Seismologists study the Earth’s structure by observing seismic waves as they travel through the planet.

Seismic waves are generated whenever an earthquake occurs, but they can also be generated, for example, by the action of sea waves hitting the coast. An underground explosion also generates seismic waves that can be recorded by seismometers.

However, the waveform characteristics of an earthquake and an explosion are very different because the source mechanisms are different. This enables scientists to distinguish between the two. While an earthquake seismogram contains two types of waves – P (longitudinal) and S (transverse) waves, the seismogram of an explosion is dominated by P waves.

The seismic waves from the nuclear bomb testing in North Korea on January 6 were recorded by seismic stations all over the world, including some in Europe. Although scientists are able to recognise the difference between an earthquake and an explosion, and calculate the magnitude of the explosion, they still cannot discriminate between an A- bomb and an H-bomb.

Underground nuclear testing started in the 1950s. In 1996, the Comprehensive Nuclear Test Ban Treaty was introduced, with the aim of banning all nuclear testing.

The CTBT Organisation in Vienna operates a worldwide network of instruments and carries out global seismic monitoring to detect and locate underground nuclear explosions. The University of Malta is the designated as the national data centre for Malta in the CTBTO.

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