How do seismic waves make an image of the interior of the earth?
An image of the Earth's internal structure can be constructed using measurements of seismic waves at the surface. Differences in the speed of the waves through the Earth allow scientists to infer what the structure might look like. The speed of the waves can change due to density differences and moving from solid to liquid for example. This technique is called seismic tomography. It is sort of like taking a CAT scan of the Earth!