When an earthquake occurs, energy is released in the form of seismic waves. This activity uses slinkys, toothpicks and marshmallows to explore two types of seismic waves (p-waves and s-waves) and their effect on the vibration of structures.
Developed as part of a teacher workshop organized by Catherine French by:
Leslie Bucar, 7-12 Science Teacher, Fond du Lac Ojibwe School, B.S. Biology, B.A.S. Teaching Biology, B.A.S. equiv Teaching Chemistry
Dan Johnson, 7-12 Math Teacher, Fond du Lac Ojibwe School, B.A.S. Teaching Math
Topics: structural dynamics
When rocks move past each other along faults, it creates stress at points where the rocks' irregular surfaces catch each other. The stress continues to build up until the energy is released in the form of seismic waves. The point where this energy release first occurs is the focus (plural, foci) of the earthquake. The foci of most earthquakes are within 65 km of Earth's surface. Seismic waves are produced and travel outward from the earthquake focus.
When earthquakes occur, three different types of seismic waves are produced. All the waves are produced at the same time, but they all behave differently as they go through the earth. When earthquake waves travel they also change speed, traveling faster through denser materials like rock than through looser materials like most other soils.
Primary waves (p-waves) move the fastest through the Earth. They move through rock and soil at speeds of about 6 km/sec (3.8 mi/sec). Primary waves cause rock particles to move back and forth in the same direction that the wave is traveling. Primary waves move through Earth the same way that a wave travels through a coiled spring, as if you squeeze one end of a coiled spring and then release it.
Secondary waves (s-waves) travel about half the speed of p-waves. S-waves travel only through solids at about 3 km/sec (1.9 mi/sec). They have lower speed because they have higher amplitudes-the wave is higher- making them more dangerous than primary waves. S-waves move through Earth by causing particles in rocks to move at right angles to the direction of wave travel.
Surface waves are produced when earthquake energy reaches the surface of the Earth. Surface waves travel outward from the epicenter. The earthquake epicenter is the point on Earth's surface directly above the earthquake focus. Surface waves are confined to the Earth's surface and outer layers because, like waves on the ocean, they need a free surface to ripple in order to exist. Their speed is slightly less than S- waves. The amplitudes, or heights, of surface waves are greater than those of primary and secondary waves. Surface waves cause the most damage during an earthquake.
Earthquake Engineering Component
Earthquake engineers design structures to withstand the forces and displacements that are transferred to the structure as a result of the ground motion. Researchers use shake tables to test structures and investigate how they will perform in future earthquakes.
This 1 minute video shows a full-scale wind turbine being tested on the large outdoor shake table at the NEES lab at UC San Diego http://www.youtube.com/watch?v=o_xLYnWAA1M
This newsclip shows researchers at the NEES at Buffalo lab testing a 70 ton bridge on a sets of shake tables http://www.youtube.com/watch?v=VnqY-h947Nc&feature=related
This 40 second video shows what happens to a child's bedroom during an earthquake. The test was of a full-scale two story wood building tested on a shake table as part of the NEESwood project http://www.youtube.com/watch?v=P7gde8KOLJU
Learning Objectives and Standards
Links to the National Science Standards and to individual State Science Standards are available by using this link:
Compare and contrast different types of seismic waves.
Observe the effect of different waves on structures.
- 5 slinkys
- 4 eyehooks
- 1 piece of wood 8' x 8'
- Two people hold the ends of a slinky about 3-4 meters apart.
- One person should cup his or her hand over the end (the last 3-4 coils) of the slinky and, when the slinky is nearly at rest, hit that hand with the fist of the other hand.
- Because the other person is holding the slinky firmly, the P wave will reflect at that end and travel back along the slinky.
- Demonstrating the s-wave is performed in a similar fashion except that the person who creates the s-wave does so by moving his or her hand quickly up and then down.
Effects on Structures:
- Using marshmallows and toothpicks, build a tower structure that will fit on your board.
- Screw 1 eyehook into each side of the wooden board.
- Attach one slinky to one side of the board.
- Place your structure on the board.
- Slowly gather a few of the end coils of the slinky into your hand.
- When a few coils have been compressed, release them suddenly (holding on to the end coil of the slinky). What happens?
- Next, stretch out the slinky again.
- Hold the end of the stretched slinky should use your other hand to grab one of the coils about 10-12 coils away from the end of the slinky.
- Slowly pull on this coil in a direction perpendicular to the direction defined by the stretched slinky.
- After the coil has been displaced about 10 cm or so, release it suddenly. What happens?
Watching the Waves
- Remove the structure from the board.
- Attach a slinky to each eyehook.
- Four people stretch the Slinkys out to between 3 and 4 meters.
- One person hit the block with a closed fist. What happens to the Slinkys?
- Now, move the block quickly up and down. What happens to the Slinkys?
- Now, move the block from side to side. What happens to the Slinkys?
- Attach one more Slinky to the end of one of the slinkys, making a double slinky. Stretch it out to between 6 and 8 meters.
- One person hold another Slinky and collapse about half of the coils and hold them in your hands, forming a half slinky, stretched out about 1/2 - 2m.
- One person move the block up and down, while the four people holding the slinkys say "Now" when the wave reaches their end of the slinky. What do you notice?
Links and Resources
Feather, R., Snyder, S., and Zike, D. 2002. Earth Science. Glencoe McGraw-Hill, New York, NY. Maryland.
Levy, M. and Salvadori, M. 1995. Why the Earth Quakes. W.W. Norton and Company, New York.
Press, F. and Siever, R. 1994. Understanding Earth. W.H. Freeman and Company, New York.
Collecting and Analyzing Data:
- What happened to your structure when a p-wave hit it?
- What happened to your structure when an s-wave hit it?
- What happened to the four slinkys when the board was hit with a fist?
- What happened to the four slinkys when the board was moved up and down?
- What happened to the four slinkys when the board was moved side to side?
- When the slinkys were different lengths, did everyone say "Now" at the same time?