American Geophysical Union. Adapted with permission from Investigating Earth Systems, American Geosciences Institute.
An earthquake occurs when massive rock layers slide past each other. This motion makes enormous vibrations, which travel from the site of the earthquake in waves.
The waves (seismic waves) travel all the way through the Earth. Seismologists can record these waves when they reach Earth’s surface using seismographs.
Earthquakes generate three kinds of waves: Compressional waves (P waves) travel the fastest. Shear (S) waves travel more slowly than P waves. Surface waves are the slowest of the three.
- Two Slinkys™
- Flat, smooth surface
- Notebook and pen
- Safety goggles
- Work with a partner. Put on safety goggles before starting this activity. With a partner, stretch out a Slinky™ on the floor about as far as it can go without making a permanent bend in the metal.
- Have one partner make waves by gathering several coils at the end of the Slinky™ and then releasing the coils, while still keeping hold of the end of the Slinky™. Observe the direction of wave movement relative to the Slinky™. Does it move in the same direction (parallel to the Slinky™) or in the opposite direction (perpendicular to the Slinky™)? Record your observations.
- This kind of wave is called a P (primary) or compressional wave. (To compress means to squeeze together.) From your observations, explain why this is a good name for this wave. You may wish to use diagrams to illustrate your answer.
- Stretch out the Slinky™ again. This time, have one partner make waves by moving the Slinky™ from side to side (left to right or right to left). Again observe the direction of wave movement, relative to the Slinky™. Does the wave move in the same direction (parallel to the Slinky™) or in the opposite direction (perpendicular to the Slinky™)? Record your observations.
- This kind of wave is called a secondary or shear (S) wave. (To shear means to slide one thing sideways past another thing.) From your observations, explain why this is a good name for this wave.
- Stretch out the Slinky™ a third time. This time, move one end of the Slinky™ up and down to generate a wave. This shows how the surface waves from earthquakes behave. What effect could this type of wave have on houses anchored to the Earth’s surface? Why is that?