clandestine test, the greater the chance that it would be detected. This is even more true for a series of tests of a new nuclear device of military significance.4
Strong views exist today as they have for 50 years—both pro and con—of the feasibility of conducting a secret decoupled explosion of significant yield. In general, experts agree that seismic signals from an underground nuclear explosion can be reduced by a large amount but that the technique is impractical for yields above 10 kilotons (Turnbull, 2002). In this appendix, the Seismology Subcommittee argues that decoupled testing with yields of 1 to 10 kilotons with decoupling factors of 50 to 100 is not credible for countries of concern to the United States and that such tests likely would be detected with present monitoring capabilities.
The following sections discuss three decoupled nuclear explosions and what can be learned from them, cavities created by past nuclear explosions in salt that might be used for future clandestine testing, use of large cavities in thick salt deposits, and testing in mined cavities in hard rock. Salt is emphasized because cavities likely exist in that material from past nuclear explosions in the Former Soviet Union, and very large cavities at depth are easiest to construct in salt. Thick salt deposits at suitable depths for decoupled testing exist in some countries but not in others, such as North Korea. The feasibility of evasive testing is very much a function of the size or yield of an explosion a country wishes to test. Finally, this appendix lists the several significant obstacles a country would face in deciding to conduct a decoupled test and have a high likelihood that it would not be detected.
Known decoupled nuclear explosions
The database of decoupled nuclear explosions is very meager. It includes the only one that was nearly fully decoupled (Sterling), one partially decoupled (Azgir), and one small U.S. nuclear explosion that may have been decoupled significantly but by an unknown amount. They are the following:
• Sterling, a 0.38 kiloton (380 ton) nuclear explosion, was detonated in 1966 in the cavity generated by the 5.3 kiloton fully-coupled Salmon nuclear explosion in a salt dome in Mississippi. A decoupling factor of about 72 + 8 was calculated. A factor of about 70 has occurred repeatedly since then in discussions about evasive testing. Assertions such as “This means a 70 kiloton test can be made to look like a 1-kiloton test, which the CTBT monitoring system will not be able to detect” is doubly false, in that a 70 kt explosion cannot be fully decoupled, and the IMS will confidently detect a signal produced by a 1 kt test. What Sterling showed, in fact, was that a 0.38-kiloton test could be decoupled by a factor of about 70. Detection and identification, which we address in this report, have improved immensely since 1966. Both decoupling and detection of larger decoupled explosions are discussed.
• In 1976, the Soviet Union conducted a partially decoupled nuclear explosion of 8 to 10 kilotons in a huge cavity of mean diameter of 243 feet (74 m) in a salt dome at Azgir, which is now in the Republic of Kazakhstan (Sykes, 1996; Sultanov et al., 1999; Murphy, 2009). That cavity had been created at a depth of nearly 3,000 feet (1,000 meters) by a well-coupled nuclear explosion in 1971 with a yield of 64 kilotons (magnitude 6.06). Even in 1976—prior to the subsequent increase in deployed seismic instruments—that event was well recorded by many stations in Europe and Asia and as far away as Canada with
4 This appendix emphasizes decoupling factors larger than 3, since reduction in seismic amplitudes at about this level can be obtained by testing in regions where either seismic waves propagate less efficiently, explosions are detonated at greater depths than past nuclear weapons tests or in weak rock geologies.