Scientists can use the speed of the waves generated by earthquakes to determine what the interior of the Earth is made of. The speed of an earthquake wave (also called seismic waves) depends on the density of the rock it is traveling through. Higher density means the waves will travel slower, lower density means the waves will travel faster. So, if you know the speed of a seismic wave, you could use that to find the density of the rock and then draw a conclusion about the composition of the rock.

Seismic waves were also used to determine the state (i.e. solid or liquid) of the interior layers of the Earth. Earthquakes generate four different types of waves; there are waves that travel through the Earth, called P-waves and S-waves, and waves that travel along the surface, called Love waves and Rayleigh waves. P-waves can travel through any kind of material, solid or liquid. S-waves however can only travel through solids, they cannot travel through liquid. This is how we discovered that the outer core of the Earth is actually liquid- there were no S-waves traveling through it. The other layers, the crust, the mantle and the inner core, are all solid.

So, seismic waves can be used to find both the composition of the rock (what kind of rock it is) and the state of the material (is it solid or liquid).