That seismic noise that probes the Earth

A "light" illuminates the interior of the Earth : seismic waves. Seismologists know how to image the structure of the underground world from these vibrations that propagate in the depths of the ground. To do this, they use the waves emitted by earthquakes or artificial sources such as explosions. However, this active seismic imaging is only practiced in certain areas, or only on a small scale and at certain times.

This is why geophysicists also seek to use seismic "noise" that weakly, but constantly, agitates the interior of the Earth. It comes mainly from microseismic events due to the interaction of the seas with the earth’s crust (see Figure 1). Judiciously exploited, long recordings of this noise reveal the underground structure. After explaining how, we will present some of the early geological applications of this method, including the search for early warning signs of earthquakes and eruptions.

Seismic waves are a special case of elastic waves within solids (see Figure 2). There are two main types : P waves, which are compression waves, and S waves, which are shear waves (vibration is transverse to propagation). These so-called volume waves combine to give waves on the surface, which can be either Love or L...

– In french : Article paru dans "Pour la Science" N°409 novembre 2011, "Ce bruit sismique qui sonde la Terre" (dernière consultation le 25/02/2013).