As the strength of rocks are exceeded and the break to form a fault, the stored energy is released as a shock wave. Shock waves created by earthquakes are referred to as seismic waves. Shock waves are of two types, shear and compression. In the case of shear waves, the movement of the Earth is perpendicular to the direction of propagation in the form of a sine wave. While the illustration shows only the vertical and horizontal components of movement, remember that the actual movement is in all directions perpendicular to the direction of propagation. The amplitude of the movement is determined by the amount of energy released by the earthquake. In the case of the compression wave, the movement of the earth is back and forth parallel to the direction of propagation. Although our discussion is strictly related to seismic waves, I might point out that the sounds we hear are created by the response our ears to the compressive shock waves. As the shock wave moves down the ear canal and encounters the ear drum, the eardrum moves back and forth parallel to the movement of the shockwave which, in turn, is detected by the cochlea and transmitted to the brain where it is interpreted as a sound. Note that since the shear wave component of the shock wave is moving parallel to the surface of the eardrum, it does not contribute to what we hear.
shock waves, seismic waves