Note that with P waves (pressure waves), the particle motion is along the direction of travel of the wave. On the other hand, for S waves (shear waves), the particle motion is perpendicular to the direction in which the waves travel. Thus, a P wave is an example of a longitudinal wave, while an S wave is an example of a transverse wave.

Seismic waves are generated naturally by earthquakes, by volcanoes, and by impacts, and may be produced artificially by explosions and mechanical devices. They tell us about the earth's interior in several general ways.