The study of earthquakes, their nature, characteristics and consequences is called seismology.
Seismology also studies the propagation of seismic energy through the Earth.
What makes this study possible is the fact that seismic energy is propagated as waves. An earthquake generates different types of waves. Their classification is based on how they make matter vibrate as they travel. How fast a wave travels depends on the properties of the materials it is traveling through. The resistance of the material to be compressed or expanded.