The Earth is basically made up of layers, with the Earth's crust being one of the last layers (the surface on which we stand). However, this crust is not continuous, it is composed of pieces called tectonic plates, and the places where these plates joint together are called faults.
When theses tectonic plates move or when sudden displacements in faults or cracks in the interior of earth’s crust occur; seismic waves are originated.
The seismic waves are the propagation of disturbances generated by small movements in a medium, they propagate towards the earth's surface causing earthquakes or low intensity seismic movements.
Now, the speed of the waves depends of the properties of the medium; which are fundamentally:
(1) the elasticity and (2) the density of the materials by which it is propagated.
