Although there are synergies among these different CTBT monitoring technologies and all of them are needed, seismology is the most effective for monitoring against the underground testing environment, which is the one most suited to attempts at clandestine treaty evasion. From the viewpoint of a sophisticated evader, this environment is also the one most suited to evaluating the performance of nuclear-weapon components via explosive testing. We therefore give emphasis to seismic monitoring of the underground environment.

Starting in the late 1940s, the United States developed a capability to monitor atmospheric nuclear tests and was successful in detecting the first, unsuspected Soviet nuclear test in late August 1949 by routine air sampling over the Pacific Ocean. Over the next decade the system for air debris sampling and infrasound detection was developed and an initial network of seismic stations was established to monitor anticipated underground testing. The Limited Test Ban Treaty (LTBT) of 1963 (banning signatories from nuclear testing underwater, in the atmosphere, and in space) did not incorporate an independent international monitoring system, but depended on the Nuclear-Weapon States’ independent national technical means (NTM), which were directed at keeping track of each other’s nuclear programs and possible testing by Non-Nuclear-Weapon States. The international community at the time appeared satisfied that this system of NTM monitoring was adequate.

For the next decade the United States (and presumably the Soviet Union) improved its worldwide monitoring system with substantially improved seismic capabilities and a variety of satellite sensors to monitor atmospheric and space nuclear explosions. In 1974, the United States and the Soviet Union signed a bilateral Threshold Test Ban Treaty, banning underground nuclear tests of yield greater than 150 kt,¹ which involved extensive, close cooperation between the two states, and led to mutual understanding of the relationship between seismic magnitude² and yield of tamped underground nuclear explosions at their respective test sites³—a subject in longstanding dispute.⁴ As an example of informal cooperation, in 1977 the Soviet Union called the attention of the United States to apparent South African preparations for an underground test in the Kalahari desert that had been missed by U.S. intelligence and that led to actions that prevented further activity at the site.

With international efforts to negotiate a comprehensive test ban in the mid-1990s, the international community was no longer satisfied to rely on the NTM capabilities of the Nuclear-Weapon States (primarily the United States) to monitor the treaty, but wanted it to be based on a truly internationally-operated system with information available to all parties. While the new system in many ways duplicates existing NTM capabilities, it adds significantly to those capabilities and makes the CTBT a genuinely international undertaking. This makes possible the establishment of a much denser network of monitoring stations and the international acceptance of challenge inspections. The treaty is designed so that both the international system and NTM can be used to carry out the monitoring function.

A significant difference between international monitoring efforts and monitoring by NTM is that the former must treat the whole world more or less equally, while the latter can concentrate on particular nations that are of concern to the United States. NTM can also focus on