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. The many small refractions cause the body waves to assume a straight travel path through the earth. Rocks of greater elasticity and their speeds, therefore increase steadily as they move downward. Rocks that are dense at the earth's surface, namely granites and basalts. Rocks that are uniformly less dense in material at the earth's surface.