Seismic monitoring

Seismic technology is an 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 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 are of crucial importance for the analysis of a suspicious event, providing essential information on its location, strength and nature. 

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. 

Measuring the azimuths of both P-waves and S-waves makes it possible to identify the direction from which the waves were emitted. Since the waves travel at different speeds, it is also possible to determine the distance to the source by measuring the different arrival times of the waves. 

Surface waves travel along the surface of the Earth, are slower than body waves and tend to be destructive. Technically, surface waves result from interference between the two types of body waves. During analysis, surface wave measurements help identify the depth and magnitude of an event. 

Seismic monitoring is significant in preparation for an on-site inspection (OSI). The analysis of seismic data provides the necessary information on the location of a suspected underground nuclear explosion, which is a prerequisite for the identification of an inspection area.