Scientists analysing seismic signals detected after the blast on Fangataufa atoll say that it created an earthquake of exactly the same magnitude as the first French test at nearby Mururoa. But the official French Defence Ministry statement described the yield of the test as "less than 110 kilotons" - which was taken as meaning it was five times more powerful than the 20 kiloton Mururoa test. The seismic shock should therefore have been significantly bigger.
The speculation arises that the main force of the blast came from the atomic bomb used to "trigger" the hydrogen fusion reaction of the H-bomb, which may not have gone off properly.
Roger Clark, a seismologist at Leeds University, said the French had consistently overstated the yields of their tests. "I have a cynical enough mind to say it [Fangataufa] could even be less than 20kt," he said.
In testing nuclear weapons, it is not enough simply to get a bang. The explosive yield and the radiation produced are closely measured to ensure that the device has performed according to its designers' specifications. If the yield is lower than forecast, that may indicate a fundamental flaw in the design.
Patricia Lewis of the London-based Vertic group, which monitors the explosions, said that the difficulty of measuring yields properly underscored the need for any test ban treaty to be completely comprehensive and not a "threshold" treaty, banning all tests above a certain size. Diplomats hope that a treaty might be attainable next year, after the French have finished, but some countries have suggested small nuclear explosions should be exempt.
Dr Clark said that if the device exploded at Fangataufa had been buried deeper than the one at Mururoa, then it was possible for a bigger bang to have given the same magnitude, and it was possible that the rocks might somehow have further attenuated the signal. But, he said, "It would be startling and unnerving from a seismologist's point of view if the calibration is different for Fangataufa, which is only 40km from Mururoa. There won't be a clear resolution of this until we have details of the yields of previous explosions. Past experience of analysing official announcements suggests that Fangatuafa could be a huge amount less than 110 kilotons."