Assumptions used in establishing seismic monitoring requirements are subjective and no independent assessment of these requirements has been made. As demonstrated in chapter 5, differences in judgment over the credibility of a first-time tester successfully conducting and containing an explosion in a large underground cavity can change the assessment of monitoring requirements by over a factor of 10. Different assumptions about the required level of confidence for the detection of a seismic event can also change the monitoring threshold by a factor of 10. What is required, therefore, is not necessarily to have the lowest possible detection threshold everywhere in the world, but rather to assess the credibility of various evasion scenarios and the appropriate level of confidence required for each part of the world.
Where the objective is to monitor the preliminary test of a first generation nuclear weapon of simple design, the task can be readily accomplished with data from existing seismic networks. As discussed in chapter 2, the decoupling evasion scenario is not credible for a first-time tester detonating such a device. Where the objective is to monitor advanced nuclear weapon states with testing expertise and experience, such as the United States, China, and Russia, the effort will be considerably more difficult. For such nations, the evasion scenario of a sub-kiloton nuclear weapon being secretly tested in a large underground cavity under the guise of a legitimate mining operation, for example, becomes more credible. An intermediate level of effort may be required for monitoring a nation with an advanced secret nuclear weapons program, or a nation that may be able to obtain tactical nuclear weapons from an advanced nuclear state and wishes to subject one or more of them to a proof test. While such an explosion may have only a subkiloton yield, the nation may or may not have the necessary capability and be able to meet all the requirements for successfully conducting a secret decoupled nuclear test. In the relatively few areas of special concern where evasion scenarios are credible for such countries, one might want to densify the distribution of broadband and high frequency seismic stations to record low magnitude seismicity from regional distances.
In the case of a CTBT, the monitoring task becomes increasingly difficult as the concern extends to smaller and smaller yields. At lower magnitudes, there are more naturally occurring earthquakes, industrial explosions, and means for evading the monitoring system. Inevitably, there will be a level below which we have little confidence in monitoring nuclear tests.
In the same manner as other treaties have been evaluated, a Comprehensive Test Ban Treaty will be considered verifiable, if it is judged that the "benefits" of the treaty (in this case, the benefits to strengthening the nonproliferation regime) outweigh the "costs" of any potential small undetected violations. Through a similar "cost-benefit" analysis, the United States and other concerned countries will have to determine the level of effort we wish to apply to increasing the confidence with which we monitor small seismic events around the world. The question becomes, how many resources are we willing to allocate to the task, and how many ambiguous events are we willing to accept? There will probably be about a ten-fold increase in cost and the number of ambiguous events for each magnitude unit that we lower the detection/identification threshold.
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