How do seismic waves travel through the Earth?

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