141. Memorandum From Secretary of State Rusk to President Johnson 1


  • Proposed U.S. Initiative on Threshold Test Ban (U)

A new arms control initiative aimed at inhibiting the spread of nuclear weapons has been given serious consideration by the Arms Control and Disarmament Agency and discussed with the Committee of Principals at its meeting on June 17. The Arms Control and Disarmament Agency suggestion is for a Presidential proposal for a new test ban agreement, expanding the present Limited Test Ban Treaty to cover verifiable underground tests.

No consensus was reached at the Principals’ meeting on the desirability of making a proposal for a threshold test ban at this time. It is suggested that you may wish to discuss this personally with the Principals.

The following is a summary of the major issues which must be considered in reaching a decision on the U.S. proposal in this area. A more detailed draft position paper prepared by the Arms Control and Disarmament Agency is forwarded as an annex to this memorandum2 along with comments thereon by the Joint Chiefs of Staff, Atomic Energy Commission, and Central Intelligence Agency.3

A. Technical and Political Effects of a Threshold Test Ban Treaty

While all agree that as a technical matter a non-nuclear country could test its first weapon under a threshold test ban which could be reasonably verified at the present time, it would nevertheless increase the technical difficulties and cost and, at the same time, reduce the political and military gains from such a development. Key potential additional nuclear countries, i.e., India, Israel, Japan, Sweden and Germany, would doubtless sign a threshold test ban and as a result probably would be [Page 343]constrained to test at about the 5 KT level, since they seemingly have neither the soil conditions nor the experience to conduct tests at much higher yields without serious risk of violating the threshold. The threshold would prevent the development of an advanced military nuclear capability.

There are differences of opinion as to the importance of these constraints and on the willingness of such countries to risk a violation if they had signed such a treaty. The Department of State and the Arms Control and Disarmament Agency, however, believe that these constraints might be a significant factor in preventing a decision by these countries to go nuclear.

We cannot be sure at this time what the Soviet reaction to such a U.S. offer would be, since previously the Soviets have always linked the threshold test ban to a moratorium on all underground tests. A moratorium is completely unacceptable to the United States. A Soviet response of this type would limit the political gains obtained by offering a threshold ban but on balance the U.S. position of being willing to ban all tests it can verify would be better than it would be otherwise, particularly when knowledge of the improved detection capability became public.

B. Verification of Compliance with a Threshold Test Ban Treaty

Recent developments in the field of seismic detection and identification of underground events indicate that all contained underground explosions with seismic magnitudes above 4.75 generate seismic signals with characteristics common to all such explosions and that only a very small number of earthquakes above magnitude 4.75 (on the average of one per year in the USSR) have signals which cannot be distinguished from explosions and which therefore might provide false alarms. The suspiciousness of even these few false alarms would be further reduced by investigation of their location and by the use of other intelligence sources. Furthermore, the number of these natural events which cannot be identified by seismic means decreases rapidly with increase in magnitude and is essentially zero at magnitude 5.

There are differences of opinion among the Principals on the adequacy of this degree of verification at the magnitude 4.75 level. Some feel that even a single Soviet nuclear test above the threshold could have a significant effect on U.S. security and that either the low natural false alarm rate could be used as a cover for such a test or that the United States would not choose to denounce a violation and abrogate the treaty based only on seismic evidence supported by unilateral intelligence. They believe that some type of on-site inspections would be required as well in order to provide demonstrable proof to the world. Others believe that this verification capability would provide the United States the ability to satisfy itself that a violation had occurred and that the United States could take the necessary international actions if it felt its security were affected.

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[1 paragraph (6 lines of source text) not declassified]

C. The Military Implications of a Threshold Test Ban

Although a 4.75 threshold test ban could preclude the proof-testing of certain advanced warhead designs the major effect in terms of existing requirements would be on the development of warheads for ABM systems. [4 lines of source text not declassified] The Department of Defense has had conducted a study which indicates that the presently planned effectiveness of the Nike-Zeus system designed against a Chinese Communist threat could be regained under the constraints of a threshold test ban by system changes at some additional cost.4 Maintaining the effectiveness of a system designed against a sophisticated Soviet threat would be more difficult and would require considerably greater costs and could not as certainly be obtained. The Atomic Energy Commission does not believe that the warheads postulated in the Department of Defense study for the revised systems could be developed under a threshold test ban but notes that certain nuclear tests could probably be carried out within the next year or two in order to achieve such developments. There are also a number of differences of opinion as to the validity of the detailed systems aspects of the DOD-sponsored study on this ABM problem.

The USSR is believed to lag somewhat behind the United States in terms of nuclear warhead development in the yield ranges which would be forbidden under the threshold test ban. In a recent NIE the belief is stated that for most of the Soviet military development programs which can be foreseen over the next few years a threshold test ban treaty would impose no greater restrictions than those already imposed by the partial test ban.5 [9-1/2 lines of source text not declassified]

D. Threshold Magnitude

The magnitude of an event is proposed to be defined as the mean of the measurements obtained by at least nine standard stations located between 1,500 and 6,000 miles from an event. The United States now operates more than twelve stations, some of which are classified, which could be used to make a determination of magnitude by the United States with respect to events taking place in the USSR.

Few stations now exist outside the United States jurisdiction which meet the qualifications for a standard station. Twenty-five to thirty properly distributed standard stations would be required in order to assure that at least nine stations at the desired distances will be able to measure the magnitude of events of m4.75 and above in the USSR, United States, India and Israel. Such stations could be constructed with equipment readily procurable in 1-5 years depending upon the intensity of effort [Page 345]and cooperation of host countries. In the absence of such a network of stations and the public availability of data therefrom, there would be no internationally accepted data base for a magnitude determination for a given event, leaving the way open to possible international controversy concerning the magnitude of a given event.

The threshold magnitude proposed by the Arms Control and Disarmament Agency and used as a basis for most of the studies was 4.75 which might permit tests up to 30 KT on the part of the United States and the USSR without significant risk of violations, and about 5 KT on the part of the key non-nuclear nations. The Atomic Energy Commission feels that the United States might be constrained to tests at lower yields to avoid risking inadvertent violation. Consideration has been also given to establishing the threshold at 5.0 which would allow U.S.-USSR tests at about 80 KT and non-nuclear country tests at about 12 KT. Such a threshold would reduce the number of false alarms from natural earthquakes in the USSR to essentially zero. It would also permit the development of some of the more conventional type Nike-Zeus warheads by the United States and presumably the USSR. On the other hand, raising the threshold would reduce the effect of the treaty in discouraging the proliferation of nuclear weapons. Moreover, it would be difficult for the United States to justify raising the threshold to 5.0 in light of current detection and identification capabilities at 4.75, and there would be strong sentiment on the part of other countries to retain the familiar 4.75 threshold.

E. Explosions for Peaceful Uses

The present Limited Test Ban Treaty provides severe limitations on explosions for peaceful excavations purposes and a threshold treaty would add some additional limitations. The Arms Control and Disarmament Agency has proposed provisions in these treaties which might permit peaceful uses explosions but many believe that such provisions would provide legal or illegal opportunities for weapons development intended to be forbidden by the treaty. Studies are now underway in the Atomic Energy Commission and the Arms Control and Disarmament Agency to elaborate on the significance of such loopholes and to develop additional procedures to close them. The Atomic Energy Commission believes that the United States should not make any proposals on a threshold test ban until an agreed position is reached within the Government concerning the extent to which nuclear explosions for peaceful purposes would be permitted under such a treaty, and the specific treaty provisions relevant to such purposes which would be acceptable to the United States.

Dean Rusk 6
  1. Source: Washington National Records Center, RG 330, OASD/ISA Files: FRC 70 A 6649, 388.3, January 1966. Top Secret; Restricted Data. A cover memorandum from Foster to members of the Committee of Principals, August 3, states that all the members of the Committee of Principals concurred in the memorandum with the exception of the Atomic Energy Commission, which suggested a number of minor changes that were incorporated in the memorandum sent to the President.
  2. Not found with source text. A copy is attached to June 9 memorandum from Foster to members of the Committee of Principals; ibid., OSD Files: FRC 70 A 4443, 388.3, June 1966.
  3. The cover memorandum states that the comments of the Joint Chiefs of Staff dated June 29 (not found), the comments of the Atomic Energy Commission dated June 16 (copy ibid.), and the comments of the Central Intelligence Agency, June 17 (Document 139) were not circulated with this paper because they were previously made available to the Committee of Principals by the originating agencies.
  4. The Department of Defense study has not been found.
  5. Document 130.
  6. Printed from a copy that indicates Rusk signed the original.