How are seismic waves used to provide evidence about Earth's interior?

Answer 1

Three different types of waves travel away from the focus of a seismic event. These waves travel at different speeds. In addition to surface waves, the waves that are proving critical to providing evidence about the interior of the Earth are 'P' waves (P standing for primary or pressure), and 'S' waves (S for secondary or shear). P-waves are faster than S-waves.

P-waves can travel through solid rock and fluids, and their speed will vary depending on the density of the rock through which they travel; increased speed will indicate increased density, thus providing information about the density and possible mineralogy of Earth's various layers.

S-waves cannot travel through fluids, thus when an S-wave encounters the outer core of the Earth (liquid iron and nickel), it is disrupted, indicating the boundary between the mantle and outer core and the boundary between the inner core and outer core. The effect observed from the motion of S-waves reveals a 'shadow zone', opposite to where they originate.

An animation of P and S-waves moving through Earth's interior and the resultant S-wave shadow due to the presence of the Earth's core is viewable in the related links section.
Geologists use seismic waves to learn about the earth's interior by using the reflection or refraction of the seismic wave to investigate subsurface structures and locate the epicenter.