The seismic waves are Onde S rubber bands which can cross a medium without modifying it. The starting impulse “will start” the elementary particles present in the medium, which “will push” other particles before taking again their place, being propagated according to a chain reaction.

The vibrations at the time of a Séisme are propagated in all the directions. One distinguishes two types of waves, the waves of volume which cross the Ground and the waves of surface which are propagated on its surface. On the recordings of the Seismograph S, they follow one another or are superimposed. Their propagation velocity and their amplitude are modified by the geological structures which they cross, this is why, the recorded signals are the combination of effects related on the source, the crossed mediums and the measuring instruments.

Various types of wave

Waves of volume

They are propagated inside the sphere. Their propagation velocity depends on crossed material and, generally, the latter increases with the depth because the crossed material becomes denser increase speeds. Attention \ rho is with the denominator-->.

One distinguishes:

• the waves P or primary waves called also longitudinal compression waves or waves. The displacement of the ground which accompanies their passage makes by successive dilations and compressions. They parallel to move the direction of propagation wave. They are fastest (6 km.s^-1 close to surface) and thus the first with being recorded on the Sismogramme S. They are responsible for grondement deaf that one can hear at the beginning of a Earthquake.