SURFING ROCK WAVES

Summary:

Earthquakes happen all over the world. On the surface we do not always feel them. The movement we do feel can be described in two motions: P waves and S waves. Students will use slinkies to imitate and better understand the two types of waves.

Instructional Method: Experiment

Goal: Explain wave motions associated with earthquakes.

Objectives: Students will be able to:

• Describe earthquake movements.
• List the names of the two waves.

Time:

Activity time: 30 minutes
Discussion: 15 minutes

Materials Needed:

• Large sheet of paper
• Slinky
• Crayons or marker

Vocabulary:

epicenter
hypocenter
P wave
seismic waves
seismograph
S wave

Background:

When movement occurs between two pieces of crust, called fault blocks, we feel an earthquake. To create a mini-earthquake, slide two blocks of rock against each other. Can you feel the roughness of the two rocks as they pass by each other? The friction between the two rocks is what creates the vibrations in an earthquake except on a much greater scale. These vibrations in nature are called seismic waves.

Most earthquakes are too small to be felt by people. Larger earthquakes are not only felt by people, but can severely damage structures. The motion from an earthquake travels through rock in the form of waves. There are two types of waves associated with earthquakes: P waves and S waves.

The fastest of the waves is a primary wave, or P wave. These waves feel like a push-and-pull movement on the crust as it is stretched and compressed. P waves are also called compressional waves. The slower of the two waves is called a secondary wave, or S wave. These waves travel in a side to side motion and produce more shaking than P waves. The side-to-side motion of S-waves causes the most damage to structures during an earthquake.

In large earthquakes it is possible to feel the difference between P and S waves. P waves feel like someone has run a semi-truck into your house and S waves feel more like you are riding waves at a lake or ocean. The following schematic shows how these waves travel through rock.

It is possible to simulate the motion of both P and S waves by using a Slinky. The push-pull motion of a P wave can be seen by stretching and compressing a slinky. The shaking motion of an S wave can be seen by moving a stretched Slinky side to side.

Instructional Procedures:

1. Divide class into groups of no less than three and provide each group with a slinky.
2. Cover tables or desks with a piece of butcher paper long enough to span the distance of a stretched slinky.
3. Have two students hold one end of the stretched slinky.
4. Place a student in the middle with a marker or crayon.
5. Have one student at the end of the slinky create waves, first simulating P-waves (push-pull) and then S waves (side to side). S waves should move up and down. Drawing them in a side to side is the same motion if the paper is held on end.
6. Instruct students to observe the slinky's reaction. Then have the student in the middle mark on paper the path the slinky took as the different waves passed through it. The mark should be in the form of squiggles that compress and stretch (close together and farther apart) or squiggles that move side to side.
7. Have students change positions so each one can "make waves".

Discussion:

Have you ever experienced an earthquake? What did it feel like to you? Could you distinguish between the P waves and the S waves? Which do you think is more destructive to houses, P waves or S waves? Why? What do you think you should do to protect yourself during an earthquake?

Variation:

Instead of using slinkies you can create P and S waves in a box of sand. Fill a rectangular box with 2-3 inches of sand. Be sure the sand is very dry. On the sand place small sticks or Lego houses. Shake the box or hit the side of it and observe what happens to the house and sand. It may help to place toothpick markers throughout the sand standing on end. Watch what happens to the toothpicks when the box is shaken. Does this work? Can you distinguish between P and S waves? How? Do you have to shake the box differently to create both types of waves?

Included National Parks and other sites:

Aniakchak National Monument and Preserve
Bryce Canyon National Park
Denali National Park
Glacier National Park
Great Basin National Park
Hawai`i Volcanoes National Park
North Cascades National Park
Point Reyes National Seashore
USGS Earthquakes Hazard Program

Photos:

Normal fault near Bryce Canyon
San Andreas Fault
Seismograph
Thrust Fault

Utah Science Core:

5th Grade Standard 2 Objective 1,2,3

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