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.

That’s a great question! Most hypotheses about the internal structure of the earth are developed by studying seismic waves that travel through the earth and are measured at seismometer stations. Seismic waves are generated in earthquakes and they travel differently through different types of material.

There are two main types of seismic body waves:
primary “p” waves and secondary “s” waves. P- waves can move through solids and liquids. S- waves can only move through solids. We have determined that the mantle is solid because both P- and S-waves that travel into it can be detected in seismometers. We know that the outer core is “molten” or liquid because S-waves that travel into it are not detected on the other side. We know that the inner core is solid by identifying a “phase-shift” of seismic waves that travel through it. Some seismic waves travel to and from the inner core at P-waves, but turn into S-waves when they travel through the core (Mussett & Khan, 2000). Really smart geophysicists can recognize this phase-shift by analyzing the waves measured at a seismometer. Some times we can even image some parts of the earth’s interior using something called “seismic tomography”. It is based on the same principle as “computer-aided tomography” or CAT-scans, which are commonly used in medicine to image the insides of people’s bodies. Seismic tomography is possible because seismic waves travel at different speeds through different material. In general, waves travel more slowly through parts of the earth that are soft or partially liquid. Usually, these are areas that are hot and may be partially molten (partially melted to a liquid).

We know how fast seismic waves travel through “normal” parts of the earth, so we know when to expect a wave to arrive at a seismometer a certain distance from where an earthquake happened. If a wave arrives “late”, we know that it passed through a hot, soft part of the earth. The diagram that I included shows a simple example of how seismic tomography works.

tomography-1.jpg

The grids in a, b, and c represent the same area within the earth. In a one ray moves through each row. Ray 3 arrives late, but it could have slowed down in any of the four small squares, so we don’t know what square is hot and soft. In b one ray moves through each column. Ray 6 arrives late. We can now identify which small square (a small area inside the earth) is hot and soft, that is the area of overlap of the two slow rays. This is a simply 2-dimension example, but if we measure seismic waves from a lot of earthquakes at a lot of seismometer stations we can make 3-dimension images. Geophysicists have used this method to image things like “hot spots” under Hawaii and southern Africa. I hope this information helps!

Sound waves travel at different speeds through different materials. Because sound radiates out from an origin point, we can know where and when an earthquake happened, and from the time it takes the waves to reach different points on the Earth's surface, we can figure out what the waves must have traveled through in order to get there.