Seismic waves are the waves of energy that travel through the Earth as a result of an earthquake and can tell 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.
Thus, if there is an earthquake somewhere, the first waves that arrive are P-waves. In essence, the gap in P-wave and S-wave arrival gives a first estimate of the distance to the earthquake.
- Scientists are able to learn about Earth’s internal structure by measuring the arrival of seismic waves at stations around the world. For example, they know that Earth’s outer core is liquid because s-waves are not able to pass through it.
- Seismic waves travel in curved paths through the Earth (because of the increasing pressure, materials are more dense towards the core, travel velocity of seismic waves increases).
- Refraction of seismic waves causes them to curve away from a direct path.
- Reflection causes them to glance off certain surfaces (e.g. core mantle boundary) when they hit it at too shallow of an angle.
- The result of this behavior, in combination with the fact that S-waves cannot travel through liquids is the appearance of seismic shadows, opposite of the actual earthquake site.
- When an earthquake occurs there is a “shadow zone” on the opposite side of the earth where no s-waves arrive.
- Similarly 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, they 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.