Seismic waves, the waves of energy that travel through the Earth as a result of an earthquake can tell us a lot about the internal structure of the Earth because these waves travel at different speeds in different materials. There are two types of waves that travel through the Earth: p-waves and s-waves.
P- waves are faster and they can travel through both solids and liquids. S-waves are slower and cannot travel through liquids. For both kinds of waves, the speed at which the wave travels also depends on the properties of the material through which it is traveling.
Scientists are able to learn about Earth’s internal structure by measuring the arrival of seismic waves at stations around the world. For example, we know that Earth’s outer core is liquid because s-waves are not able to pass through it; when an earthquake occurs there is a “shadow zone” on the opposite side of the earth where no s-waves arrive. Similarly, we know that the earth has a solid inner core because some p-waves are reflected off the boundary between the inner core and the outer core. By measuring the time it takes for seismic waves to travel along many different paths through the earth, we can figure out the velocity structure of the earth. Abrupt changes in velocity with depth correspond to boundaries between different layers of the Earth composed of different materials.
