usually allows them to be distinguished from single-fired contained explosions. There are a limited number of mines in the world that routinely generate seismic events greater than REB mb = 3.5, with the largest signals over the past several years being about 4.0. These large open pit mines take many years to develop and are visible by commercial satellite. Such mines and their blasting signals will be the subject of extra attention under the CTBT, noting that the Treaty provides means for voluntary reporting of shots greater than 300 tons (0.3 kilotons). Since the 1990s, the United States has voluntarily provided a separate list of mining events (times, locations, and magnitudes) through a website of the U.S. Geological Survey. The U.S. should encourage other countries with national seismic networks to do the same.
Work done in the 1990s indicates that a masked underground contained explosion would need to be smaller than 10 percent the size of the masking large mine blast. For the largest mine blasts, this leads to estimates for fully coupled shots of a few hundred tons of nuclear yield or less (decoupled explosions are covered in another section). For an evader, mining operations provide cover for extensive excavations needed to contain a nuclear test and reasons for seismic signals. However, mine operations that routinely produce large seismic events (magnitude > 3.5) can be among the best calibrated areas on Earth due to their past record of numerous events. The record of many routine signals allows techniques such as waveform correlation and joint seismic and infrasound analysis to provide ways to flag and identify unusual signals. Mines that routinely produce large seismic signals offer opportunities to better calibrate the seismic network for improved detection, location and identification.
Mine collapses that generate seismic signals greater than magnitude 3.5 are infrequent and are not easily controllable for masking purposes. Their unusual seismic signals make them a potential source of false alarms, but new algorithms have been developed that can distinguish such events from both explosion and earthquakes.
Unsuccessful Proposals for Clandestine Underground Nuclear-Explosion Testing
Several other scenarios have been proposed for clandestine underground testing, but none is considered likely to be successful:
• Hiding signals of an explosion in that of an earthquake—Modern instruments detect seismic waves from earthquakes and nuclear explosions over a very broad range of frequencies and at many different distances. Signals from small nuclear explosions can be separated from those of large earthquakes by simple filtering in the frequency domain, using arrays to separate signals arriving from different azimuths and wave speeds, and looking at data from regional stations closest to the presumed explosion. Because earthquakes cannot be predicted, a wait of years typically would be involved after emplacing a nuclear device and then determining in a very short amount of time that a large nearby earthquake had occurred.
• Set off series of explosions so seismic waves look like those from an earthquake—This idea was proposed about 40 years ago when identification was made using seismic waves with only two periods, about 1 and 20 seconds—the Ms/mb technique. This evasion proposal does not work when digital data are used, as they are today, with a broad range of periods.
• Absorb energy by placing carbon in cavity containing an explosion—very small U.S. tests of this concept many decades ago were not successful.
• Reduce size of seismic waves by testing in rubble zone of a previous nuclear explosion— Sites of past nuclear explosions that generated rubblized zones of any significant size are known and can be monitored. Rubblized zones are likely to be conduits for noble gases and other bomb-produced radionuclides.