The nuclear explosion set off by North Korea this week is bad news for would-be nuclear nations. The network of blast detectors intended for the Comprehensive Test Ban Treaty, which has not yet come into force, seems to have perfectly identified the explosion as a nuclear test, despite its small size.

The timing is critical. President Obama wants the US Senate to ratify the 1996 treaty, which bans all explosive nuclear tests, to demonstrate US commitment to nuclear non-proliferation ahead of crucial international meetings next year.

However, in 1998, the Senate rejected the CTBT partly over fears that countries could cheat, by claiming small covert weapons tests were earthquakes. The detection of the North Korean test raises hopes that the Senate will no longer be able to object.

North Korea’s test was no secret – Pyongyang announced it shortly afterwards. But it demonstrated that the CTBT’s only partly built monitoring system could alert member states to a test within 90 minutes, says Tibor Tóth, head of the CTBT secretariat in Vienna.


The last time North Korea set off a nuclear explosion, in 2006, 22 CTBT seismographs tracked it. This time 39 pinpointed the blast to “a couple of kilometres away from the 2006 test site,” Tóth says.

Bigger blast

“The seismographs were in a variety of countries, so there was good geopolitical saturation” – a key factor for the system’s political credibility, he says. In 2006, it took 12 days for tell-tale radioactive elements to reach a CTBT detector in Canada. Now the system has more than twice as many in place and detection might take only a few days, says Tóth.

The signal from the closest seismograph at Mudanjiang, China, “is obviously very similar to the signals of the 2006 test, and different from the signals of earthquakes” in the same region, says seismologist Paul Richards of the Lamont-Doherty Earth Observatory in Palisades, New York.

The signal was also bigger than the 2006 test. Richards says that if you compare the signal at seismic stations that measured both blasts, this one appears to be “about 5 times stronger”.

The 2006 test was estimated at only 0.6 kilotonnes (TNT equivalent), which would make this week’s blast only about 3 kt. However, definitive yield estimates require further analysis, says Richards. By comparison the 1945 Hiroshima bomb yielded about 15 kt.