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The size or magnitude of earthquakes and other seismic events is measured using the Richter scale. Several thousand earthquakes larger than magnitude 4 on the Richter scale occur each year around the globe. A magnitude 4 earthquake is a fairly light earthquake which can cause windows and doors to rattle, but which does not result in significant damages.

A seismic event generates two types of seismic waves: body waves and surface waves. The faster body waves travel through the interior of the Earth while the slower surface waves – as the name suggests – travel along its surface. Both types of wave are looked at during analysis to collect specific information on a particular event.

The instruments employed to measure seismic waves are seismometers, which are sensors converting ground motion into electrical voltage.

Seismic monitoring is one of the three waveform technologies used by the Comprehensive Nuclear-Test-Ban Treaty (CTBT) verification regime to monitor compliance with the Treaty. The objective of seismic monitoring is to detect and locate underground nuclear explosions. Data resulting from seismic monitoring are used to distinguish between an underground nuclear explosion and the numerous natural and man-made seismic events that occur every day, such as earthquakes and mining explosions.

Underground nuclear testing began in the 1950s and provoked growing concern. It was, however, soon recognized that seismic observations could provide a method of verifying the strength and location of these events.

Seismic technology is a very efficient means of detecting a suspected nuclear explosion. Seismic waves travel so fast that an event creating these waves can be registered by seismic stations distributed worldwide in a time span ranging from a few seconds to about ten minutes.

A seismic event generates body waves and surface waves. Both are of crucial importance for the analysis of a suspicious event. They provide essential information on the location, the strength and the nature of an event.

There are two types of body waves emanating from a seismic event, P-waves and S-waves. P-waves are primary or compressional waves that alternately compress and expand the ground in the direction of the wave’s propagation. These waves can move through any material.

S-waves are secondary or shear waves in the ground that move perpendicular to the direction of the wave’s propagation. S-waves can only move through solids as this kind of movement is impossible in liquid or gaseous materials.