Adapted with permission from L.W. Braile and S.J. Braile and the Incorporated Research Institutions for Seismology (IRIS).
Push away from those paper seismograms and get outside to make your own earthquake waves! You're going to learn about earthquake location kinesthetically. In the activity below, you will model how earthquake waves travel through the Earth at different speeds. You also will construct and utilize a graph to characterize the relationship between distance and time of travel of seismic waves (a travel-time curve). Finally, you'll use the constructed travel-time curves to locate the epicenter of a simulated earthquake by triangulation.
While seismologists conduct much of their research indoors at a computer terminal, going outside to collect data is a vital aspect of their work, as in the activity below. Seismologists travel to some of the most remote places on Earth to install seismographs far away from the vibrations of human civilization. Here they can record the high-quality data necessary for them to conduct their research, including locating earthquakes.
- 3 stopwatches
- Paper and pens
- Outside, simulate P and S waves by jogging (to model the faster "primary waves") and walking (to model the slower "secondary waves"). Practice jogging and walking at a constant velocity to ensure consistency.
- To determine the velocity of student P and S waves, station timers (students using stopwatches) at distances 10, 20 and 30 meters from the source. Time how long it takes these students, traveling a straight path, to arrive at timers. To improve the accuracy of the P and S wave velocity measurements, complete several trials. Write down your findings.
- Compute average travel times for the student P and S waves at various distances, and graph the data. Like the travel-time curve traditionally found in a standard earthquake location exercise, you can plot travel times for each wave versus distance. By plotting how long it takes seismic waves to travel various distances, you're modeling the way scientists create travel-time curves.
- Next, mark the corners of a 30-meter-square space as well as the "epicenter," the place within that square that will be the source of student P and S waves. Create "seismic stations" by having students with stopwatches standing, backs to the center, at three corners of the square.
- Assemble six student P and S waves at the epicenter marker, with S waves standing back-to-back and their associated P waves standing directly in front of each of them. Until the earthquake occurs, they represent stored potential energy in rocks.
- Representing an earthquake, have students jog and walk outward from the epicenter toward the seismic stations. Because student seismometers have their backs to the epicenter, they register P wave arrival by starting their stopwatches, and they register S waves by stopping the stopwatches. Record the time differences between P and S wave arrivals.
- Use this measurement along with the travel time curves they created earlier to calculate the distance from each station to the epicenter, then combine all three distances to locate the epicenter by triangulation.
- Once you've calculated the location of the playground earthquake, compare your result with the actual epicenter in the 30-meter-square space. Discuss possible reasons for any inaccuracies in determining actual earthquake location.
For the complete activity, please see http://web.ics.purdue.edu/~braile/edumod/walkrun/walkrun.htm.
For seismology education resources, see http://www.iris.edu/about/ENO/ and http://web.ics.purdue.edu/~braile/indexlinks/educ.htm.