• 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.