Natural earthquake vs. nuclear test: waveform graphs tell the story

North Korea claimed on Jan. 6 that it had tested a hydrogen bomb. Before the official announcement, however, news agencies were already reporting a possible nuclear test after an "artificial" earthquake was detected with its epicenter around North Korea's nuclear testing site in the northeast of the country.

How can earthquake monitoring agencies tell the difference between a natural quake and one caused by a nuclear blast? The answer is in the incident's waveform. The undulations in the ground produced by a natural quake build to a sudden crescendo, while those produced by an underground nuclear test spike at the very beginning and then trail off.

For example, the waveform of a natural earthquake recorded by the Japan Meteorological Agency (JMA) in 2002 looks like this:

Meanwhile, the waveform for the alleged Jan. 6 North Korean H-bomb test looks like this:

That's very similar to the waveform the JMA recorded for North Korea's Feb. 12, 2013 atom bomb test:

(By Robert Sakai-Irvine, Staff Writer)