Energy travels through the Earth in seismic waves.
As a seismic wave travels it causes the particles in the Earth to oscillate. These oscillations are related to the 'rock stiffness' or rigidity of the rock. Eventually seismic waves dissipate their energy to the earth and lose amplitude. Like other waves, seismic waves obey the laws of physics. Waves also have different speeds in different media because each medium has its own rigidity.