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.
Different speeds give rise to refraction effects, including critical angle.
C'' and refracts back up towards the surface at the critical angle. D'' while the rest of the waves energy is transmitted into the lower medium but at a new or refracted angle. E Vertical wave - straight down perpendicular to the boundary E' a small part of the waves energy is reflected straight back up E'' while most is transmitted. The direction does not change but the speed does. We can calculate the seismic wave velocities in both layers and the depth to the boundary.