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On October 9, North Korea announced that it had carried out an underground nuclear test In subsequent days, the apparent low yield of the device and initial lack of reports of detection of radioactivity from the test raised questions about whether North Korea had actually tested a nuclear device or if a test had failed.
One week later, the Office of the Director of National Intelligence issued a statement confirming the detection of radioactive debris and stating that North Korea had conducted a nuclear explosion with a yield of less than 1 kiloton (1,000-ton TNT equivalent).1 Our analysis of the available public information is consistant with this conclusion. We also judge that, although the test did not succeed as planned, North Korea might have been testing a lower-yield design than many commentators have assumed. This imperfect test may well lead North Korea to test again.
Our analysis concerns three questions: How powerful was the explosion? Was it a nuclear test? If nuclear, was the test successful?
How Powerful Was It?
North Korea reportedly informed China that it would be conducting a nuclear test, with a yield in the range of 4 kilotons.2 Such an explosion in hard rock would produce a seismic event with a magnitude of about 4.9 on the Richter scale.3 By contrast, the U.S. Geological Survey reported the explosion to have a seismic magnitude of 4.2.4 South Korea's state geology research center reported the magnitude as lower, between 3.58 and 3.7, and estimated a yield equivalent to 550 tons of TNT.5 An uncertainty in seismic magnitude of 0.5 translates into a shift in yield of about a factor of 4.6. Compounding this uncertainty is that the relationship between seismic magnitude and yield depends on the hardness of the rock in which the explosive is buried.6
Given all these uncertainties, it is not surprising that a range of yields has been reported. Terry Wallace, a seismologist at Los Alamos National Laboratory, based on an unclassified analysis of open data, estimates a yield of between 0.5 and 2 kilotons, with 90 percent confidence that the yield is less than 1 kiloton.7 Lynn R. Sykes of Columbia University estimates a yield of 0.4 kilotons, with 68 percent confidence that the yield is between 0.2 and...