1. Divide students into pairs or groups of four. Hand each group/pair a slinky and some plastic tape.
2. Direct students in each group/pair to mark two spots on their slinky near the center with plastic tape at the top of adjacent loops. Students mark the coils so that they can see the movement of energy along their length.
3. Have two students each hold one end of the slinky for their group. Stretch out the slinky to about 3 meters along a floor, table, or other flat surface. Have students take turns compressing 10–20 coils and then releasing them rapidly while they hold the end of the slinky, making sure to watch the energy wave travel the length of the slinky.
4. After several repetitions, ask students to describe their observations of the coil and the tape: the coils move back and forth along the length of the slinky as they compress and expand it. Ask students what kind of earthquake waves this slinky motion resembles. The answer is compressional waves. Remind them that in compressional waves particles of material move back and forth parallel to the direction in which the wave itself moves. As a compressional wave passes, the material first compresses and then expands. P-waves (P stands for primary) are compressional earthquake waves that pass through the interior of the Earth. P-waves change the volume of the material through which they propagate.
Note: P-waves in the air are sounds. P-waves can move faster through the ground than the air, but not all of this energy is in the range of human hearing. When the sound waves are in the audible frequency range, some people may hear them.
5. Now tie one end of a 2-meter rope to the door knob of the classroom door. Ask one student to hold the free end of the rope in his/her hand. Ask the student to back away from the door until the rope is straight with a little bit of slack and start to gently shake the rope up and down. Allow each student to create this motion. After several repetitions, ask students to describe the rope motion. Ask them what kind of earthquake wave motion this resembles. The answer is shear waves. Remind them that in shear waves particles of material move back and forth perpendicular to the direction in which the wave itself moves. S-waves (S stands for secondary) are shear earthquake waves that pass through the interior of the Earth. S-waves don't change the volume of the material through which they propagate, they shear it.
Note: The motion of the rope due to shear waves is much easier to observe than the compression waves, but the shear waves travel more slowly than compression waves. In an earthquake, scientists can observe the arrival of compression waves before the arrival of shear waves using seismographs. You may choose to show a close-up of a record (seismogram) for a single earthquake event, and ask your students to point out different seismic waves. In addition, shear waves cause much more damage to structures since it is easier to shake surface rocks than it is to compress them.
6. Encourage your students to critically evaluate their slinky and rope setup. Ask them if they see any limitations associated with their setup. Ask them to compare and contrast their simple setup with actual vibrations caused by seismic waves traveling through the Earth or along its surface. For instance, seismic waves carry energy from the source of the shaking outward in all directions (not in one direction only as the setup shows).
7. (Optional) Both primary and secondary waves are body waves (pass through the interior of the Earth). Surface waves travel along the Earth's surface. Two examples of surface waves are Rayleigh waves and Love waves. Explain to your students that Rayleigh waves cause the ground to ripple up and down (like water waves in the ocean before they break at the surf line) whereas the Love waves cause the ground to ripple back and forth (like the movement of a snake).
8. Ask the students to recall how scientists use seismic wave observations to investigate the interior structure of the Earth. This is similar to checking the ripeness of a melon by tapping on it. To understand how scientists see into the Earth using vibrations, one needs to understand how waves or vibrations interact with the rocks that make up the Earth. Introduce to your students the two simplest types of wave interactions with rocks: reflection and refraction. Ask students to define reflection. They should be able to give simple examples like echoes or reflection in a mirror. Explain to your students that echoes are reflected sound waves, and that students' reflections in a mirror are composed of reflected light waves. Tell students that a seismic reflection happens when a wave impinges on a change in rock type. Part of the energy carried by the wave is transmitted through the material (refracted wave) and part is reflected back into the medium that contained the wave. Refraction can be demonstrated by dropping a coin in a bottle filled with water. The coin changes direction when it hits the water's surface and won't sink to the bottom vertically. In other words, the path of the coin refracts (changes direction) when moving from the air into the water.
9. Explain to students that seismic waves travel fast, on the order of kilometers per second. The speed of a seismic wave depends on many factors. Ask students to think about a few factors that can change the speed of a seismic wave (e.g., rock composition, temperature, pressure) Ask them to explain how these factors can change the speed of a seismic wave. Students should be able to answer these questions based on the knowledge they acquired throughout this lesson. Seismic waves travel faster in denser rocks; temperature tends to lower the speed of seismic waves; and pressure tends to increases the speed.
Caution: The speed of a seismic wave generally increases with depth, despite the fact that the increase of temperature with depth works to lower the wave velocity.
