Explore 2: Locating an Epicenter Using Triangulation (“Sizing Up
Earthquakes”)
Target Concepts
• Seismic waves called “P” waves, “S” waves, “Love” waves and “Rayleigh” waves travel differently
than each other. They travel different paths, move the ground differently from one another, and move
at different speeds. This has consequences for how much damage they do. It also allows scientists
to pinpoint an earthquake’s epicenter.
• The epicenter of an earthquake is the point on Earth’s surface directly above the place where the
earthquake originated; that is, directly above the place where the stored energy in the rock was first
released as wave energy.
Assessment Task
Students will determine the location of the epicenter of an earthquake using triangulation.
Vocabulary
seismic waves
Materials
Slinky (at least one)
Copies of student reading, “Earthquake Wave Background”
Background
The Good Friday earthquake of 1964 was the strongest earthquake on record in North America, and one
of the strongest in the world. Its effects in Alaska are well-known: the trees whiplashing back and forth
to the ground, the mountains visibly rolling in the distance, the tsunamis and the landslides. One
hundred fourteen people were killed.
Furthermore, the earthquake affected large areas outside of Alaska. It cracked ice on lakes and rivers
throughout an area of about 100,000 square miles. It caused the water levels in wells to fluctuate as far
away as Georgia, Florida, Puerto Rico and Denmark. People in Oregon and California, as well as people
in Alaska, were killed by its tsunami. It’s hard to believe, but the earthquake caused small wave activity in
lakes on every continent.
There were no seismographs located in southern Alaska when the Great Alaskan Earthquake of 1964 hit.
However, the different arrival times of the different types of waves at seismographs outside of Alaska
allowed scientists to calculate the epicenter of the earthquake. Your students will use a similar technique
in Part III of this exercise.
Procedure
1. Ask students to read the article, “Earthquake Wave Background.”
2. Demonstrate P and S waves with a slinky.
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3. Use a line of students to demonstrate how P and S waves behave differently in a solid than they do in
a liquid. Have about ten people stand at the front of the room, side by side. (This demonstration
was designed by L Braile of Purdue University and is used by permission.)
P-waves in a solid: to represent wave propagation in a solid, have each person put their arms
over the shoulders of the person next to them (the “molecules” of the solid are tightly bonded).
Push on the person at the end of the line and the deformation (leaning to the side and then
straightening up) will propagate down the line of people approximating a P-wave. Note that the
propagation down the line took some time (there is a velocity for the wave propagation) and that
although each person was briefly subjected to a deformation or disturbance, the individuals did
not move from their original location.
S-waves in a solid: For the S-wave, make the person at the end of the line bend forward
slightly at the waist and then stand up straight. The transverse or shear motion will propagate
down the line of people.
P-waves in a liquid: Next, to represent wave propagation in a liquid, have the students stand
shoulder to shoulder without their arms around each other. Push on the shoulder of the end
person and a P-wave will still propagate down the line.
S-waves in a liquid: Now, make the person at the end of the line bend forward at the waist: a
transverse or shear disturbance. This time, because the “molecules” of the liquid are more
loosely bound, the shearing motion will not propagate through the liquid (along the line of
people).
3. Do at least Part III of the activity, “Sizing Up Earthquakes.” This activity is used by permission of the
Federal Emergency Management Agency. (FEMA/AGU. “Seismic Sleuths—Earthquakes—A
Teacher’s Package on Earthquakes for Grades 7-12.” American Geophysical Union, Washington,
DC, 1994, 367 pp.)
Generalize
Discuss the ‘64 Alaskan earthquake.
Assess
Circulate among the students as they do their triangulations.
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