Plate Tectonic - Earthquakes (5)
Pre Lab 

   
OBJECTIVES:
  • Analyzing the type of waves produced by earthquakes.
  • Comparing S and P waves recorded on a seismogram.
VOCABULARY:
  • earthquake "event"
  • lithosphere
  • primary wave
  • secondary wave
  • seismic waves
  • seismogram
MATERIALS:

Students learn about P and S waves.


Ground rupture caused by an earthquake

BACKGROUND:

Earthquakes and volcanoes are evidence for plate tectonics. Earthquakes are caused when energy is released as the lithosphere (crust and upper mantle) of the Earth moves. Energy is emitted in the form of waves. There are different types of waves, some move faster, slower, sideways, or up and down. A seismograph records these waves on a seismogram. When an earthquake is recorded it is called an earthquake "event."

 There are two types of waves you will discuss with the students, P and S waves. P waves or primary waves, are the first waves that the seismogram records. The P wave is the "fast" wave and can be called a push-pull wave, because it moves by contracting and expanding along a horizontal path. A P-wave travels through a material as a compressional force. For example, when you speak, your voice compresses a volume of air. One of the properties of air (and just about any other material) is that it resists being compressed into a smaller volume. When your voice compresses the air, it resists by pushing against neighboring volumes of air. These volumes then resist compression, and they push back against their neighbors. This generates a wave of compression that travels through all the volumes of air between your mouth and the person hearing you. 

The second major type of seismic wave is called an S-wave. S-waves are shear waves. S-waves are slower than P-waves. The particle motion in shear waves is perpendicular to the direction of the wave.

PROCEDURE:
  1. Review the divisions of inside the Earth. Earthquakes occur in the upper part of the crust and mantle. Earthquakes release energy in the form of waves.
      
  2. Demonstrate P- and S-wave motion to the class. P-waves can be demonstrated with a slinky. Pull the slinky apart and then pull in about 6 coils. Let them go. The wave will oscillate through the slinky, alternately compressing and expanding the coils.
      
  3. The S wave can be shown by using a rope attached to a wall. Hold onto the rope and move your wrist up and down. This whipping motion will generate S-waves. The motion will be up and down as the energy goes through the rope. Although you can demonstrate both types of wave with a slinky, we have found that students can distinguish the two types of waves more readily if you use different materials. If you cannot attach a rope to your classroom walls, try this demonstration with two people.
      
  4. Draw a P and S wave on the board as illustrated below. Make sure the students understand how to identify them. In addition, explain that the greater the height of the lines on the seismogram, the larger the earthquake. This holds true unless a seismograph is located very close to the epicenter of the earthquake. This causes the wave height to be exaggerated.

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