Nuclear mattersSeismographs accurately detected North Korea's nuclear test

Published 27 May 2009

In 1998 the U.S. Senate rejected the Comprehensive Test Ban Treaty (CTBT) partly owing to fears that countries could cheat by claiming that small covert weapons tests were earthquakes; the quick and accurate detection of the North Korean test shows that the currently deployed system of sensors works

There are many thing to worry about as a result of the North Korean nuclear test on Monday — but there is a silver lining. As New Scientist’s Debora MacKenzie writes, the nuclear explosion — or, rather, its detection — is bad news for would-be nuclear nations. The network of blast detectors intended for the Comprehensive Test Ban Treaty (CTBT), which has not yet come into force, appears to have accurately identified the explosion as a nuclear test, despite its small size.

The timing is critical. President Obama wants the U.S. Senate to ratify the 1996 treaty, which bans all explosive nuclear tests, to demonstrate U.S. commitment to nuclear non-proliferation ahead of crucial international meetings next year, a meeting in which the extension of the Nuclear Nonproliferation Treaty (NPT) will be discussed. In 1998 the Senate rejected the CTBT partly owing to fears that countries could cheat by claiming that small covert weapons tests were earthquakes. The detection of the North Korean test raises hopes that the Senate will no longer object to the United States joining the CTBT.

North Korea’s test was no secret — Pyongyang announced it shortly afterward. It demonstrated, however, that the CTBT’s only partly built monitoring system could alert member states to a test within ninety minutes, says Tibor Tóth, head of the CTBT secretariat in Vienna. The last time North Korea set off a nuclear explosion, in 2006, twenty-two CTBT seismographs tracked it. This time, thirty-nine seismographs pinpointed the blast to “a couple of kilometers away from the 2006 test site,” Tóth says.

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 twelve 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 may 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.”

MacKenzie writes that the 2006 test was estimated at only 0.6 kilotons, which would make Monday’s blast only about 3 kt. Definitive yield estimates require further analysis, says Richards. By comparison, the 1945 Hiroshima bomb yielded about 15 kt.