Bang!: How is the Energy of Seismic P-Waves Transmitted Through the Earth?

Problem

How is the energy of seismic P-waves transmitted through the earth?

Materials

• Scissors
• Ruler
• String
• Masking tape
• 5 marbles

Procedure

1. Cut 5 pieces of string, each 12 inches (30 cm) long.
2. Tape one piece of string to each of the marbles.
3. Tape the free end of each string to the edge of a table. Adjust the position and length of the strings so that the marbles are the same height and touching each other.
4. Pull one of the end marbles to the side, and then release it.
5. Observe any movement of the marbles.

Results

The marble swings down, striking the closest marble in its path, and stops moving. The marble on the opposite end swings outward, and strikes its closest neighboring marble when it swings back into its original position. The cycle of the end marbles swinging back and forth continues for a few seconds.

Why?

Raising the end marble gives it energy, which is transferred to the marble it strikes. This energy is passed from one marble to the next, as each marble pushes against the next. The end marble is pushed away from the group. The transfer of energy from one marble to the next simulates the transfer of energy between particles of the earth during a seismic P-wave (primary pressure wave of an earthquake).

The first sign that an earthquake has occurred is the hammerlike blow felt and heard as a P-wave exits through the earth's surface. Before that, P-waves move through liquids and solids by compressing (pressing together) the particles of earth directly in front of them. The compressed particles quickly spring back to their original position as soon as the energy moves on. The crust of the earth moves upward as it is hit with the energy of the P-wave, and then settles back into place when the energy moves on.

Let's Explore

1. Would it affect the transmission of energy if the marbles were not in line? Stick pieces of clay under the strings on the side of the table in order to change the position of the marbles. Be sure that the marbles touch at some point, but that each marble is at a different height.
2. Would changing the distance between particles affect the transfer of energy? Repeat the original experiment, moving the pieces of tape supporting the marbles farther apart so that there is a slight separation between each marble.

Show Time!

1. Use a Slinky to demonstrate the particle movement of a seismic P-wave as it moves from the focus (starting point) of an earthquake to the epicenter (the point on the earth's surface directly above the focus). The Slinky can be used as part of a project presentation by slightly stretching it vertically and attaching its top and bottom loops to the display. Compress four to five loops together at one end, then release.
2. Seismic waves move more slowly through sand because the energy of the waves moves forward in different directions as the sand particles move outward in all directions. To demonstrate this, cover the end of a paper towel tube with a paper towel. Secure the paper towel to the tube with a rubber band. Fill the tube with uncooked rice or bird seed. Use your fingers to press down on the rice as you try to push the rice down and out through the paper towel.

Check it Out!

P-waves are the swiftest seismic waves. Find out the speed of P-waves as they travel through the different layers of the earth's interior: crust, mantle, and core. Display a diagram of a cross section of the earth, with speeds of P-waves indicated for each layer.

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