Whose Fault Is It Anyway?
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This page first made public: Oct 28, 2005
This material was originally created for Starting Point:Introductory Geology
and is replicated here as part of the SERC Pedagogic Service.
- An intuitive understanding of the difference between S and P waves and how they allow seismologists to determine the hypocenter of an earthquake
- An opportunity and motivation to make deductions based on quantitative data
- No chance to sleep through your lecture
Context for Use
Description and Teaching Materials
Have the students get up from their chairs and stand in lines of 10-25 people
- Students at the end of the line are seismometers. Each of them should take a time piece (either a watch with a second hand, or better yet a stopwatch) in their free hand.
- All but one of the students between them are mediums. They represent sections of Earth's interior that carry seismic waves.
- One student, secretly determine, is the earthquake hypocenter. How to choose a hypocenter without anyone else knowing: have all the students close their eyes, and tap one of them on the head. Be sure to announce what that means.
Have all of the students join hands and look up and straight ahead, not at their hands or at anyone next to them
- When you call out "Earthquake", the earthquake hypocenter sends a P-wave in both directions. The P-wave is a gentle squeeze of the hands of the people on either side of the hypocenter.
- As they feel their hand squeezed, each medium immediately but gently squeezes the hand of the next person in the line.
- When the P-wave reaches the seismocenter, they should note the time, but don't stop the stopwatch.
- Two seconds after sending the P-wave, the hypocenter sends out an S-wave to both sides by gently and briefly shaking the hands of the people on either side up and down.
- Upon receiving this signal, each medium waits 2 seconds before sending it down the line
- Now, the seismometers again note the time.
Practice wave propagation before beginning the game.
- Try sending S and P waves individually from a known hypocenter first.
- Do a trial run of with both from a known hypocenter, have the seismometers announce how long it took the P and S waves to reach them, and then ask students to estimate what the lag is between them per person between each seismometer and the epicenter.
- Ask the students how they could use this lag to determine who the epicenter was if they didn't know who it was at the start.
Now, try again with a mystery hypocenter.
- Write the names of the participants on the board in the order in which they stood in line Have the seismometers post the times for the S and P waves on the board next to their own names.
- Give the students five minutes to work out who the hypocenter is, showing their work (only really important for the hypocenter)
- Check through the answers. Have the real hypocenter stand up and take a bow. If most of the students didn't figure it out correctly, form the line up again with new seismometers and choose a new hypocenter to repeat the game.
Teaching Notes and Tips
References and Resources
This exercise is based on Muller, 1993 .