Different types of seismic waves travel in different manners. As they move or propagate, they cause vibration in the body of the rocks through which they pass. 

• P-waves vibrate parallel to the direction of the wave. This exerts pressure on the material in the direction of the propagation. As a result, it creates density differences in the material leading to stretching and squeezing of the material.
• S-waves is perpendicular to the wave direction in the vertical plane. Hence, they create troughs and crests in the material through which they pass.

Further Reading:

• Earthquake – Body Waves, Causes & Types
• Geography Notes