The P waves are compression waves. A wThis type of wave progresses due to a succession of movements of expansion and compression along its longitudinal axis. In the case of S waves, the vibrating movement takes place in the plane defined by the wavefront and thus perpendicular to the propagation direction, by which they are called transversal waves. A good analogy for this type of wave is a guitar string that is set in vibration. The passage of the transverse wave requires that the vertical planes of the medium to move "up and down" and therefore the adjacent elements of the medium suffer variations in shape, which alternate between a rectangle and a lozenge. Assuming that both the P wave and S wave are mechanical waves, ie, requiring a means for spreading material, are subject to the discontinuity existing in the propagation medium, and may occur phenomena of reflection or refraction of the waves, which leads to changes in trajectory of the waves.