosion

The effect of partial decoupling scaled for the case of a

1 kt explosion,

SOURCE: Modified from R.W. Terhune, C.M. Snell, and H.C. Rodean, ‘r Enhanced

Coupling and Decoupling of Underground Nuclear Explosions, ” LLNL

Report UCRL-52806, Sept. 4, 1979,
106

B

might actually produce seismic signals plosion would have to be detonated in a

larger than a well-coupled explosion. cavity near the maximum possible depth

Partial decoupling creates greater pres- to minimize the pressure on the cavity

sure on the wall of a cavity thin- full

decoupling. For example, a 20 kt explosion set off in a cavity suitable for full

decoupling of 10 kt will result in a doubling of the cavity pressure compared to

that for the 10 kt shot. Partial decoupling

damages the cavity wall and this makes

it more difficult to be confident that noble gases and other bomb-produced isotopes will not leak out of the cavity, reach

the surface, and be detected. A 20 kt exwall. Risks of deformation or collapse increase with both the yield and the depth

of the cavity. A risk trade off would be

involved: the desire to minimize the escape of bomb-produced gases leads the

evader to construct a cavity as deep as

possible, whereas construction difficulties, the time needed for construction, and

the risk of cavity deformation or collapse

all become increasing problems at greater

depths.

MONITORING CAPABILITIES CONSIDERING EVASION

The previous two chapters discussed the

various thresholds for the detection and identification of seismic events within the Soviet

Union. These thresholds, combined with the

feasibility of successfully conducting clandestine decoupled nuclear explosions, effectively

determine what levels of nuclear test restrictions can be monitored with high confidence.

As discussed in chapters 4 and 5, well-run

seismic monitoring networks can detect and

identify underground nuclear explosions with

yields well-below 1 kt if no attempt is made

to evade the monitoring network. However, a

country wanting to test clandestinely above

the allowed threshold would presumably attempt to reduce the size of the seismic signal

created by the explosion. For example, a country might attempt an evasion scenario where

the explosion is secretly decoupled in a large

underground cavity and the muffled signals

are then masked by or attributed to a large

chemical explosion that is simultaneously detonated under the guise of legitimate industrial

activity. The problem for the monitoring network is to demonstrate a capability to distinguish such an evasion attempt from the background of frequent earthquakes and legitimate

industrial explosions that occur at low yields.

The monitoring burden placed on the seismic network by various evasion scenarios can

be greatly lessened if seismology gets some

help. Countering the various evasion scenarios

needs to be approached through a combination

of seismic methods, treaty constraints, and

other monitoring methods that reduce the

difficulties and uncertainties of applying seismic monitoring methods to every conceivable

test situation. Specifically, the structure of any

treaty or agreement should create a testing

environment such that a combination of prohibitions, inspections, and seismic methods

will provide the desired high levels of verification capability. Examples of the type of treaty

constraints that have been proposed to improve the capability of various monitoring networks include the following:

Limitations on Salvo-Fired Chemical Explosions: All large salvo-fired chemical explosions above a certain size (depending on

the threshold being monitored and the

area) would be limited and announced well

in advance with inspections/monitoring to

be conducted on-site by the monitoring

side at their discretion.

Limitations on Ripple-Fired Chemical Explosions: All ripple-fired chemical explosions above a certain size (depending on

the threshold being monitored and the

area) would be announced in advance with

a quota of on-site inspections available to

the monitoring party to be used at their

discretion.
107

. Limitations to One Inspected and Calibrated Test Site: All tests would be conducted within the boundaries of one defined test area in hard rock or below the

water table. Further, several calibration

tests recorded by in-country monitoring

stations would be allowed with yields

spanning the threshold yield. Inspections

of the test site would be allowed before

enactment of the treaty to ensure that no

large cavities suitable for decoupling were

present.

● On-going Test Site Inspections: A yearly

quota of on-site inspections by the monitoring party would be allowed at the designated test site.

● Joint On-site Inspections of Sites of Possible Violations: A yearly quota of on-site

inspections would be allowed at sites designated by the monitoring party with

prompt access by a U.S.-Soviet technical

team.

● Country-wide Network Calibration Tests:

Agreement to conduct a number of large

chemical explosions, of both salvo and

ripple-fired types, would be allowed to

evaluate signal propagation characteristics and detection-identification capabilities in particular critical areas.

If these types of testing constraints can be

negotiated within a treaty, reduced thresholds

could effectively be monitored. Keeping in

mind the various types of networks and negotiated treaty constraints that are possible, the

following sections give a sense of the treaty

thresholds that could be monitored.

Monitoring Capability Within The

U.S.S.R. Using No Internal Stations

The threshold for detecting and locating seismic events within the Soviet Union (90 percent probability of detection at four or more

stations), using a seismic network with no internal stations, is at least as low as mb 3.5 (ch.

4). The associated threshold for the identification of 90 percent of all seismic events within

the Soviet Union is at least as low as mb 4.0

(ch. 5). An mb of 4.0 corresponds to a wellcoupled nuclear explosion with a yield of about

1 kt. Consequently, clandestine nuclear explosions above 1 kt would need to be decoupled

to evade the monitoring network. Several considerations limit the threshold at which such

clandestine nuclear tests might be attempted.

Holes large enough to decouple explosions

above a few kilotons would have to be made

in salt domes. Almost all of the known salt

dome regions of the U.S.S.R. and regions that

have any known types and thicknesses of salt

deposits are situated in areas of low natural

seismicity and good seismic transmission. The

detection of seismic events from such areas

would probably be better than average.

Even if an explosion were successfully decoupled and the seismic signal muffled down below the 90 percent identification threshold, it

might still be identified. Decoupled explosions

produce seismic signals that are very

explosion-like. Because there is no breaking of

rock or tectonic release, the signals from decoupled explosions do not look like earthquakes.

This makes the identification of a detected

event as a decoupled explosion likely. Even

though the magnitude of the clandestine test

is below the identification threshold, the test

would in many cases still be well-detected and

located. Also, note that the identification

threshold is for 90 percent identification, that

is, 90 percent of all events above this magnitude will be positively identified. There is no

sharp boundary between identification and

non-identification. Even if the seismic magnitude from a specific event fell somewhat below the identification threshold, there is a good

chance that it would be identified. As discussed

earlier, the identification threshold is largely

set by the problem of distinguishing large

chemical explosions from small decoupled nuclear explosions, so that treaty constraints for

handling large chemical explosions would be

very helpful.

The largest air-filled cavity that could reasonably be created and remain stable would

fully decouple a nuclear explosion of no more

than about 10 kt. While large explosions of up

to 20 or more kt could theoretically be partially
108

decoupled to produce a seismic signal below

the cautious identification threshold, such evasion scenarios are considered implausible because of the practical considerations of containment and predicting the decoupling. In

fact, evasion scenarios for explosions above 10

kt are not considered credible by most experts.

This means that the monitoring of the Soviet

Union with only an external network can be

accomplished down to a threshold of about 10

kt. However, for accurate monitoring of a 10

kt treaty, all experts agree that it would be

desirable to have stations within the Soviet

Union for accurate yield estimation, plus treaty

restrictions for handling the identification of

large chemical explosions in areas where decoupling could take place.

Monitoring Capabilities Within The

U.S.S.R. With Internal Stations

The detection threshold (90 percent probability of detecting a seismic event at four or more

stations) is mb 2.() - 2.5 using a seismic network with internal stations (ch. 4). The associated threshold for the identification of 90 percent of all seismic events is at least as low as

mb 3.5 (ch. 5) and could be reduced depending

on what provisions are negotiated to handle

chemical explosions. This identification threshold corresponds to a well-coupled nuclear explosion with a yield below 1 kt.

Seismic stations within the Soviet Union

would permit lower thresholds to be monitored

by reducing the opportunities for evasion.

Decoupling is possible for explosions with

yields below 10 kt. Consequently, the network

of internal stations would be designed primarily to reduce the opportunities for decoupling.

The most challenging evasion scenario for such

a network would be the situation where a small

(1-5 kt) nuclear explosion is decoupled and the

reduced seismic signal masked by or attributed

to a simultaneous detonation of a legitimate

industrial explosion. As noted above, several

considerations limit the threshold at which

such clandestine nuclear tests might be attempted.

Almost all of the known salt dome regions

of the U.S.S.R. and regions that have any

known types and thicknesses of salt deposits

are situated in areas of low natural seismicity

and good seismic transmission. The exceptions

include salt deposits in the Caucasus, Tadjikistan, and near the Chinese border. An internal monitoring network should involve the

placement of more seismic stations at closer

spacing in those few areas. In addition, those

areas are all near the southern border of the

U.S.S.R. where the detection and identification thresholds either are currently better or

can be made better than the average identification threshold by monitoring from nearby

countries (i.e., Turkey for Caucasus) and stations inside the U.S.S.R.

Many seismologists feel that the discrimination threshold of mb 3.5 is too cautious a

prediction for the capability of an internal seismic network. This identification threshold is

mostly set by the large numbers of chemical

explosions that occur below this level. The limitations imposed by identifying chemical explosions can be approached in two ways: first,

limiting them by treaty (limiting their size and

requiring on-site observers and monitoring)

and second, by further developing techniques

to make use of the expected differences between the signals created by distributed ripplefired chemical explosions and the concentrated

point explosions characterizing decoupled nuclear tests. While chemical explosions in the

U.S.S.R. of mb 3.0 are likely to be more common than those of mb 3.5, the monitoring need

only be concerned with those chemical explosions of mb 3.0 and larger that are located in

areas of known or possible salt domes. This

excludes very large areas of the Soviet Union.

A monitoring network with stations internal

to the Soviet Union should concentrate on

areas of poor transmission and areas where

decoupling opportunities would be possible.

Through such a strategy and with constraints

on chemical explosions, many predict that the

identification threshold will be closer to mb

3.0. This would significantly reduce the size

of decoupled explosions that could be clandestinely attempted.
109

All of the considerations so far have not

made allowances for increased verification capability afforded by high-frequency recording.

The recent N.R.D.C. recordings to very high

frequencies at distances of 200-650 km from

three chemical explosions with yields of 0.01

-0.02 kt are very impressive in this regard.

From these data, it appears that explosions

with yields comparable to a fully decoupled 2.6

-3.8 kt explosion (corresponding to magnitude

mb 3.0) in areas of good transmission will produce large signals with frequencies of 10-20

Hz. This is also a frequency band in which the

decoupling factor will be small.

The decoupling reduction that is assumed

for these evasion scenarios is a factor of 70.

If the monitoring system has even a modest

capability to record frequencies as high as 10

-15 Hz, the effectiveness of the decoupling

would be greatly reduced. Figure 6-2 indicates

a decoupling factor of 30 for frequencies of 1

-2 Hz and 50 as averaged from 1 to 5 Hz. At

high frequencies, the decoupling factor will

probably be reduced from 70 to below 10.

Smaller decoupling factors will result in a lower

(better) threshold for the detection and identification of decoupled explosions of a given

yield.

Decoupling combined with masking remains

a challenging evasion scenario even with a

high-quality internal network. Opportunities

for such evasion, however, would be limited

by the many practical considerations described

above and throughout this report. Attempting evasion by this complicated scenario would

entail further risk when viewed in conjunction

with all types of intelligence gathering, rather

than purely as a problem for seismic discrimination. Detected seismic events in areas of possible decoupling would be suspicious and presumably focus attention. On-site inspections

could play an important role as opportunities

for cheating could be still further reduced by

negotiated agreements requiring prior announcement and possible on-site inspections of large

chemical explosions in areas of potential decoupling.

Small differences of opinion concerning monitoring capability will always remain because

parts of the debate are comparable to discussions of “half-full” versus “half-empty’ glasses

of water. Some will review the complex operation of seismic monitoring and will conclude

that a country could cheat if any step in the

process is uncertain. The chain is only as good

as its weakest link. Others will review the complicated evasion scenarios that have been

postulated and conclude that evasion is too difficult and uncertain to be credible. Cautious

assumptions about seismic monitoring capability and generous assumptions about the

likelihood of successfully conducting clandestine decoupled nuclear explosions can be combined to produce the conclusion that even with

an internal network an explosion of up to 10

or 20 kt could be partially decoupled in the

largest hole (capable of fully decoupling a 10

kt explosion) to create a seismic signal below

the mb 3.5 identification threshold. On the

other hand, generous assumptions about monitoring capabilities and favorable assumptions

about uncertainty and the role of other intelligence gathering systems can be combined to

produce the conclusion that even explosions

of a fraction of a kiloton fully decoupled can

be effectively monitored with high confidence.

Considering all of the arguments, however, a

few general statements can still be made concerning monitoring capability with an internal seismic network.

Most experts agree that a high-quality network of internal stations combined with stringent treaty constraints, could monitor a threshold of around 5 kt. Differences of opinion range

from 1 to 10 kt and are due to judgments about

the level of constraints that can be negotiated

into the treaty and what levels of motivation

and risk the Soviet Union would be willing to

take to test clandestinely slightly above the

threshold. Experts further agree that below

1-2 kt, monitoring would become much more

difficult because additional methods of evasion

are possible. Explosions of 1 or 2 kt could be

decoupled not only in salt but also in other media such as granite and alluvium. At present,

there is not a consensus that an internal net-
110

work would be capable of positively identify- perience of low-yield monitoring within the Soing with high confidence all such evasion at- viet Union together with a high level of negotempts. If such a capability is possible, it will tiated supplementary measures to limit certain

require demonstration through practical ex- evasion opportunities.
Chapter 7

Estimating the Yields of

Nuclear Explosions
CONTENTS

Page

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...........113

Measuring the Size of Seismic Signals.. . ............................113

Determining Explosive Yield From Seismic Magnitude . ...............115

Sources of Uncertainty. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...........,.116

Bias Correction for Soviet Nuclear Explosions. . . . . . . . . . . . . . . . . . . . . ...117

Reducing Uncertainties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..............120

Random Uncertainty. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. ... ... ... ..120

Systematic Uncertainty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .........,122

Yield Estimation Capabilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..122

Soviet Test Practices and Test Ban Compliance . . . . . . . . . . . . . . . . . . . . ..124

Box

Box Page

7-A. Calculations of the Six Largest East Kazakhstan Explosions.

By Sykes et al., Based on Unclassified Data . ....................124

Figures

Figure No. Page

7-l. Computation of P-Wave Magnitude . . . . . . . . . . . . . . . . . . . . . . .......114

7-2. Explosions in Granite . . . . . . . . . . . . . . . . . . . . . . . . .................115

7-3. Yield Data From the Nevada Test Site . . . . . . . . . . . . . . . . . . . . .. ....116

7-4. Yield Estimate Distribution . . . . . . . . . . . . . . . . . . . . . ...............116

7-5. Schematic Illustration of Attenuation-Related Magnitude Bias .. ....118

7-6. Comparisons of Explosions in Eurasia With Explosions in North

America. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..119

7-7. Uncertainty in Yield Estimates . . . . . . . . . . . . . . . . . . . . . . . . . . .......121

7-8. mb Versus Time for Large Soviet Explosions . . . . . . . . . . . . . . . . . . . . .125

7-9. U.S.S.R. Nuclear Tests 1966-86 According to Sykes and Davis .. ....126
Chapter 7

Estimating the Yields of Nuclear Explosions

For treaties that limit the testing of nuclear weapons below a specific

threshold, the yield of the explosion must also be measured.

INTRODUCTION

Once a seismic event has been detected and

identified as a nuclear explosion, the next step

is to estimate the yield of the explosion. This

is particularly important for monitoring treaties that limit the testing of nuclear weapons

below a certain threshold. The process of estimating the yields of Soviet explosions involves

three steps: 1) calculate the magnitude of the

seismic signal; then, 2) make corrections to adjust for the different geology at each test site;

and finally, 3) convert the magnitude into a

yield estimate.

The final yield measure describes the explosive energy of a nuclear explosion in terms of

kilotons, where 1 kiloton (kt) was originally defined as the explosive power equivalent to

1,000 tons of TNT. This definition was found

to be imprecise,’ however, and so it was agreed

in the United States during the Manhattan

project that the term “kiloton” would refer to

the release of 1012 calories of explosive energy.

1 kiloton = 1,000,000,000,000 = 1012 calories

While this convention is also followed in the

Soviet Union, it does not necessarily mean that

*The definition is not precise for two reasons. First, there is

some variation in the experimental and theoretical values of

the explosive energy released by TNT (although the majority

of values lie in the range from 900 to 1,100 calories per gram).

Second, the term “kiloton” could refer to a short kiloton (2x106

pounds), a metric kiloton (2.205x106 pounds), or a long kiloton

(2.24x106 pounds).

the United States and the Soviet Union calculate explosive yields in the same way. Only that

part of the total energy released in a nuclear

explosion that is immediately available (the socalled prompt energy release) is counted in the

yield, and there does not appear to be any generally accepted precise definition of what energy

release time scale is considered “prompt.” Also

a complication arises in determining the yield

of underground nuclear explosions due to

energy released by interaction of the neutrons

from the explosive device with the surrounding ground. Consequently, there might be

slight differences in how the United States and

the Soviet Union measure yields.

Yields can be estimated not only through

seismic means, but also by other methods. The

other methods are based on analysis of the nuclear byproducts of the explosion (radiochemical methods) or measurements of the speed

of the shockwave generated by the explosion

in the surrounding rock (hydrodynamic methods). Neither radiochemical nor hydrodynamic

methods are currently used by the United

States to measure routinely the yields of Soviet explosions because they require access to

the test site during the test, and in the case

of radiochemical methods, may require information about the weapon design that could reveal sensitive information concerning the characteristics of the weapon.

MEASURING THE SIZE OF SEISMIC SIGNALS

At present, U.S. estimates of Soviet yields waves, Rayleigh waves, and Lg waves.

2 The

are generally made using seismic waves re- various magnitudes are averages based on

corded at teleseismic distances (distances

greater than 2,000 km). Seismic magnitudes 2A description of the various types of seismic waves is precan be determined from the amplitudes of P sented in Chapter 3, The Role of Seismology.

113
114

recordings at several stations. The magnitudes

are then converted to explosive yields using

formulas developed through past experience.

The formulas used are based on testing experience at the Nevada test site and at the test

site operated in the Sahara by France in the

1960s. The three magnitude measures most

often used in yield estimation are: the P-wave

magnitude mb the surface wave magnitude

Ms, and the Lg

-wave magnitude mb(Lg).

The mb is computed from measurements of

P-wave recordings by the use of the formula

m b = log (A/T) + B.

As illustrated in figure 7-1, A is the largest

P-wave amplitude in nanometers (0.000000001

meters) measured peak-to-peak from a seismic

short-period recording during the first few seconds of the P wave and correcting it for the

instrument magnification, T is the duration of

one cycle of the wave in seconds near the point

on the record where the amplitude was measured (for P waves the period is typically 0.5

to 1.5 seconds), and B is a distance-dependent

correction term that compensates for the change

of P-wave amplitudes with distance.

The surface wave magnitude is determined

by measuring the Rayleigh-wave amplitude

near the point where the dominant period of

the wave is nearest to 20 seconds on longperiod vertical component records. The formula used is

Figure 7-1.–Computation of P-Wave Magnitude

A

 second s

o 1 2 3 4 5

Measurement made on P waves to obtain the magnitude of

a seismic event. The peak-to-peak amplitude (A) in the first

few seconds of the P wave is corrected for the instrument

magnification at the dominant period T.

Ms = log (A/T) + D,

where A and T are the amplitude and the

period measured off long-period vertical component seismic recordings, again in nanometers

and seconds, and D is a distance-dependent correction term for Rayleigh waves.

The magnitude measure derived from measurements of Lg waves is computed from the

formula

mb (Lg) = 5.0 + log [A(10km)/110],

where A( 10 km) is the maximum sustained amplitude of Lg on short-period vertical records

in nanometers extrapolated backwards to a distance 10 km from the source by dividing by

the geometrical spreading factor of d-5/6, where

d is the source-to-receiver distance, and by the

estimated attenuation along the path. The empirical mb

(Lg) v. log Yield (Y) relationship also

includes a small second-order term, giving

mb

(Lg) = 3.943 + 1.124 log Y -0.0829 (log Y)2

for explosions in water saturated rocks such

as those at the Nevada Test Site.

The mb magnitude is routinely used at teleseismic distances for yield estimation because

P waves are detectable at large distances, even

for small seismic events. This measure can

almost always be obtained for any seismic

event that is detected. The measurement of

Ms requires a larger event, because Rayleigh

waves are small for nuclear explosions. For explosions below 50 kt, Ms may be missed altogether at teleseismic distances. The Lg amplitude is similarly weak for small explosions.

Consequently, it maybe important for seismic

stations to be close to the explosion if surface

waves and Lg waves are to be used for yield estimation of explosions less than 50 kt. This is one

of the reasons why seismic stations within the

territories of the treaty participants are desirable. The distance correction factors can be quite

variable regionally, and hence, some of the

magnitude-yield relationships will need to be

adjusted for different regions.

In addition to the conventional surface wave

magnitude Ms, a new measure of source

strength for surface waves is coming into wide
715

use. Called seismic moment (MO), it is an estimate of the strength of a compressional (explosion-like) force at the explosion site. Seismic moment gives a direct description of the

force system, acting in the Earth, that would

make seismic waves of the size and shape actually recorded. The advantage of using seismic moment is that the computation can correct for the estimated effects of contamination

of the seismic signals due to earthquake-like

motion triggered by the explosion. This is useful because nuclear explosions often release

stress that has been built up in the area of the

explosion by geological processes. The release

of built-up stress by the explosion creates a

surface wave pattern similar to that observed

for earthquakes, which is seen superimposed

on the signals of the explosion. Characteristics of an earthquake, such as Love waves and

reversed polarities in the Rayleigh waves, are

often observed from a nuclear explosion, indicating release of pm-existing stress. If not

removed, this release of natural stress by the

explosion, called tectonic release, can distort

yield estimates obtained from conventional

Ms.

DETERMINING EXPLOSIVE YIELD FROM SEISMIC

MAGNITUDE

Once the seismic magnitude measurements

have been made, the next step is to relate the

magnitude measurements to the yield of the

explosion. Because we know the actual yields

only of U.S. tests and some French nuclear explosions (the Soviets have announced yields

only for a few of their tests), our knowledge

is based on data other than Soviet data. The

actual data used to derive this relationship are

shown in figure 7-2. The relationship between

the yield of a nuclear explosion and the measFigure 7-2.—Explosions in Granite

I

4.0

I

Yield (kt)

Data for explosions in granite from which the magnitude v.

yield equation is derived.

SOURCE: Modified from Air Force Technical Application Center.

ured seismic magnitude can be described using

an equation of the general form

M = A + B log Y + Bias Correction

where M is a magnitude measure (or moment)

from surface waves, body waves, or Lg waves,

A and B are constants that depend on which

magnitude measure is used, and Y is the yield

in kilotons. The specific constants used by the

United States for these calculations are classified. The “Bias Correction” term is an adjustment made to correct for the differences

in how efficiently seismic waves travel from

the various test sites. This correction is particularly important for mb

, because shortperiod body waves are strongly affected by the

physical state (especially temperature) of the

medium through which they travel.

The empirical magnitude-yield relationships

for mb that are used to estimate yields at inaccessible test sites in the U.S.S.R. and elsewhere have been revised several times during

the last two decades. These revisions were improvements in yield formulas and computational procedures to correct for such problems

as difficulties in merging magnitude sets from

different station configurations and instruments, clipping (limiting the maximum recordable amplitudes) of large signals by the recording systems, and not correctly accounting for
116

differences in the geology at different test sites.

The early magnitude-yield formulas were based

on the simplifying assumption that all nuclear

explosions in granite at any site follow a simple

linear relationship between mb and log(yield).

After the factors listed above were properly

considered, however, it became obvious that

bias corrections for each test site were needed.

SOURCES OF UNCERTAINTY

Under the ideal conditions of a perfectly uniform and symmetrical Earth, it would be possible to estimate yields of nuclear explosions

at any site from measurements at a single

seismic station. In practice, however, seismic

magnitudes and magnitude-yield plots show

scatter. Using data from the International

Seismological Centre3 as an example, individual mb measurements typically have a standard deviation of 0.3 to 0.4 magnitude units before station corrections are applied. When

station corrections are applied, the standard

deviation is reduced to 0.1 to 0.15 units. Figure 7-3 illustrates typical scatter in a magnitude-yield plot.

‘The International Seismological Centre is an organization

based in England that gathers data for the research community from thousands of seismic stations operated all over the

globe.

Figure 7-3.—Yield Data From the Nevada Test Site

7.0 

6.0

5.0

4.0

Yield (kt)

Mb versus yield for explosions at the Nevada Test Site. The

scatter is characteristic of yield measurements when only

P-wave magnitudes are used.

SOURCE: Modified from Air Force Technical Applications Center.

One reason for this variation is the smallscale geologic contrasts in the Earth that cause

focusing and scattering of seismic waves. Focusing effects near the recording seismometers

can create differences in estimated magnitudes

even when the stations are closely spaced.

Focusing effects near the explosion can cause

broad regional variations of seismic amplitudes

so that seismic observatories over whole continents may observe higher or lower average

amplitudes than the global average. Fortunately, the uncertainty introduced by focusing

effects can be reduced by averaging measurements from numerous stations if the stations

are well distributed around the test sites in

both distance and direction.

In addition to the scatter due to focusing,

geological structures under individual stations

may amplify seismic waves. Such effects may

Figure 7-4.—Yield Estimate Distribution

95% of

total area

—

o 75 150 225 300 375

Yield estimate. in kilotons

50 % of

Yield estimate, in kilotons

Probability of yield estimates for a 150 kt explosion measured with a factor-of-2 uncertainty.
117

be corrected for by applying statistically derived station corrections that compensate for

any such local effects.

After averaging many measurements and

applying appropriate corrections, estimates of

the yield of a nuclear explosion are expected

to be distributed about the “true” value in the

manner indicated in figure 7-4. The horizontal

axis in this figure is the yield estimate while

the vertical axis is the probability that the estimate is correct. The area under the curve between 2 yield values represents the probability that the actual yield is in this interval (the

percentage chance is 100 times the probability and the total area under the curve is 1, giving a 100 percent chance that some value of

magnitude will be measured). This figure shows

that it is most likely that the central yield value

(150 kt in this case) will be close to the actual

value and that outcomes become increasingly

less likely the larger the difference between the

estimated value and the central value (see chapter 2 for a more detailed discussion of uncertainty and what it represents). The yield distribution is asymmetric due to the normal

distribution of mb and the logarithmic relationship between the yield of the explosion and

the measured seismic magnitude. Figure 7-3

is a typical empirical magnitude-yield curve

obtained from actual data at the Nevada test

site that shows the measurements do not follow a single line but scatter around it because

of measurement errors and variations in rocks

surrounding the explosions.

Some of the uncertainty described above is

due to variations in how well explosions are

coupled to the surrounding rock. Also, explosion depth can influence the amplitudes of the

seismic waves emitted, as can variations in the

physical properties of the Earth. For inaccessible test sites, these effects result in increased

uncertainty in estimating yields. However, if

data were exchanged and calibration shots performed, corrections could be made that would

greatly reduce the uncertainty. Nevertheless,

there will always be some uncertainty in estimates of the yields of Soviet explosions, as in

estimates of any physical quantity. This is not

unique to seismology. Some uncertainty will

exist no matter what type of measurement system is used. Such uncertainty should not necessarily be considered to represent opportunities for cheating. Chapter 2 discusses the

meaning of the various uncertainties and their

implications for cheating.

BIAS CORRECTION FOR SOVIET NUCLEAR EXPLOSIONS

In estimating the yields of Soviet explosions,

a major concern is how well the magnitudeyield formula for U.S. tests can be applied to

Soviet test sites. Geophysical research has

shown that seismic P waves traveling through

the Earth’s mantle under the main U.S. test

site in Nevada (and many other areas of the

world as well) are severely attenuated when

compared to most other continental areas,

especially those with no history of recent plate

tectonic movements. If not corrected for, this

attenuation will cause a sizable systematic error in estimates of the yield of Soviet explosions.

The apparent reason for this attenuation is

the high temperature in the upper mantle under Nevada and many other tectonically active regions. Regions of high temperatures

change the elastic and absorptive properties

of the rocks, causing a large loss in the amplitudes of seismic waves traveling through them.

Similar phenomena are thought to occur under the French test sites in Algeria and the

Pacific, though not under either the Soviet test

sites in Kazakh and Novaya Zemlya or the U.S.

test sites in Mississippi and Amchitka. If the

P-wave magnitudes observed from U.S. tests

in Nevada are used as a basis for estimating

yields, most Soviet explosions which have been

exploded in areas where the upper mantle is

cool and there is little attenuation of P waves

will appear considerably larger than they actually are.
118

The evidence for such attenuation effects

comes from many studies, including:

●

●

●

●

comparisons of P-wave amplitudes observed at the Nevada test site with observations made at other sites in areas underlain by colder mantle;

studies of short period S-wave amplitudes,

which are very sensitive measures of mantle temperature variations;

studies of the frequency content of both

P and S waves, i.e., the relative loss of high

frequency energy in waves traveling

through the upper mantle in both directions under Nevada; and

studies of P- and S-wave velocities, which

In addition, there is a large amount of independent geophysical evidence supporting the

notion of anomalously high temperatures under most of the western United States. This

evidence includes:

●

●

●

measurements of anomalously high heat

flow,

measurements of electrical conductivity,

and

the low velocity P waves (Pn) and the absence of S waves (Sn

) that propagate just

under the Earth’s crust.

These “symptoms” of high attenuation have

been observed in many other areas of the world

and are recognized as such by most geophysiare also influenced by temperature. cists. The sketch in figure 7-5 illustrates how

Figure 7-5.—Schematic Illustration of Attenuation. Related Magnitude Bias

Signal from Signal from

Soviet test U.S. test

Soviet Union’s

test site

Us.

test site

Seismic body waves crossing parts of the upper mantle with high temperatures become anonymously reduced in amplitude.

Seismic signals from the Soviet Union’s test site appear much larger than signals from an identical explosion conducted at

the U.S. test site.

SOURCE: Modified from Defense Advanced Research Projects Agency.
119

this attenuation is created in the Earth and

affects estimates of the size of the wave source.

Seismic body waves crossing the hatched high

attenuation zones in the upper mantle are reduced in amplitude and high frequency components of wave motion relative to waves that

bypass such zones.

Magnitudes derived from Rayleigh waves

and Lg waves are less influenced by temperature variations in the mantle because they

travel along the surface and largely bypass the

high attenuation zones in the upper mantle.

A plot of P-wave magnitudes (rob) against surface wave magnitudes (Ms

) should, therefore,

show the attenuation of P waves relative to

surface waves. By plotting the Ms - mb ratio

of explosions for different test areas, the attenuation indifferent regions can be compared.

Figure 7-6 shows the results of an early study

that compared the Ms - mb for explosions in

Eurasia with the Ms - mb for explosions in

North America. It can be seen that the two

groups are offset, with explosions of the same

Ms value having lower mb values in North

America than in Eurasia. Results like this led

to early speculation about the existence of high

attenuation zones in the upper mantle.

In general, if the P-wave magnitudes are

plotted against the Rayleigh or Lg magnitudes for the Nevada and Soviet test sites, the

2 sets of data are offset by about 0.3 to 0.4

magnitude units (or an amplitude factor of

about 2 for P waves).4 The most likely explanation for this offset is the reduction of P-wave

magnitudes due to attenuation at the Nevada

test site. Such data constitute additional support for the idea of an attenuation bias for P

waves. Offsets can also be brought about by

other factors such as contamination of the MS

measurement by tectonic release. However,

such contamination can be detected by the

strong Love waves the release generates, and

“See for example P. Il. Marshall and P. W. Basham, “Discrimination Between Earthquakes and Underground Explosions

Employing an Improved M. Scale, ” GeophysicaZJoumal of the

libyzdAstronomicaJ Soa”ety, vol. 28, pp. 431-458,1972, and Otto

W. Nuttli, “L~ Magnitudes of Selected East Kazakhstan Underground Explosions, ” BulZetin of the Seismological Society

of Ame~”ca, vol. 76, No, 5, pp. 1241-1252, October 1986.

Figure 7-6.—Comparisons of Explosions in Eurasia

With Explosions in North America

7.0

6.0

5.0

I I 1 I

3.0 4.0 5.0 6.0

M s

Explosions

● Eurasia o North America

Explosions with the same MS value have lower mb values in

North American than in Eurasia. This led to early speculation about the existence of high attenuation zones in the

upper mantle.

SOURCE: Modified from P.D. Marshall and P.W. Basham, Geophysical Journal

of the Royal Astronomical Society, 1972, vol. 28, pp. 431-458.

reduced by using seismic moment instead of

surface wave magnitude (Ms) for yield estimation.

Various government-supported scientific

panels of seismologists, after considering the

totality of the geophysical evidence, have

repeatedly recommended during the last decade that U.S. yield estimates of Soviet explosions be reduced by subtracting a larger “Bias

Correction” term from the magnitudes to account for the attenuation effect on mb

. As a

result, the bias correction has been increased

on several occasions over the last decade as

new scientific evidence indicated that such

changes are appropriate.

The size of the bias correction was determined simply by averaging the correction inferred from a number of independent or semiindependent estimates of the attenuation effect made by different researchers. Most evidence for an attenuation “bias” has been indirect thus far, although the evidence from

76-584 0 - 88 : QL 3 - 5
120

global seismic studies and seismological experience gives strong support to the idea. More

direct measurements of this bias may soon become available. The United States and Soviet

Union have recently agreed on experiments to

calibrate seismic yield estimation methods

through measurements at each others test

sites. Explosions will be measured with seismic methods and the yields confirmed independently by hydrodynamic methods. In addition, several seismic stations have been set up

recently in the Soviet Union near the Kazakh

test site by a group of U.S. scientists supported

through the Natural Resources Defense Council. Data from these stations will help improve

estimates of the bias correction and assess the

efficiency of seismic wave propagation at high

frequencies to regional distances.

The bias correction is currently used as a simple, yield-independent adjustment to the intercept, A, in the rob-log Y curve. The value

currently used by the U.S. Government is intended to be the most appropriate value for

yields near the 150 kt threshold of the 1974

Threshold Test Ban Treaty. A different bias

may be appropriate for yields that are either

much larger or much smaller.

REDUCING UNCERTAINTIES

The estimated yield of an underground of nuclear explosion, like any quantity derived from

measurements, has some error associated with

it. The error comes from a variety of sources.

Some of the error is considered to be random

in that it varies unpredictably from one measurement to another. Other errors are not random but are systematic. Systematic errors are

those that always act in the same way, for example, the bias between test sites. If systematic errors are understood, corrections can be

made to reduce or eliminate the error.

The distinction between random and systematic errors, however, has no clear boundary.

If everything about the Earth and seismic

waves were known, almost all errors in seismology would be systematic. In general, random errors usually turn out to be systematic

errors once the reason for the error is understood. However, if the systematic errors are

not understood, or if there are lots of systematic errors all operating indifferent ways, then

the systematic errors are often approximated

as random error. In such cases, random uncertainties are inflated to encompass the unexplained systematic uncertainties.5

‘As discussed in Chapter 2, random errors do not provide opportunities for cheating. However, if systematic errors are found

to be 1) of sufficient size, 2) usable for an advantage by one side,

and 3) unrecognized as being systematic by the other side, then

such errors can be exploited under some situations. A treaty

should, therefore, contain provisions to reduce the uncertainty

of yield estimates and counter evasion opportunities.

Random Uncertainty

Different methods of yield estimation have

different accuracies and uncertainties. At the

Nevada test site, the most precise method uses

P-wave magnitudes (rob). Less precise methods use Lg waves (mb

(Lg)), surface waves (MS),

 Ms and seismic moment ( At the Nevada test )

site, yields estimated from mb alone have a

random uncertainty factor of 1.45 at the 95

percent confidence level, whereas those from

mb

(Lg) have an uncertainty factor of 1.74, and

those from Mo have an uncertainty factor of

2.13.

Recently, the Defense Advanced Research

Projects Agency (DARPA) has been able to

reduce the random uncertainty in seismic yield

estimates at the Nevada test site by combining measurements made by the three different methods. Scientists have shown, using

data from U.S. explosions at the Nevada test

site, that the random errors of the three types

of magnitude measures for a given event can

be considered statistically independent. Consequently, an improvement in the accuracy of

yield estimates can be achieved by combining

several methods to produce a “unified” magnitude measure. By forming a weighted average of the three magnitudes, a “unified seismic magnitude” with an uncertainty factor of

1.33 (figure 7-7) has been derived. Most seismologists believe that if this method were now
121

Figure 7-7.—Uncertainty

6.2

5.4

1.74

in Yield Estimates

/

100 200 300400

/

2.13

Yield (kt)

95%

Factor

1.33

1 I

100 200300400

Yield (kt)

Uncertainty in yield estimates can be greatly reduced through the use of a unified seismic yield estimate. On the left are three

plots of mb

, mb(Lg), and MO versus yield at the Nevada Test Site. On the right is a similar plot of the unified seismic yield estimate versus the actual yields. The 95 percent uncertainty factors are shown to the right of each plot.

SOURCE: Modified from Defense Advanced Research Projects Agency.

applied to estimating yields at the Soviet test

site in Eastern Kazakh, the uncertainty would

be reduced to a factor of 1.6- 1.5 for explosions

around 150 kilotons.6 What limits the uncertainty from being reduced to the level of the

‘Consider, as an example, the situation where there are 3 statistically independent methods of calculating the yield of an

explosion, all of which (for the sake of this example) have a factor of 2 uncertainty in a log normal distribution:

# of Methods Resulting Uncertainty

1 2.0

2 1.6

3 1.5

By combining methods, the resulting uncertainty can be reduced. This methodology, however, can only reduce the random

uncertainty. Systematic uncertainty such as differences between

the test sites will remain and limit the extent to which the uncertainty can be reduced unless calibration is performed.

Nevada test site (a factor of 1.3) is the systematic uncertainty or bias correction. As we will

see later, however, this systematic uncertainty

can be reduced through calibration shots.

The expected precision given above are only

for explosions where all waves are used for the

estimation. For smaller explosions, the regional Rayleigh and L waves are not always

strong enough to travel the long distances required to reach seismic stations outside the Soviet Union. Consequently, for monitoring lowyield explosions, stations within the Soviet

Union may be necessary to obtain the improved

accuracy of the “unified seismic yield” estimation method. The relationship between magnitude and yield for the stations within the Soviet Union will also have to be established.
122

As noted above, the formulas derived from

the Nevada test site data that describe the relationships between yield and mb

(Lg) and moment MO are not directly applicable to the Soviet test site in Kazakh without some as yet

unknown adjustments. The values of these adjustments can be determined if stations are

placed within the Soviet Union and the Soviet

test sites are calibrated. Test site calibrations

suitable for lower yields could only take place

after an internal network is installed. If the

mb(Lg) and MO v. yield curves are suitably

calibrated, the absolute yields of explosions

at Kazakh should be measurable with the “unified seismic method” just as accurately as at

the Nevada Test Site.

The above analysis applies only to explosions

at known test sites observed by a large set of

well-distributed seismic stations for which the

appropriate station corrections and bias correction have been determined. The accuracy

with which the yields of “off-site” nuclear tests

could be estimated would be less than that

stated above. Therefore, to maintain high accuracy in yield estimation, nuclear testing

should be prohibited outside specified, calibrated test sites.

Most yield estimation research has concentrated on yields around 150 kt, so the accuracy

that could be achieved by seismic methods at

lower yields is not yet well known. In any future low threshold test ban treaty, it might be

expected that the initial uncertainties in yield

estimation for explosions below 10 kt would

be large. These uncertainties would then be reduced as more data were gathered, as our

knowledge of wave propagation properties for

various paths in the monitored regions was refined, and as calibration information was obtained.

Systematic Uncertainty

The yield estimation precision described

above for teleseismic data are limited because

of systematic uncertainties. As discussed

above, the systematic uncertainty can be reduced by calibrating the test site. Calibrating

a test site involves exploding devices whose

yields are either known or accurately determined by independent means, and then measuring the magnitudes at a large number of

monitoring stations. By doing so, the yields

of other events can be determined by comparing the amplitudes of the seismic waves at common seismic recording stations with those

originating from the events with known yields.

This approach reduces the systematic uncertainties caused by having to estimate the varying properties of the rocks surrounding the

explosion and any focusing effects near the explosion sites. As long as these factors remain

approximately unchanged within a geologically uniform area, the calibration improves

the estimation of yields.

The sizes and numbers of geophysically distinct subdivisions in any test site depend on

the geological structures of the area. A specific calibration maybe valid only for a limited

area around the shot if, at larger distances, the

rock properties and focusing effects change.

The distances over which the relevant conditions change vary, depending on the local geology. Testing areas that are large or contain

varying geology would obviously need more

calibration shots than areas that are geologically uniform. If calibration were performed

at the Soviet test site, the expected seismic

yield estimation capability would be comparable to the existing seismic capability at the Nevada Test Site.

YIELD ESTIMATION CAPABILITIES

In considering the capability of all methods is radiochemical methods. Radiochemical methof yield estimation, it must be kept in mind ods of yield estimation have an uncertainty of

that it is never possible to determine a yield about 10 percent (a factor of 1.1). Also, experiwithout some uncertainty. The standard against mental devices often detonate with yields that

which yield estimation methods are measured are slightly different from what was predicted.
123

This uncertainty in predicted yield was recognized during the negotiations of the Threshold Test Ban Treaty and provisions were established for unintended breaches (see ch. 2).

The yields of Soviet underground explosions

can be seismically estimated with a much better capability than the factor-of-2 uncertainty

that is commonly reported.7 New seismic methods have greatly improved yield estimation capabilities. Further improvements would occur

if the test sites were calibrated and, for small

tests, if stations were present within the Soviet Union during the calibration. The capabilities depending on these variables can be

summarized as follows:

●

●

Without Calibration: For large explosions

(above 50 kilotons) seismic yield estimation could be improved with the additional

use of the other methods including: surface waves, Lg waves, and seismic moment. Through such a combined method,

it is estimated that without calibration Soviet yields can be seismically measured

with present resources to a factor of 1.6

to 1.5 uncertainty.

With Calibration: Further reductions in

the uncertainty of yield estimates can be

accomplished if the Soviet test site were

calibrated. At a defined, well-calibrated

Soviet test site, it is estimated that yields

could be seismically measured with the

same factor of 1.3 uncertainty that is

found for seismic estimates at the Nevada

‘See, for example, VerifyingIVuclear TestingLim.itations: Possible U.S.-Soviet Cooperation (Washington, DC: U.S. Department of State, Bureau of Public Affairs, Special Report No. 152,

Aug. 14, 1986)

●

Test Site. In fact, Soviet seismologists

have told U.S. seismologists that they are

able to estimate yields seismically at their

own test site with only a factor of 1.2 uncertainty.

Small Explosions: For small explosions

(below 50-kt), the regional seismic waves

may not always be strong enough to travel

long distances to seismic stations outside

the Soviet Union. Consequently, seismic

stations within the Soviet Union may be

necessary (in addition to calibration) to obtain the 1.3 factor of uncertainty from

combined seismic methods for explosion

with yields below 50 kt. At yields below

10 kt small variations of the physical environment may produce greater uncertainty.

Therefore, at yields below 10 kt, the uncertainty may be inherently greater.

A 1.3 factor of uncertainty (for yields above

50 kt) is the claimed capability of the hydrodynamic yield estimation method using CORRTEX data8 that has been proposed as an alternative means for improving yield estimation.

Consequently, hydrodynamic yield estimation

will not provide a significantly superior yield

estimation capability over what could be obtained through well-calibrated seismic means

(also a 1.3 factor of uncertainty). Hydrodynamic yield estimation is, however, one of the

methods that could be used to provide independent estimates of the yields of calibration

shots to improve seismic methods. Once a test

site was calibrated using hydrodynamic methods, there would be no need to continue the

use of those intrusive methods.

‘See appendix, Hydrodynamic Methods of Yield Estimation.
124

SOVIET TEST PRACTICES AND TEST BAN COMPLIANCE

Specific concern over compliance with test 1. The mb of several Soviet tests at their

ban treaties has been heightened with findings Shagan River (E. Kazakhstan) test site are

by the Reagan Administration that: significantly l arger than the mb for U.S.

Soviet nuclear testing activities for a num- tests with yields of 150 kt.

ber of tests constitute a likely violation of the 2. The pattern of Soviet testing indicates

legal obligations under the Threshold Test that the yields of Soviet tests increased

Ban Treaty.9 after the first 2 years of the treaty.

Such findings are presumably based on net

assessments of all sources of data. In measuring yields near the 150 kt limit of the Threshold Test Ban Treaty, however, seismic evidence

is considered the most reliable basis for estimating the yields of Soviet underground nuclear explosions.10 It is, therefore, the seismic

evidence that has received particular attention.

Concern about whether the Soviet Union is

actually restricting its testing to a maximum

yield of 150 kt is motivated by two arguments:

“’The President’s Unclassified Report on Soviet Noncompliance with Arms Control Agreements, ” transmitted to the Congress Mar. 10, 1987.

Conclusion of the Defense Intelligence Agency Review Psnel

as stated in a letter from Roger E. Batzel, Director of Lawrence

Livermore National Laboratory to Senator CMbome Pen on

Feb. 13, 1987.

The validity of the first argument is dependent on how the Soviet yields are calculated.

Because of the uncertainty in measuring the

yields of Soviet tests using only mb and because of differences in opinion as to what the

correct bias value for Soviet tests should be,

there is disagreement as to whether the mb

values of the largest Soviet tests do, in fact,

represent violations of the 150 kt limit of the

Threshold Test Ban Treaty. For example, when

calculations such as those in table 7-1 are made

using both mb and MS measurements and a

bias correction of 0.35, they indicate that the

few remaining yields estimated as above 150

kt are well within the expected random scatter, and do not support claims of a violation.

The second argument is dependent on assumptions about probable Soviet behavior.

Two years after the signing of the Threshold

Box 7-A.—Calculations of the Six Largest East Kazakhstan Explosions. By Sykes et al.,

Based on Unclassified Data

6 Largest East Kazakhstan Explosions 1976-1986

Yield from Yield averaged from

Date mb only mb & MS

23 June 79. . . . . . . . . . . . . . . . . . . . . . . . . . 152 149

14 Sept 80 . . . . . . . . . . . . . . . . . . . . . . . . . . 150 150*

27 Dec 81 . . . . . . . . . . . . . . . . . . . . . . . . . . 176 161

4 July 82 . . . . . . . . . . . . . . . . . . , . . . . . . . . 158 158*

14 July 84 . . . . . . . . . . . . . . . . . . . . . . . . . . 140 140*

27 Oct 84. . . . . . . . . . . . . . . . . . . . . . . . . . . 140 140*

Average: 152.7 (± 13.4 kt) Average: 149.7 (± 8.8 kt)

(*based on mb only; no Ms determined)

All estimated yields are well within the uncertainty expected for observance of the 150 kt threshold limit.

SOURCE: Calculations of the six largest East Kazakhstan explosions made by Sykes et al., based on unclassified data. Body wave measurements from International Seismological Centre Bulletins and United States Geological Survey Reports. Station corrections determined to be 0.02 to 0.04 from mean. Surface wave calculations made by Sykes et al. Calibration corrected for bias using a value

of 0.35 to make body wave data consistent with surface wave data.
125

Test Ban Treaty, the size of the largest Soviet

explosions at their eastern Kazakh test site

increased markedly (see figure 7-8).11 This increase has been interpreted by some to infer

that the Soviets have been violating the 150

kt threshold limit in the later tests. The argument assumes that the Soviets were testing

up to the limit for the first 2 years and, therefore, by inference, have been testing above the

limit in violation of the treaty ever since 1978.

Alternate interpretations for this apparent

yield increase have been offered. It has been

pointed out that a similar pattern of testing

occurred at Kazakh for the 2 years prior to the

treaty. It has also been speculated that this

increase in yields may reflect a Soviet decision

to move their high yield testing from the Novaya

Zemlya test site to the Kazah test site.12 As

~There w= &IO speculation that this increase was coincident

with a change in the U.S. official method of yield estimation.

For example, Jack Anderson, “Can’t Tell If Russia Cheats On

Test Ban,” The Waslu”ngton Post, Aug. 10, 1982, p. C15.

‘*See for example, “Nuclear Test Yields” (Letter to the Editor), J. F. Evemden and L. R. Sykes, Science, vol. 223, Feb. 17,

1984, p. 642.

anon-technical consideration, it can be argued

that if the Soviets had tested above the limit

of the Threshold Test Ban Treaty at the Kazakh

test site, they would never have offered to allow the United States to calibrate their test

site using CORRTEX and Soviet test explosions. The calibration will reduce the uncertainty of yield estimates, a reduction that applies to past as well as future explosions and

hence can provide more accurate evidence concerning past compliance.

Because of the statistical nature of all yield

estimates, the question of compliance can be

addressed best not by looking at individual

tests but rather by examining the entire pattern of Soviet testing. It is particularly useful

to compare the testing programs of the United

States and the Soviet Union. It can be seen

from figure 7-9 that if a bias value lower than

0.35 is used, there appears to have been about

10 (out of over 200) Soviet tests since the signing of the Threshold Test Ban Treaty in 1974

with yield central values above the 150 kt

threshold limit. When the same method of yield

Figure 7-8.— mb Versus Time for Large Soviet Explosions

E

5.0 ‘

4.4 “ ‘

*o ● .

I I I I 1 I

1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 198’1 1982 1983

. Kazakhstan O Novaya Zemlya

The mb versus time for all large Novaya Zemlya and Kazakhstan explosions. It can be seen that a large increase of the maximum yield for explosions at the Eastern Kzazkh test site occurred about 2 years after the Threshold Test Ban Treaty was signed.

SOURCE: T.C. Bache, S.R. Bratt, and L.B. Bache, “P Wave Attenuation mb Bias, and The Threshold Test Ban Treaty,” SAIC-86-1647, submitted to AFGL, March 1966, p. 5.
126

Figure 7-9.—U.S.S.R. Nuclear Tests 1966-86 According to Sykes and Davis

5,000

2,000

1,000

 500

100

50

20

1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985

● Kazakhstan O Novaya Zemlya

These yields were calculated by combining P-wave and surface-wave magnitudes and using a bias correction of 0,35. Bars

denote the estimated standard deviations of the estimates. The few tests that do appear to have exceeded 150 kt are well

within the expected scatter. If a lower bias correction is applied and only P-wave determinations are used, then slightly higher

yield estimates will result and additional central values will be greater than 150 kt.

SOURCE: Modified from L.R. Sykes and D.M. Davis, Scientific America, vol. 258, No. 1, January 1987, pp. 29-37.

estimation is applied to U.S. tests, approximately the same number of U.S. tests also appear to be above the 150 kt threshold limit.

This, however, does not mean that one or the

other or both countries have cheated; nor does

it defacto mean that seismology is an inadequate method of yield estimation. It is inherent in any method of measurement that if both

countries are testing up to the yield limit, the

estimated yields of some tests will have central values above the yield limit. Because of

the uncertainty of measurements using any

method, it is expected that about half the Soviet tests at 150 kt would be measured as

slightly above 150 kt and the other half would

be measured as slightly below 150 kt.

All of the estimates of Soviet tests are within

the 90 percent confidence level that one would

expect if the yields were 150 kt or less. Extensive statistical studies have examined the distribution of estimated yields of explosions at

Soviet test sites. These studies have concluded

that the Soviets are observing a yield limit.

The best estimate of that yield limit is that

it is consistent with compliance with the 150

kt limit of the Threshold Test Ban Treaty .13

I$such gtatistic~ studies have been carried out extensively

by Lawrence Livermore National Laboratory, The conclusion

of these studies was reported in open testimony before the Senate Armed Services Committee on Feb. 26, 1987 by Dr. Milo

Nordyke, Leader of the Treaty Verification Program.
Appendix
Appendix

Hydrodynamic Methods of Yield Estimation

Hydrodynamic methods could be used to complement

seismic methods of yield estimation

Introduction

The yield of an underground nuclear explosion

may be estimated using so-called hydrodynamic

methods. These methods make use of the fact that

larger explosions create shock waves that expand

faster than the shock waves created by smaller explosions. Three steps are involved in making a yield

estimate. First, the properties of the geologic media at the test site that may affect the expansion

of the shock wave are determined. Second, the expansion of the shockwave caused by the explosion

of interest is measured during the hydrodynamic

phase, when the ambient medium behaves like a

fluid. Finally, the yield of the explosion is estimated

by fitting a model of the motion of the shock front

to measurements of the motion.

Although the algorithms used by different individuals or groups can (and usually do) differ in detail, most of the algorithms currently in use are

of four basic types: insensitive interval scaling,

similar explosion scaling, semi-analytical modeling,

and numerical modeling. Before considering these

algorithms and their application to test ban verification, it will be helpful to have in mind how the

shock wave produced by an underground nuclear

explosion evolves during the hydrodynamic phase

and how this evolution is affected by the properties of the ambient medium.

Shock Wave Evolution

The hydrodynamic evolution of the shock wave

produced by a large, spherically symmetric explosion underground may be usefully divided into

three different intervals. These are listed in table

A-1, along with the times after detonation at which

they begin for 1 and 150 kiloton (kt) explosions in

granite. The characteristics of these intervals

follow.

Self-Similar Strong-Shock Interval

At the very earliest times, the energy of the explosion is carried outward by the expanding

weapon debris and by radiation. Soon, however,

Table A-1.—Characteristic Times in the Evolution of

a Shock Wave Caused by an Underground

Nuclear Explosiona

Event in the evolution Time (μs) for a Time (μs) for a

of the shock wave 1 kt explosion 150 kt explosion

Beginning of the Self-Similar

Strong Shock Intervalb – - 1 0

Beginning of the Transition

Interval c. . . . . . . . . . . . . - 2 0 - 8 0

Beginning of the Plastic

Wave Intervald. . . . . . . . . . . . . . . -2,000 -10,000

aFOr an idealized Spherically-syrnrnelrlc exploslon In granite

bf+ere fhe shock wave is assumed 10 become self-similar when If reaches a radws of 1 m (see

text) No time is gwen for a 1 kt explosion because the shock wave caused by such an exploslon

fyplcally weakens before it has time to become self -s!mllar

coeflned as the time when the denslfy just behind the shock front falls to 80 percent Of Its maXU’rWM

limiting value

d~fined as the time when the speed of the shock wave falls to 120 perCent Of the plaStlC Wave

speed In reality, granite undergoes a phase transition sllghlly before the shock speed reaches

this value, a complication that has been neglected In this Illustration

SOURCE’F K, Lamb, “Hydrodynamic Methodsof Yield Estlmatlon ‘‘ Report Iorthe U S Congress

office of Technology Assessment, Feb 15, 1988

a shock wave forms and begins to move outward.

At this time the speed of the shock wave is much

greater than the speed of sound in the undisturbed

ambient medium, the pressure behind the shock

wave is predominantly thermal pressure, and the

ratio of the density behind the shock wave to the

density in front is close to its limiting value. This

is the strong shock interval.1

If the shock wave envelops a mass of material

much greater than the mass of the nuclear charge

and casing while it is still strong, and if energy

transport by radiation can be neglected, the shock

wave will become self-similar, expanding in a particularly simple way that depends only weakly on

the properties of the medium.’ The time at which

‘See Ya. B. Zel’dovich and Yu. P. Riazer, Physics of Shock Waves

and High-Temperature Phenomena (New York, NY: Academic Press,

1967 ~nglish Translation]), ch. XI. As the strength of a shock wave

is increased, the ratio of the material density immediately behind it to

the material density immediately in front of it generally increases, until

a value of the ratio is reached beyond which an increase in the strength

of the shock wave produces little or no further increase in the density

of the post-shock material. This density ratio is referred to as the limiting density ratio. In typical rocks, pressures behind the shock front of

about 10-100 Mbar are needed to produce a density ratio close to the

limiting value.

‘Ibid., ch. I and XII.

129
130

the motion becomes self-similar depends in part

on the design of the nuclear charge and diagnostic

equipment and on the size of the emplacement hole.

As the shockwave expands, it weakens and slows,

the density behind the shock front drops, and the

wave enters a transition interval in which the motion is no longer self-similar. No time is given in

table A-1 for the beginning of the self-similar

strong-shock interval for a 1 kt explosion because

the shock wave produced by such an explosion

typically weakens before it has time to become selfsimilar.

Transition Interval

As the shock wave weakens and slows, it enters

a broad transition interval in which the thermal

pressure is not much greater than the cold pressure of the medium. The motion of the shockwave

changes only gradually and so the time at which

the transition interval is said to begin is purely conventional. In this report the shockwave is considered to have entered the transition interval when

the density just behind the shock front has fallen

to 80 percent of its maximum limiting value. The

speed of the shock front is only a few times greater

than the relevant sound speed-the speed of the

so-called plastic wave—in the medium over much

of the transition interval and hence the motion of

the shock wave in this interval is more sensitive

to the properties of the medium than it is in the

strong shock interval.3

Plastic-Wave Interval

In the absence of phase transitions and other

complications,4 the shock wave weakens and slows

still further, entering an interval in which the pressure behind the shock front is predominantly the

cold pressure of the compressed ambient medium

and the shock speed is close to the plastic wave

speed in the medium. Again, the motion of the

shock wave changes only gradually and so the time

at which the plastic-wave interval is said to begin

is purely conventional. In this report the shock

wave is considered to have entered the plastic-wave

interval when its speed is less than 120 percent of

the plastic wave speed in the medium. In practice,

phase transitions and other effects complicate the

evolution of the shock wave in this interval for

rocks of interest.

‘Ibid., pp. 741-744,

‘Ibid., ch. XII.

Theoretical models and experimental data show

that the evolution of the shock wave in all three

intervals depends on such properties of the rock

as its chemical composition, bulk density, plastic

wave speed, and degree of liquid saturation. These

properties vary considerably from one rock to

another. As a result, the shock wave generally develops differently in different rocks. For example,

the characteristic radius at which the shock wave

produced by a 150 kt explosion changes from a

strong, self-similar wave to a plastic wave5 varies

from about 30 meters in wet tuff to over 60 meters

in dry alluvium.

Measuring the Position of the

Shock Front

Several techniques have been used to measure

the position of the shock front as a function of time.

During the 1960s and early 1970s, extensive measurements were made using the so-called SLIFER

technique. 6 In the mid-1970s an improved technique, called CORRTEX, was developed.7 This is

the technique the Reagan administration has proposed as a new technique to monitor the Threshold Test Ban Treaty (TTBT).8

In the CORRTEX technique, an electrical sensing cable is lowered into a vertical hole to a depth

greater than the depth at which the nuclear explosion will take place, typically hundreds of meters

for explosives with yields near 150 kt. The hole may

be the one in which the nuclear explosive is placed

(the emplacement hole) or one or more other holes

(so-called satellite holes) that have been drilled specifically for this purpose. The latter geometry is

shown in figure A-1. If satellite holes are used, they

must be drilled at the proper distance(s) from the

emplacement hole, typically about ten meters for

yields near 150 kt. Then, if the sensing cable is

strong enough that it is not crushed by other disY3ee F. K, Lamb, ACDIS WP-2-87-2 (University of Illinois Program

in Arms Control, Disarmament, and International Security, Urbana,

IL, 1987).

‘M. Heusinkveld and F. Holzer, Review of Scientific Instruments, 35,

1105 (1964). SLIFER is an acronym for “Shorted Location Indication

by Frequency of Electrical Resonance.”

7C.F. Virchow, G.E. Conrad, D.M. Holt, et.al., Review of Scientific

Instruments, 51,642 (1980) and Los Alarnos National Laboratory public information sheet on CORRTEX (April 1986). CORRTEX is an acronym for “Continuous Reflectometry for Radius v. Time Experiments. ”

W.S. Department of State, Bureau of Public Affairs, “U.S. Policy Re

garding Limitations on Nuclear Testing, ’’Special Report No. 150, August 1986; and U.S. Department of State, Bureau of Public Affairs,

“Verifying Nuclear Testing Limitations: Possible U.S.-Soviet Cooperation,” Special Report No. 152, Aug. 14, 1986.
131

CORRTEX

recorder

Sensing

cable

Figure A-1 .—Use of the CORRTEX Technique

Surface

Sensing

cable

point

Typical cable emplacement

in satellite hole

CORRTEX

recorder

4

tic

Working

point

/

Surface

nostic

I

/

/

\

\

Shock front /

progression

/

Moving shock wave from

nuclear detonation

crushes and shortens cable

SOURCE: U.S. Department of State, Bureau of Public Affairs, “U.S. Policy Regarding Limitations on Nuclear Testing,” Special Report No 150, August 1986
132

turbances but weak enough that it is crushed by

the pressure peak at the shock front, it will be electrically shorted close to the point where the shock

front intersects the cable (see figure A-l). As the

shock front expands with time, the changing distance from the surface to the shallowest point at

which the shock wave intersects the sensing cable

is measured at preset time intervals by electrical

equipment attached to the cable and located above

ground. The CORRTEX technique is much less affected by disturbing early signals from the explosion than were earlier techniques.

The time at which the explosion begins is taken

to be the time at which the first signal, produced

by the electromagnetic pulse (EMP) from the explosion, arrives at the CORRTEX recorder. If the

explosion is spherically symmetric, the length of

the unshorted cable decreases rapidly and

smoothly with time as the shock front expands

away from the center of the explosion and the radius of the shock front at a given time can be calculated using simple geometrical equations. If the

explosion is not spherically symmetric, due to the

shape of the canister, the design of the nuclear

charge, or inhomogeneities in the ambient medium,

the interpretation of CORRTEX data is more complicated and could be ambiguous or misleading under the conditions encountered in treaty verification. Problems of this kind can be prevented by

cooperative agreements, as discussed below.

An error of 1 meter in the measured distance of

the crushing point from the center of the explosion will cause an error of about 50 kt in the yield

estimate, for yields near 150 kt. Thus, an accurate

survey of the satellite hole is required in order to

make an accurate yield estimate. Surveys are currently made with special laser or gyroscopic equipment. In some yield estimation algorithms, the

lateral displacement from the center of the explosion can be treated as one of the unknowns in estimating the yield.

Insensitive Interval Scaling

Once measurements of the radius of the shock

front as a function of time are in hand, an estimate

of the yield of the explosion can be made by comparing the measurements with a model of the motion of the shock front away from the center of the

explosion. The simplest algorithm currently in use

is insensitive interval scaling. This is the algorithm

that the Reagan administration has proposed to

use in analyzing CORRTEX data as an additional

new method of monitoring compliance with the 150

kt limit of the TTBT.

Insensitive interval scaling is based on the assumption that the radius of the shock wave for an

explosion of given yield is independent of the

medium during a certain interval in time and radius called here the insensitive interval. Indeed,

studies indicate that for the collection of rocks

within U.S. test experience (mostly silicates), rock

properties are correlated in such a way that there

is a time during the transition interval when the

radius of the shock front produced by an explosion of given yield varies relatively little from one

rock to another.9

In using insensitive interval scaling, the shock

wave sensing cable must be placed close enough

to the center of the explosion that it samples the

insensitive interval. Yield estimates are then derived by fitting a simple empirical formula, called

the Los Alamos Formula, to the shock radius

versus time data in this interval.10 The Los Alamos

Formula is a power law that approximates the actual radius versus time curve during the insensitive interval. This is illustrated by figure A-2,

which compares the Formula with a model of the

evolution of the shock wave produced in granite

by a spherically-symmetric point explosion with

a yield of 62 kt.

Yield Estimation Algorithms

Hydrodynamic methods of yield estimation are

evolving as research aimed at gaining a better understanding of underground explosions and improving yield estimation methods continues. At

present, four basic types of algorithms are commonly in use. In order to simplify their description, the explosion will be assumed to be spherically symmetric (the complications that can arise

if it is not will be addressed later).

The rocks for which the United States had good data or models are

the dry alluvium, partially saturated tuff, saturated tuff, granite, basalt, and rhyolite at the test sites used, almost all of which are at the

Nevada Test Site. At present, the reason for the correlation of rock properties that gives rise ta the insensitive interval is not well understmd

from a fundamental physical point of view. Moreover, it is known that

the radius of the shock wave in this interval is very different for other

very different kinds of rocks. Thus, the existence of an insensitive interval must be established by test experience or modeling, and is only

assured for certain geologic media.

1’The Los Alamos Formula for the shock radius in meters is R(t) =a

.W%(t/W1/3)6, where W is the yield of the explosion in kilotons, t is the

elapsed time since the beginning of the explosion in milliseconds, and

a and /3 are constants. Different values of a and 6 have been used by

different individuals and groups and have changed with time. The values

of a and /3 used here are 6.29 and 0.475 (see M. Heusinkveld, Journal

of Geophysictd Research, 87, 1891, 1982).
133

Figure A-2.—Comparison of the Los Alamos Formula

With a Semi= Analytical Model of the Shock Wave in

Granite Caused by a Spherically asymmetric Point

Explosion With a Yield of 62 Kilotons

1000

100

10

1

— Granite Model

— Los Alamos Formula

.0001 .001 .01 .1 1 10 100 1000

Time (ins)

SOURCE: F.K. Lamb, “Hydrodynamic Methods of Yield Estimation,” Report for

the US. Congress, Office of Technology Assessment, Feb. 15, 1988.

In practice, the Los Alamos Formula is usually

first fit to a broad interval of radius versus time

data that is thought to include the insensitive interval. The result is a sequence of yield estimates.

Due to the departure of the Formula from the actual radius versus time curve at both early and late

times, the sequence of yield estimates typically

forms a U-shaped curve. This is illustrated in figure A-3, which shows the sequence of yield estimates obtained by applying the Formula to the

relatively high-quality SLIFER data from the

Piledriver explosion in granite. If the assumptions

on which the algorithm is based are satisfied, the

yield estimates near the bottom of the curve approximate the actual yield of the explosion, In the

usual form of the algorithm, only the radius versus

time data that fall within a certain predetermined

interval chosen on the basis of previous experience

(the so-called algorithmic interval) are actually used

to make the final yield estimate. The length of the

algorithmic interval and the time at which it occurs are both proportional to W1/3, where W is the

yield of the explosion (see table A-2). The algorithFigure A-3.—Application of the Los Alamos Formula

to an Explosion in Granite

200

100

■ ✎

1 1

1 2 3 4 5

Time (ins)

SOURCE: F.K. Lamb, “Hydrodynamic Methods of Yield Estimation,” Report for

the U.S. Congress, Off Ice of Technology Assessment, Feb. 15, 1988.

mic interval is indicted in figure A-3 by the two

vertical bars at the bottom of the figure. In this

example, the assumptions of the algorithm are

satisfied and the average of the yield estimates

that lie within the algorithmic interval is very close

Table A-2.—Algorithmic Intervals for Various Yieldsa

Time Time interval (ins) Radius interval (m)

1 0.1-0.5 2-4.5

10: : : : : : : : : : : : : : : : 0.2-1.1 4.5-1o

50. . . . . . . . . . . . . . . . 0.4-1.8 8-17

100. . . . . . . . . . . . . . . 0.5-2.7 10-21

150. . . . . . . . . . . . . . . 0.5-2.6 11-24

aThe time intewals used by various individuals and groups varies. Throughout

this report the algorithmic interval Is taken to be from O.l W’~ milliseconds to

0,5W1~ milliseconds after the beginning of the explosion, where W is in kilotons,

SOURCE: F.K. Lamb “Hydrodynamic Methods of Yield Estimation” Report for

the U.S. Congress, Office of Technology Assessment, Feb. 15, 1988.
134

to the announced yield of 62 kt. Studies indicate

that yield estimates made using this algorithm

have a precision of about a factor of 1.2 at the 95

percent confidence level for spherically-symmetric

explosions with yields greater than 50 kt conducted

at the Nevada Test Site (NTS). Estimates of the

accuracy of the algorithm made by comparing results with the usually more accurate radiochemical method are similar. According to official statements, the insensitive interval algorithm is

expected to be accurate to within a factor of 1.3

at Soviet test sites for explosions with yields

greater than 50 kt in media within U.S. test experience.” Some scientists believe that the uncertainty would be somewhat larger.

The insensitive interval algorithm does not work

as well if the assumptions on which it is based are

not satisfied. This is illustrated in figure A-4, which

shows the yield estimates obtained by fitting the

Los Alamos Formula to good-quality SLIFER

data from atypical low-yield explosion in alluvium.

In this example the radius and time data have been

scaled using the actual yield so that the derived

yield should be 1 kt. However, the yield estimates

given by the Los Alamos Formula are systematically low, ranging from 30 to 82 percent of the actual yield, and do not forma U-shaped curve. The

average of the yield estimates that lie within the

algorithmic interval is about 60 percent of the actual yield. The overall appearance of the yield

versus time curve shows that the assumptions of

the algorithm are not satisfied.

A common misconception has been that the algorithmic interval lies within the strong shock region and that the relative insensitivity of yield estimates to the properties of the medium stems from

this.12 As explained earlier, radius versus time data

in the algorithmic interval would indeed be relatively independent of the medium if this were so,

and would follow a power-law curve similar to the

Los Alamos Formula. However, the shock wave

is not strong during the algorithmic interval, because in this interval the shock speed is only a few

times the speed of sound and the post-shock pressure is much less than the pressure required to

achieve the maximum limiting density. Indeed, the

exponent of time usually used in the Los Alamos

Formula is significantly greater than the value

“U.S. Department of State, op. cit., footnote 8.

“For example, “The accuracy of the method is believed to be relatively, but not wholly, independent of the geologic medium, provided

the satellite hole measurements are made in the ‘strong shock’ region

. . . “ (Ibid.) This misconception may have arisen from the fact that the

interval formerly used to estimate the yields of nuclear explosions in

the atmosphere using hydrodynamic methods is within the strong shock

region.

Figure A-4.—Application of the Los Alamos Formula

to a Low-Yield Explosion in Alluvium

O.8

0.7

0.6

0.5

.

.

■

.

●

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

m

.

.

.

.

0.1 0.3 0.5 0.7 09 1.1

Time (scaled ms)

SOURCE: F.K. Lamb, “Hydrodynamic Methods of Yield Estimation,” Report for

the US. Congress, Office of Technology Assessment, Feb. 15, 1988.

appropriate for a strong shock wave. The Los

Alamos Formula is, as noted earlier, an empirical

relation, which was obtained by fitting a powerlaw expression to data from a collection of explosions in a variety of different rocks and approximates actual radius versus time curves over a portion of the transition interval.

Similar Explosion Scaling

If a given explosion occurs in the same medium

as a previous explosion at a different site, and if

radius versus time data and the yield are available

for the previous explosion, then the yield of the

given explosion can be estimated by similar explosion scaling. The reason is that for explosions in

the same medium, the radius versus time curve depends only on the yield of the explosion, and this
135

dependence is known and is simple.13 Hence, an

estimate of the yield of the given explosion can be

made by comparing the two sets of radius versus

time data. This algorithm can make use of data

outside the insensitive interval and works well if

the ambient media at the two explosion sites are

sufficiently similar. However, in practice it has

sometimes proved difficult to ascertain whether

the relevant properties of the media are similar

enough to give the desired accuracy. Similar explosion scaling has been proposed as a supplement

to insensitive interval scaling for TTBT verification.

Semi-Analytical Modeling

Semi-analytical modeling is another approach

that is useful for studying the evolution of shock

waves in geologic media and for estimating yields.

In this approach both the properties of the ambient

medium and the motion of the shock front are

treated in a simplified way that nevertheless includes the most important effects. The result is a

relatively simple, semi-analytical expression for the

radius of the shock front as a function of time. If

the required properties of the ambient medium are

known and inserted in this expression, the yield

of an explosion can be estimated by fitting the expression to measurements of the shock wave motion with time.14 Semi-analytical algorithms can in

principle make use of more of the data than can

the insensitive interval algorithm and can also be

used to estimate the uncertainty in the yield caused

by uncertainties in the properties of the ambient

medium.

Numerical Modeling

If a treatment that includes the details of the

equation of state and other properties of the ambient medium is required, or if the explosion is

asymmetric, modeling of the motion of the shock

front using numerical hydrocodes may be necessary.15 In principle, such simulations can Provide

“See H. L. Brode, Annual Review of Nuclear Science, 18, 153-202

(1968).

“For an early semi-analytical model, see M. Heusinkveld, Report

UCRL-52648 (Lawrence Livermore National Laboratory, Livermore, CA,

1979). For improved semi-analytical models, see F. K. Lamb, ACDZS

WP-87-2-1 (University of Illinois Program in Arms Control, Disarmament, and International Security, Urbana, IL, February 1987); W. C.

Moss, Rep. UCRL-96430Rev. 1 (Lawrence Livermore National Laboratory, Livermore, CA, July 1987); and R. A. Axford and D. D. Helm,

Proc. NEDC (Los Alamos, October 1987).

15 For a discussion of numeric~ models currently in use ~ F. K. L~b,

“Monitoring Yields of Underground Nuclear Tests, ” Threshold Test

Ban Treaty and Peaceful Nuclear Explosions Treaty, Hearings Before

the Comnu”ttee on Foreign Relations, Um”ted States Senate (Washington, DC: U.S. Government Printing Office, 1987), pp. 359-370.

radius versus time curves that extend over much

of the shock wave evolution, making it possible to

base yield estimates not only on data from the transition interval but also data from later phases of

the shock wave evolution. In practice, the yield

estimates obtained using such a procedure are

fairly sensitive to the equation of state of the ambient medium, which is known with sufficient accuracy for only a few geologic media. If adequate

equation of state data are lacking, numerical modeling may not be warranted.

In summary, the shockwave produced by an underground nuclear explosion propagates differently in different media and different geological

structures. As a result, knowledge of the ambient

medium and local geological structures is required

in order to make accurate yield estimates using

hydrodynamic methods. Several different yieldestimation algorithms have been developed. These

algorithms, like those based on seismic methods,

involve some complexity and require sophistication to understand and apply correctly. Some key

terms that have been introduced in this discussion

are listed and explained in table A-3.

Application to Monitoring Treaties

Assuring Accuracy

Ambient Medium.-The physical properties and

geologic structure of the ambient medium enter

directly into yield estimates based on hydrodynamic methods. Incorrect assumptions about

the average properties of the ambient medium may

bias the yield estimate, decreasing its accuracy,

while small-scale variations will cause scatter in

the radius versus time data, decreasing the precision of the yield estimate. Thus, it is important to

gather information about the types of rock present

at the test site and their properties, including their

chemical composition, bulk density, and degree of

liquid saturation, as well as the speed of sound in

the ambient medium and any specific features of

the local geologic structure that could affect the

yield estimate. Availability of the required data

would need to be assured by appropriate cooperative measures.

Some information about the geologic medium at

the test site could be obtained by examining the

contents of the hole drilled for the CORRTEX sensing cable. Verification could be improved by cooperative arrangements that would also allow observation of the construction of the emplacement hole,

removal and examination of rock core or rock frag-
136

Table A-3.–Glossary of Hydrodynamic Yield

Estimation Terms

Term/Explanation

Strong Shock Interval: The interval in radius and time during

which the speed of the shock wave is much greater than

the speed of sound in the unshocked medium

Transition Interval: The interval in radius and time outside

the strong shock interval in which the speed of the shock

wave approaches the speed of sound in the unshocked

medium

Plastic Wave Interval: The interval in radius and time outside

the transition region in which the speed of the weakening

shock wave is approximately the plastic wave speed

SLIFER Technique: A technique for measuring the position

of the shock wave expanding away from an underground

explosion by determining the resonant frequency of an

electrical circuit that includes a sensing cable placed in

a hole in the ground near the site of the explosion

CORRTEX Technique: A technique for measuring the position of the shock wave expanding away from an underground explosion by determining the round-trip travel time

of electrical pulses sent down a sensing cable placed in

a hole in the ground near the site of the explosion

Insensitive Interval Scaling: A yield estimation algorithm in

which the Los Alamos Formula is fit to measurements of

the position of the expanding shock wave as a function

of time during the algorithmic interval

Los Alamos Formula: The empirical formula used in the insensitive interval scaling algorithm to make yield estimates

by fitting to shock radius versus time data

Algorithmic Interval: The special time interval used in insensitive interval scaling during which the radius of the shock

wave is relatively insensitive to the ambient medium; usually assumed to be 0,1-0.5 scaled milliseconds after the

beginning of the explosion

Similar Explosion Scaling: A yield estimation algorithm in

which data obtained from a previous explosion in the same

medium are scaled to fit measurements of the position

of the expanding shock produced by the explosion under

consideration

SOURCE: F.K. Lamb “Hydrodynamic Methods of Yield Estimation” Report for

the US. Congress, Office of Technology Assessment, Feb. 15, 1988,

ments from the wall of the emplacement hole, examination of any logs or drill core from existing

exploratory holes, removal and examination of rock

core or rock fragments from the walls of existing

exploratory holes, and if necessary, construction

of new exploratory holes.

There is precedent for such cooperative arrangements in the Peaceful Nuclear Explosions Treaty

(PNET), which explicitly established the hydrodynamic method as one of the monitoring methods

that could be used for large salvos and specified

verification measures like these.16

Test Geometry.–CORRTEX data must be

taken very close to the center of the explosion in

1aArms control md Disarmament Agreements (U.S. Arms Control

and Disarmament Agency, Washington, DC, 1982).

order to cover the insensitive interval (see table

A-2). As a result, yield estimates can be affected

by the arrangement of the nuclear charge and the

canister or canisters containing it and the diagnostic equipment.17 In particular, any properties of the

experimental set-up or the surrounding geologic

media that cause the shock front to be distorted

at the radii of interest could affect the accuracy

of the yield estimate. The reason is that a CORRTEX sensing cable measures only the depth of the

shallowest point where a pressure wave first

crushes it, at a single lateral displacement from

the explosion. Thus, unambiguous interpretation

of the data may become difficult or impossible if

the explosion is not spherically symmetric.

For example, explosions of nuclear charges in

tunnels may be accompanied by complicated (and

unanticipated) energy flows and complex shock

wave patterns. If significant energy reaches the

sensing cable ahead of the ground shock and shorts

it before the ground shock arrives, the CORRTEX

data will describe that flow of energy and not the

motion of the ground shock. Alternatively, the motion of the ground shock itself could be sufficiently

distorted that interpretation of the shock position

data becomes ambiguous or misleading. As

another example, a large canister or double explosion could short the CORRTEX cable in such away

that only part of the total yield is sensed over most

of the interval sampled by the CORRTEX cable,

as shown in figure A-5. The physical size of

lrThe imwrt~ce of these disturbing effects is less for high-yield than

for low-yield explosions.

Figure A-5.—Effect of Nuclear Test Design on

Shock Wave Radius Measurements Using CORRTEX

Equipment

CORRTEX

Recorder

SOURCE: F.K. Lamb, “Hydrodynamic Methods of Yield Estimation,” Report for

the U.S. Congress, Office of Technology Assessment, Feb. 15, 1988.
137

canisters and diagnostic lines-of-sight tend to pose

more of a problem for nuclear directed-energy

weapons than for traditional nuclear weapons.18

In using hydrodynamic methods to estimate the

yields of one’s own tests, the design and placement

of the nuclear charge and related equipment are

known and can be taken into account. This is not

necessarily the case when monitoring the nuclear

tests of another party. Cooperative agreements to

make possible optimal placement of sensing cables

and to exclude nuclear test geometries that would

significantly disturb the yield estimate would

therefore be required.19

Such agreements could, for example, limit the

length of the canister containing the nuclear charge

and the cross-sectional dimensions of the emplacement hole, and mandate filling of the nuclear

charge emplacement hole with certain types of materials. Such agreements could also provide for observation of the emplacement of the nuclear charge

and the stemming of the emplacement hole, confirmation of the depth of emplacement, and limitations on the placement of cables or other equipment that might interfere with the CORRTEX

measurement. For test geometries that include

ancillary shafts, drafts, or other cavities, additional

measures, such as placement of several sensing cables around the weapon emplacement point, may

be required to assure an accurate yield estimate.

For tunnel shots, sensing cables could be placed

in the tunnel walls or in a special hole drilled toward

the tunnel from above. Again, there is precedent

for such cooperative measures in the PNET.20

The restrictions on the size of canisters and diagnostic lines-of-sight that would be required even

with the sensing cable placed in a satellite hole

would cause some interference with the U.S. nuclear testing program at NTS. However, these restrictions have been examined in detail by the U.S.

nuclear weapon design laboratories and the Department of Energy, and have been found to be

manageable for the weapon tests that are planned

for the next several years. In assessing whether

hydrodynamic methods should be used beyond this

period, the disadvantages of the test restrictions

must be weighed against the potential contribu-

‘

8 See Sylvester R. Foley, Jr., Assistant Secretary for Defense Programs, Department of Energy, letter to Edward J. Markey, Congressman from Massachusetts, Mar. 23, 1987.

1’S. S. Hecker, Threshold Test Ban Treaty and Peaceful Nuclear Explosions Treaty, Hearings Before the Comnu”ttee on Foreign Relations,

United States Senate (Washington, DC: U.S. Government Printing Office, 1987), pp. 50-61 and 226-235; M. D. Nordyke, Ibid., 67-71 and 278-

285; Foley, ibid

200p. cit., footnote 16.

tion to treaty monitoring made by these methods.

In summary, the accuracy that could be achieved

using hydrodynamic methods to estimate the

yields of underground nuclear explosions depends

on the amount of information that can be gathered

about the medium in which the explosion occurs,

and the nature and extent of cooperative arrangements that can be negotiated to optimize the placement of sensing cables and to limit disturbing

effects.

Hydrodynamic methods for estimating the

yields have not yet been studied as thoroughly or

as widely as the seismic methods currently in

use, although they have been examined more

thoroughly than some seismic methods that have

been proposed for the future. Tests and simulations

to identify troublesome configurations have been

carried out, but only a few explosions have been

monitored with the CORRTEX sensing cable in a

satellite hole.21 Given the possibility that

hydrodynamic yield estimation may have to be

used to monitor treaty compliance in an adversarial atmosphere, the possibility of deliberate efforts

to introduce error or ambiguity, and the tendency

for worst-case interpretations to prevail, additional

research to reduce further the chances of confusion, ambiguity, spoofing, or data denial would be

very useful.

Minimizing Intrusion

Hydrodynamic yield estimation methods are

more intrusive than remote seismic methods for

several reasons:

1. Personnel from the monitoring country would

be present at the test site of the testing country for perhaps 10 weeks or so before as well

as during each test, and would therefore have

an opportunity to observe test preparations.

The presence of these personnel would pose

some operational security problems.22

2. The exterior of the canister or canisters containing the nuclear charge and diagnostic

equipment must be examined to verify that

the restrictions necessary for the yield estimate to be valid are satisfied. For tests of nuclear directed energy weapons, this examination could reveal sensitive design information

unless special procedures are followed.23

*’Op. cit., footnote 8. Approximately 100 tests have been carried out

with the CORRTEX cable in the emplacement hole, and SLIFER data

from satellite holes are available for several tens of earlier explosions.

IZR. E, Batzel, Threshold Test Ilan . . ., op. cit., footnote 19, PP. 48+0

and 210-225, and Foley, op. cit., footnote 18.

21 Batzel, ibid.
138

3. Sensing cables and electrical equipment will

tend to pick up the electromagnetic pulse

(EMP) generated by the explosion. A detailed

analysis of the EMP would reveal sensitive information about the design and performance

of the nuclear device being tested.

Intrusiveness could be minimized by careful attention to monitoring procedures and equipment.

For example, the electrical equipment required can

be designed to avoid measuring sensitive information about the nuclear devices being tested. CORRTEX equipment has been designed in this way, and

the United States could insist that any Soviet

equipment used at NTS be similarly designed. The

security problems posed by opportunities to observe test preparations are more severe for nuclear

directed energy weapon tests, since they tend to

have more and larger complex diagnostic systems

and canister arrangements which, if fully revealed

to the Soviets, might disclose sensitive information. The United States has determined that the

Soviet personnel and activities that would be required at NTS to monitor U.S. tests would be

acceptable both from a security standpoint and

from the standpoint of their effect on the U.S. test

program. Detailed operational plans have been developed to accommodate such visits without adverse impact on operations.24

Specific Applications

Threshold Test Ban Treaty.–As noted earlier,

hydrodynamic yield estimation has been proposed

by the Reagan administration as a new routine

measure for monitoring the sizes of nuclear tests,

in order to verify compliance with the 150 kt limit

of the TTBT. To reduce the cost and intrusiveness

of such verification, it could be restricted to tests

with expected yields greater than some threshold

that is an appreciable fraction of 150 kt. Hydrodynamic measurement of the yields of one or more

nuclear explosions at each country’s test site or

sites has also been suggested as a method of

calibrating seismic yield estimation methods.25

24 FOley, Op. cit., footnote 18-

ZfiIt has ban sugge9ted that the nuclear calibration chwges to be detonated at the test sites could be provided by the monitoring country.

If they were, a hydrodynamic estimate of the yield might not be needed,

since, as explained inch. 7, the yields of certain types of nuclear charges

are accurately reproducible. Knowledge of the surrounding medium and

geologic structure would still be needed to provide assurance that the

coupling of the explosion to seismic waves was understood. However,

some way would have to be found to provide assurance that no sensitive information about the design of the nuclear weapons of either country would be revealed.

From the point of view of the United States, possible advantages of being able to use hydrodynamic

yield estimation methods at Soviet test sites include the additional information on yields that this

would provide, establishment of the principle of

on-site inspection at nuclear test sites, and the possibility of collecting data on the ambient media and

geologic structures at Soviet test sites. Obviously,

the larger the number of explosions and the greater

the number of test sites monitored, the more information that would be obtained. Possible disadvantages for the United States include the potential difficulty of negotiating routine use of

hydrodynamic methods at Soviet test sites, which

could impede progress in limiting nuclear testing,

and the operational security problems at NTS

caused by the presence of Soviet monitoring personnel there.

Peaceful Nuclear Explosions Treaty.–As it

stands, the PNET does not provide for use of

hydrodynamic yield estimation except for salvos

in which the “planned aggregate yield” is greater

than 150 kt.26 Thus, if the TTBT is modified to allow hydrodynamic yield estimation for all weapon

tests with planned yields above a certain value, the

purpose of the modification could in principal be

circumvented by carrying out weapon tests as

“peaceful” nuclear explosions of “planned yield”

less than or equal to 150 kt, unless the PNET is

also modified to close this loophole.

Low-Threshold Test Ban Treaty. -Underground

nuclear explosions as small as 1 kt produce shock

waves that evolve in the same way as those

produced by explosions of larger yield. However,

such explosions can and usually are set off at shallow depths and can be set off in alluvium. As a result, the motion of the ground can be markedly

different from that on which standard hydrodynamic yield estimation methods are based, causing a substantial error in the yield estimate (see

figure A-4). There can also be significant variations

in the motion of the ground shock from explosion

to explosion under these conditions.

In addition, serious practical, operational, and

engineering problems arise in trying to use

hydrodynamic methods to estimate the yield of

such a small explosion. For one thing, the sensing

cable must be placed very close to the nuclear

charge (see table A-3). Drilling a satellite hole

within 3 meters of the emplacement hole to the

depths of typical nuclear device emplacement, as

would be required in order to use hydrodynamic

‘eArms . . . , op. cit., footnote 16.
methods to estimate the size of an explosion with

a yield near 1 kt, would be challenging, to say the

least. The need for such close placement would necessitate further restrictions on the maximum size

and orientation of the canister used to contain the

nuclear charge and diagnostic instrumentation.

Such restrictions might be deemed an unacceptable interference with test programs. However, use

of small canisters with numerous diagnostic linesof-sight to the detonation point could disturb the

CORRTEX measurements. Because the shock

wave radii to be measured are much smaller at low

yields, survey errors become much more important.

Possible solutions to these problems have not

yet been carefully and thoroughly studied. Thus,

139

at the present time hydrodynamic yield estimation

methods could not be used with confidence to monitor compliance with threshold test bans in which

the threshold is less than several tens of kilotons.

Comprehensive Test Ban.–As their name implies, hydrodynamic yield estimation methods

have been developed to measure the sizes of underground nuclear explosions. They are a potentially valuable component of a cooperative program

to monitor limits on yields, but are neither intended

nor able to detect, identify, or measure the yields

of unannounced or clandestine nuclear tests. Thus,

they are not applicable to monitoring a comprehensive test ban.

o