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.

What Is Seismology?

•Seismology is the study of earthquakes and seismic waves that move through and around the earth. A

seismologist is a scientist who studies earthquakes

and seismic waves.

Note •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 .

Seismicwaves

•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

Types of seismic waves •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 •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•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 •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 .