CTBT... At last!

Gregory E. van der Vink, David W. Simpson, Rhett Butler, Christel B. Hennet, IRIS Consortium, Terry Wallace, University of Arizona


The forty-year battle to stop the testing of nuclear weapons was won on September 24, 1996 when President Clinton signed the Comprehensive Test Ban Treaty (CTBT) at the United Nations. With the same pen President Kennedy used in 1963 to ban testing above ground and underwater, President Clinton effectively extended the ban to include underground tests; and in doing so, limited the future development of nuclear weapons.

Although it has been known from the beginning that testing is not necessary for the development of simple fission bombs, testing is recognized as the means for developing more advanced thermonuclear weapons, and thus fueling regional arms races. Despite attempts by the nuclear nations to argue that testing is necessary to maintain the safety and reliability of the nuclear stock-pile, non-nuclear nations linked the 1995 extension of the Non-Proliferation Treaty to the successful negotiation of a CTBT by the end of 1996. Through a series of complex political maneuvers, a moratorium originally proposed by Congress and signed into law by President Bush was extended by President Clinton, and ultimately lead to the achievement of what President Clinton is calling "the longest sought, hardest fought prize in arms control history."

[Station Map] Figure 1. Over 50 stations of the IRIS GSN are currently part of the CTBT verification system. GSN stations being used in the primary network are shown as circles, auxiliary stations are shown as triangles. White symbols are GSN stations for which the GSE recommends continuing operation until other stations are installed. Between the time that the United States began negotiations on the CTBT in January 1994 to the signing of the treaty in September 1996, the IRIS Consortium completed the installation of 44 Global Seismographic Network stations to support the treaty's international verification regime. Working with the USGS, UCSD, and our local operating partners through scientific channels, IRIS independently encouraged countries to contribute GSN stations to the development of the International Monitoring System, thus greatly expanding global participation.

Like the 1974 Threshold Test Ban Treaty, however, the CTBT is almost certainly destined to languish for years in legal "no-mans land". India has announced its intention not to sign the treaty, thus hindering achievement of one of the current requirements for ratification. The treaty is signed and therefore binding. The treaty, however, is not ratified, and in the United States it must go to the Senate for approval (termed 'advice and consent' to ratification). Without ratification, the verification provisions called for in the treaty may not necessarily come into force. In the case of the 1974 Threshold Test Ban Treaty, the failure to implement verification provisions that included the exchange of seismic calibration data contributed to the incorrect accusation by the Reagan Administration that the Soviet's had violated the threshold limit of the treaty.

Seismic Verification

The CTBT calls for an International Monitoring System (IMS) consisting of: a primary and auxiliary seismic network, a radionuclide monitoring network, a hydroacoustic network, an infrasound network, and on-site inspections. Although the combined interaction of these systems results in an enhanced level of deterrence, it is the capability of the seismic networks that largely defines the capability of the overall monitoring system. The CTBT calls for a seismic network consisting of 50 primary stations and 120 auxiliary stations. Such a network of 170 stations (50 primary plus 120 auxiliary) is hauntingly reminiscent of the proposal of "160-170 control posts" made forty years earlier during the negotiations between Eisenhower and Khrushchev. While the 1950's network was projected to have a capability for detecting events only down to a magnitude 4.75, the goal of the 1990's network is to have a detection threshold of around magnitude 4.25 with location uncertainty of less than 1000 square kilometers. As illustrated by the following article by Harvey, however, a significant amount of calibration work needs to be done if this goal is to be achieved.

Verification, of course, can never be 100%. In the case of the seismic monitoring system, for example, it is theoretically possible to evade the monitoring system by decoupling a nuclear explosion of say one or two kilotons in a large underground cavity. That cavity could muffle the strength of the seismic signal and reduce the magnitude of the generated seismic signal down below the magnitude 4.25 detection threshold attributed to the monitoring system. Debate, both genuine and disingenuous, over the credibility of such evasion scenarios and the required levels of confidence for the verification system has been a persistent barrier to the CTBT since 1970.

Figure 2: The primary network of the International Seismic Monitoring System consists of 50 stations, approximately 60% of which are intended to be, or are currently, arrays. The primary network is distinguished from the auxiliary network in that the stations will transmit data continuously in near real-time to the International Data Center where they are used for the initial event detection. Figure 3: The auxiliary network of the International Seismic Monitoring System consists of 120 stations. These stations will be used to refine the parameters (location, depth, magnitude, etc.) of the events detected by the primary network.

Although the official monitoring system is important, particularly for a) the rapid exchange and processing of 'official' data, and b) maintaining a political commitment to the monitoring regime; it is only the "tip of the iceberg" in terms of facilities that have the potential to record the seismic signals from a secret underground nuclear explosion. Many nations have, and will presumably continue to have, significant national means. And more important, perhaps, is that international cooperation in seismic monitoring in the form of earthquake reporting long predates concerns about nuclear tests. For many areas of the world, the dense coverage of regional networks developed by scientists provides a detection capability far better than that of the monitoring systems.

Today, in an era of digital data, low-cost modems, global telecommunications networks, and global computer communications systems (the World Wide Web), all of these scientific and environmental resources create the technological equivalent of a global neighborhood watch program that will enhance the CTBT verification regime at little or no additional cost. These resources will provide strong additional deterrent to any country considering violating the CTBT below the threshold of the monitoring system.

At some level, we will not be able to demonstrate with high confidence that the monitoring system would catch extremely small nuclear tests, if they were to occur. In this context, the sensitive issue of verification becomes a value judgment about the costs of violations weighed against the benefits of the treaty. With superpower competition being replaced by concerns over the proliferation of nuclear weapons, a consensus has emerged that the CTBT is now verifiable.

IRIS' Role

In response to Congressional and Executive Branch requests, IRIS accelerated the installation of the Global Seismographic Network beginning in 1994. In what the Director of the National Science Foundation has called "a blueprint for the support of multi-use scientific programs that serve the national interest," IRIS expanded the Global Seismographic Network with special Congressional funding so that it would contribute not only to scientific endeavors but also to the monitoring of a CTBT.

When diplomatic efforts were unable to expand participation in the International Seismic Monitoring System, IRIS worked with the USGS and UCSD through scientific channels, to encourage nations with GSN stations to contribute to the international verification regime. Through such direct contact, IRIS greatly expanded participation in the international monitoring system and over 50 of the IRIS Global Seismographic Network stations are now part of the International Seismic Monitoring System. Following the recommendation of the White House National Science and Technology Council, the National Science Foundation has enhanced its support of IRIS over the next 5 years for its role in the CTBT monitoring system.

With the establishment of the official monitoring system being caught in the legal limbo of signed but unratified treaties, it is fortunate that much of the monitoring system is composed of multi-use stations. Such stations are supported not only for treaty verification but also for the scientific exploration of the Earth's interior and the mitigation of earthquake hazards. With such a broad base of support, the operation of the IRIS Global Seismographic Network stations will almost certainly continue, now relatively impervious to political fluctuations in the diplomatic community.

[photo of President Clinton signing CTBT] President Clinton signing the Comprehensive Test Ban Treaty at the United Nations in New York on Tuesday, September 24, 1996. Clinton signed the treaty with the same pen President John F. Kennedy used to sign the 1963 Limited Test Ban Treaty. (AP Photo/Greg Gibson)


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