12 February 2013, 4:57 am (South African time): Sensors around the globe recorded a magnitude 5.0 seismic event in Asia. Within an hour of North Korea’s latest underground nuclear test, the world knew about it. Here’s how. – BY JOE PAPPALARDO
Identify: Earthquake-monitoring hardware is used to detect nuclear detonations. Paul Richards of Lamont-Doherty Earth Observatory explains: “An explosion sends out waves to a great distance if the signal is large enough. These waves travel through the earth and shake the ground.” Many nations have agreements to share seismic data; the Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organisation alerted its member nations to this blast within an hour.
Locate: Seismic sensors measure different waves, which allows scientists to determine where an explosion took place. “We use the time between the P wave (lateral compression) and the S wave (up and down motion) to tell how far away it is from that station,” Richards says. “Then we get the signal from another station, and draw intersecting circles and find out where (the source) came from.” The more stations that contribute data, the greater the accuracy. The location of the 12 February explosion was nearly identical to that of two previous North Korean nuclear tests, but the blast was twice as large as the nation’s last test, in 2009.
Confirm: Underground nuclear detonations vent trace amounts of radioactive particles into the air. Investigators rely on a global network of specialised detectors to confirm that a nuke test occurred and glean information about the source.
Earthquakes move the ground up and down, producing more S waves than P waves, which come from lateral compressions.
Where does the radioactivity come from?
Detectors can find many kinds of radionuclides, but only a few are produced by nuclear explosions.
Any detection system must screen out the presence of naturally occurring sources of radioactive isotopes in the air, earth and water. The types and levels of background radiation differ around the globe. There are more than 60 known types found in Nature.
Radioactive isotopes, or radionuclides, are used in medical tracers and industrial manufacturing. They can also come from power-plant reactors, which routinely release limited, legally permissible amounts of radioactive noble gases.
Source: power plant or medical facility
Underground tests vent noble gases, so sensors that detect them have been added to the monitoring network. Investigators can measure the amounts of xenon isotopes to determine whether a bomb was plutonium or uranium.
Source: nuclear explosion