The effect of stress is to cause strain which is a measure of the amount of body is deformed by stress. With the assistance of this instrument it is possible to isolate the major kinds of elastic wave motion involved in an earthquake. Mercalli scale Magnitude Intensity Richter scale This type of wave is represented by pulses of energy that are transmitted in such a way that the movement of rock particles is parallel to the direction of propagation of the wave itself. By using the information from the arrival time of the P and S waves at a seismic station the following can be determined. The movement of rock particles in secondary waves is at right angles to the direction of propagation of energy.
S waves travel more slowly than P waves.
Unlike P-waves, secondary waves will not pass through liquids or gases. Similar to P-waves, secondary waves will pass through liquids and gas. Body waves are subjected to refraction but they may also be reflected off the surface of a dense rock layer much as is light reflected off a polished surface. Body waves travel faster in rocks of greater elasticity. Both P and S waves are sometimes timed body waves.