Why do seismic waves curve as they travel through the earth?
Seismic waves can be modelled as wavefronts with a general ray that represents the direction in which the wave is travelling (if you are familiar with high-school optics in physics, the concept is essentially the same). When this ray reaches an interface (a boundary between two different types of material), it changes its path: if the medium that it is propagating into allows the wave to travel faster, then the ray refracts (bends) away from the normal (the right-angle intersector to the surface of the interface) – if the wave is going to slow down in the new medium, then it bends towards the normal (This is Snell’s law by the way. You can read up on it pretty much anywhere). As the Earth is essentially composed of concentric spheres (lots of spheres that fit inside one another), the waves that propagate through the Earth change direction each time they travel from one layer into another. As a general rule, the deeper one goes into the Earth, the higher the density – and since the velo
