Draft (09/23/15)
Human Waves Demonstrate How Seismic Waves Travel
Time: 10 minutes plus class worksheet
Target Grade Level: Any level
Objective Students will:
• Model how earthquake waves travel at different
speeds
• Construct a distance and time graph to represent the
P and S waves
Materials: Stop watch & group of students
Background for Seismic Waves:
Screen grab from the classroom video demo of this activity:
http://www.iris.edu/hq/inclass/video/human_wave_modeling_
seismic_waves_in_the_classroom
• Short animation (& related resource tabs) that
describes seismic waves, please visit:
http://www.iris.edu/hq/inclass/animation/1component_
seismogram_building_responds_to_p_s_surface_waves
NGSS Science Standards
Introduction
Remember the “stadium wave,” when one person stands
and raises his hands in the air and the motion is translated
completely around the arena? This simple demonstration
uses a similar principal by sending seismic waves through
a line of people to illustrate the difference between P waves
and S waves. Webster’s Dictionary defines a wave as “a
disturbance or variation that transfers energy progressively
from point to point in a medium.” Suppose the medium were
the audience in a football arena. .
Procedure
To simulate seismic wave propagation in solids and liquids
have a line of 10-30 people stand shoulder to shoulder, side
by side, with their feet about shoulder width apart. Instruct
the group to not be too rigid or too limp when pushed from
the side. They should give with the force that they will feel
from the person next to them, but not fall over, and then
return to their upright position. In other words, they should
be “elastic.” Have a “spotter” (or wall) at the end of the
line in case the last person begins to fall. (It is important
to stress these instructions to the participants so that the
demonstration will work effectively.)
To represent wave propagation in a solid, gently push the
first person in the line and the compressive “deformation”
will propagate down the line of people approximating a
P wave (Figure 1B). Although each person was briefly
subjected to a deformation or disturbance, the individuals
did not move from their original locations. The motion
• From Molecules to Organisms—Structures and
Processes: MS-LS1-8
• Energy: MS-PS3-5
• Waves and Their Applications in Technologies for
Information Transfer: MS-PS4-1, HS-PS4-1, MS-PS4-2
of each person is in the direction of propagation so that
the people moved closer together (compression), and
then apart (dilation) to return to their original positions.
Also, the propagation down the line takes time; there is a
velocity for the wave propagation.
To simulate the S wave in a solid, have each person put
their arms over the shoulders of the person next to them
(“chorus line style”; the “molecules” or “particles” of the
solid are tightly bonded). Make the first person at the end
of the line bend forward at the waist and then stand up
straight. The transverse or shear motion will propagate
down the line of people (Figure 2B). Again, the wave takes
some time to propagate and each person ends up in the
same location where they started even though a wave has
passed. Also, note that the shear motion of each particle
is perpendicular to the direction of propagation. Which
one moves faster? The teacher or one of the observers can
time the P and S wave propagation in the human wave
using a stopwatch. Because the shear wave motion is more
complicated in the human wave, the S wave will have a
Modified from Larry Braile’s “Seismic Waves and the Slinky: A Guide For Teachers.”
NGSS standards are from the Cascadia Earthscope Earthquake and Tsunami Education Program (http://ceetep.oregonstate.edu/) e-binder.
http://www.iris.edu/hq/inclass/search
a
slower velocity (greater travel time from source to the end of
the line), similar to seismic waves in a solid.
To represent wave propagation in a liquid, have the
students stand shoulder-to-shoulder, without their arms
around each other. Bump the P wave again and time the
propagation down the line. It should be the same as a solid.
For an S wave, keep the arms at sides, but now have the first
person a bend forward at the waist—a transverse or shear
disturbance (Figure 3). However, because the “molecules” of
the liquid are more loosely bound, the shearing motion will
not propagate through the liquid (along the line of people).
The disturbance does not propagate to the next person, as it
did with arms over shoulders in the solid because the liquid
1A
does not support the shearing motion. (Compare pressing
your hand down on the surface of a solid such as a table top
and on the surface of water and moving your hand parallel to
the surface. There will be considerable resistance to moving
your hand on the solid.) One could even push the entire table
horizontally by this shearing motion. However, there will be
virtually no resistance to moving your hand along the surface
of the water.) Only the first person in the line – the one that is
bent over at the waist – should move because the people are
not connected. If the next person bends, “sympathetically,”
not because of the wave propagating, ask that person if he
or she felt, rather than just saw, the wave disturbance, then
repeat the demonstration for S waves in a liquid.
1B
Figures 1A & B—LEFT: P wave through a solid or liquid.
RIGHT: When a P wave bumps the first person (molecule), the bumping
down the line happens quickly. P waves travel easily within a liquid or solid.
From Video of an P-wave demonstration taken in Roger Groom’s Mount Tabor Middle School Earthl-science class. The travel times are graphed.
2A
2B
Figures 2A & B — LEFT: S wave through a solid. RIGHT: The line of
people (molecules) forms an elastic solid by linking arms or placing
arms over shoulders to show that when the S wave “shears” the first
person pulls the next into a bend. Note that the motion of each person
is perpendicular to the direction that the wave travels. As each person
bends and straightens, they pull their neighbor into the same motion. This is a slower effect than the P wave.
Figure 3—An S wave can’t travel
through a liquid. The line of people
(molecules) forms a liquid by
unlinking their arms. Activity uses the
S wave concept of “shearing” into a
bend, but because arms aren’t linked
into an elastic solid, the rest of the
line doesn’t bend.
b
Extension Idea
have students figure out how to model
materials with different densities or
cohesiveness such as sand vs. solid
rock. Grades 9-12 might evaluate
wavelength, amplitude and frequency.
In the classroom
Review and introduce concepts for the experiment.
What is an earthquake? Define P and S waves.
What is recorded at a seismic station during an earthquake?
What do we measure? Why?
What information do we want?
What other events could generate seismic waves, like an
earthquake? (See Types of Earthquakes.)
Activity:
Explain to students that the following activity will give
them a chance to see how P and S waves travel through
liquids and solids and establish their speeds by simulating
them in the Human Wave.
• Line the students up feet apart with shoulders touching.
Measure the length of the line in feet or meters to
record on the x axis of Graph 1 (next page).
• Designate one student to record the times in Table 1.
All students will plot the data on Graph 1 and do any
calculations (simple subtraction, division) afterward.
• When ready, the instructor will both nudge the first
student with a P (compressive) wave and click the
stopwatch. When the last student is bumped, stop the
watch. Record the time. Repeat each example according
to the notes in the procedure three times.
• The P-wave times should be roughly similar. More
students will increase accuracy. Repeat with the S-wave
method.
•Ask class which one is slower? (S-waves will be about
50% slower.)
• As a class, plot the data for the trials on Graph 1. Use a
different color for P and S wave data. Fit a straight line
between zero and each point.
Questions and simple math:
1) What does any point on the graph represent?
2) Do we see the expected behavior of waves represented?
(i.e. Are P waves faster than S waves?).
Demonstrate graphically how we know (i.e., compare
two points of a constant distance, and trace to their
times – and compare with time difference line).
3) Calculate the group speeds for P and S waves in feet/
second or meters/second by dividing the distance by
seconds. Now calculate how much slower the S wave is
by dividing S m/sec by P m/sec.
4) How do the P and S waves differ? (See “Speed” in the
Vocabulary P & S wave descriptions.)
Divide 3500 m/sec by 6000 m/sec. S waves trabout 58% the speed. of P waves. How does the Human Wave
compare?
Vocabulary
Earthquake—shaking or trembling of the earth that
accompanies rock movements extending anywhere
from the crust to 680 km below the Earth’s surface.
It is the release of stored elastic energy caused by
sudden fracture and movement of rocks inside the
Earth. Part of the energy released produces seismic
waves, like P, S, and surface waves, that travel
outward in all directions from the point of initial
rupture. These waves shake the ground as they pass
Seismic Wave— an elastic wave generated by an
impulse such as an earthquake or an explosion.
P Wave—the primary body wave; the first seismic wave
detected by seismographs; able to move through
both liquid and solid rock..Also called compressional
or longitudinal waves, they compress and expand
(oscillate) the ground back and forth in the direction
of travel, like sound waves that move back and forth
as the waves travel from source to receiver. P wave is
the fastest wave.
Speed: 6000-8000 m/s (19,685-26,246 ft/sec)
S Wave—secondary body waves that oscillate the ground
perpendicular to the direction of wave travel. They
travel about 1.7 times slower than P waves. Because
liquids will not sustain shear stresses, S waves will
not travel through liquids like water, molten rock, or
the Earth’s outer core. S waves produce vertical and
horizontal motion in the ground surface.
Speed: 3500-4500 m/s (11,483– 14,764 ft/sec)
Types of earthquakes:
A) Tectonic Earthquake: earthquake that occurs when
the earth’s crust breaks due to geological forces on
rocks and adjoining plates that cause physical and
chemical changes.
B) Volcanic Earthquakes: earthquakes that result
from tectonic forces which occur in conjunction
with volcanic activity.
C) Collapse Earthquakes: small earthquakes in
underground caverns and mines that are caused
by roof collapse.
D) Explosion Earthquakes: earthquakes which
are the result of the detonation of nuclear and
chemical devices
This page modified from a “Human Waves” Lesson Plan from
Michigan Technological University
c
Table 1: Travel time for the P and S waves through solids and liquids.
Graph 1: Travel Time Graph for Times vs Distance
24
22
20
18
Distance, in feet
16
14
12
10
8
6
4
2
0
0 1 2 3 4 5 6 7 8 9 10
Time, in seconds
d