Whose Fault Is It Anyway?

Eric Muller

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This page first made public: Oct 28, 2005

This material is replicated on a number of sites as part of the SERC Pedagogic Service Project

Summary

This game has students simulate the propagation of S and P waves after an earthquake and to use the lag between these to determine where in the simulation the earthquake occurred. Students stand in lines holding hands with those at the ends working as seismometers to record how long it takes P- and S-waves to reach them. The students between them propagate the waves as handshakes, allowing an extra two-second lag for S-waves. The student who originated the waves, representing the earthquake is known only to him- or herself and the instructor until the others can figure out who it is based on the lag between the S- and P-waves, which increases with the number of people between the earthquake and the seismometer.

Learning Goals

This exercise give students:

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

This exercise could be used as a half-hour break in a lecture. It's well suited for an introductory geology or environmental hazards class.

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

The author suggests an alternative exercise for a really large class. Form a long line with the whole class, with the hypocenter (known) at one end and a seismometer every five people (use a regular watch kept in view as the seismometers won't have a hand free). Each seismometer notes and puts on the board (or adds to a projected spreadsheet) how long it took to receive the S and P waves and how many people were between him/her and the hypocenter. Have each student graph propagation times for S and P waves by the number of people.

Assessment

None formal, but if you want to do some scoring, give students or student groups who can figure out who the earthquake is points, perhaps extra points for getting the right answer first second, third, etc. Run the game a few times so points can add up, especially if not all groups get the right answer the first time.