You may think of the Earth as a solid structure, but in fact the Earth’s crust is floating on a semi-liquid layer of molten rock (magma) just below the crust. Below that, scientists believe the Earth’s core is a solid mass of nickel and iron. How can scientists know this? High temperatures and tremendous pressure in the Earth’s interior would make traveling there and remaining alive impossible. Even the deepest oil wells are only a few kilometers deep, and the diameter of the Earth is 12,756 km.

Well, scientists have used indirect measurements and seismic waves from earthquakes to determine the internal structure of the Earth, and you are going to recreate an experiment similar to the ones scientists have done to see how they did it. But first, a quick review of the different types of seismic waves.

Analysis of the Earth’s internal structure is made possible because earthquakes produce vibrations called seismic waves. These waves travel through the interior of the Earth and can be measured with sensitive detectors called seismographs. Scientists have seismographs set up all over the world to track movement of the Earth’s crust.

Seismic waves are divided into two types: Body waves and surface waves. Body waves include P and S waves, and these are the two types of waves that are used to determine the internal structure of the Earth. (There are other types of seismic waves, but they relate to surface damage and only travel through the Earth’s crust, not the entire Earth. If you are interested though, there is a short activity to show you what they are and how they move. Click here


Welcome to the Mighty Wave Maker

P Waves

P waves (pressure or primary waves) travel as a region of compression. How would this appear? Using the diagram above, make the green dots move left and right. Observe what happens to the distance between the dots. During compression, the dots move:

A) closer together or
B) further apart.
This wave is similar to the way
A) sound or
B) light waves
travel through air. As a P wave travels, the green dots vibrate back and forth
A) parallel or
B) perpendicular
to the direction of wave travel.

P waves are the fastest kind of seismic wave. A longitudinal P wave has the ability to move through solid rock and fluid rock, like water or the semi-liquid layers of the earth. It pushes and pulls the rock it moves through in the same way sound waves push and pull the air. Have you ever heard a big clap of thunder and heard the windows rattle at the same time? The windows rattle because sound waves push and pull on the glass much like P waves push and pull on rock. Sometimes animals can hear the P waves of an earthquake, but usually humans only feel the “bump” of these waves.

S Waves

S waves (shear waves) travel like vibrations in a bowl of Jello. How would this appear? Using the diagram above, make the green dots move up and down.

A) Does the distance between the green dots change, or
B) is the rectangular shape between the dots distorted?
The movement of the green dots is
A) parallel or
B) perpendicular
to the direction of the wave travel. As an S wave travels, the material is distorted but the green dots do not compress (the space between them pretty much stays the same.)

S waves are the second wave you feel in an earthquake. An S wave is slower than a P wave and only moves through solid rock. This wave moves rock up and down, or side-to-side.

Because P waves are compression waves, they can move through a liquid. However, S waves cannot move through a liquid. This is because a liquid is not rigid enough to transmit an S wave. S waves travel more slowly than P waves and, again, S waves cannot travel through a liquid.

So how can scientists use this information about wave travel to determine the internal structure of planet Earth? P and S waves, which are usually generated by earthquakes, volcanoes, or large objects like meteors hitting the earth, can also be produced by man using explosives or other large machinery. Scientists have used this method to gather evidence about the Earth’s internal structure. You need just a tiny bit more information regarding this, and then you can try it out for yourself!

P and S waves caused by an earthquake do not travel in straight lines. They also do not have a constant speed. Do you think the wave would

A) speed up or
B) slow down
as it moved further from the wave source? Waves can reflect off (bounce off) of materials that have a different density, or they can be refracted (bent) as they pass through a boundary between layers of different material. Scientists use the difference in arrival times of reflected and refracted waves from distant earthquakes to construct a picture of what the Earth’s interior looks like.

Discover more about the Earth's interior

Scientists believe that the Earth does not have the same internal structure that it did when it was originally formed approximately 4.6 billion years ago. Most scientists believe the Earth underwent a process called differentiation. This topic is covered in detail in another lesson produced by ASPIRE, called Protoplanetary Disc, which deals with planet formation.

P waves and S waves have allowed scientists to determine indirectly the internal structure of the Earth. Because these waves travel at different speeds through different material, they are also used to help determine the exact location of an earthquake (epicenter). Remember that these waves are transferring energy, and that energy is what causes the damage seen in the crust of the Earth, which is not plastic like the mantle and can break under pressure (causing faults!).


For more on waves, seismicity and earthquakes, you can check out some of the following web-sites to learn more:

United States Geological Survey: (A great resource for LOTS of inter-related topics, with an entire section on earthquakes. There is even a section to Ask-A-Geologist where you can email a question and get a response from a real geologist!)

Michigan Technological University (A great site with activities galore about earthquakes and seismic waves.)

Virtual Earthquake (Earthquake simulation)

Alaska Science Forum (A great paper about how "hearing" an earthquake before it hits: the speed of sound versus the speed of seismic waves.)

Nevada Seismological Laboratory (seismic waves and how they are deformed - great visuals)