Human Waves
Objective:
- Students experience a simulation of how P and S waves are used by earth
scientists to determine the location of seismic events.
- Students will be able to define P and S waves and describe the differences in
their behavior.
- Students will be able to describe how P and S waves are used to locate seismic
epicenters using basic triangulation methods.
Time Duration:
- 50 min. class
Level / Subject:
- Grade 8 / Earth Science
- Grade 9 / Physical Science
Curriculum Standard:
- E3.2B: Explain how scientists infer that the Earth has interior layers with
discernable properties using patterns of primary (P) and secondary (S) seismic
wave arrivals.
Materials Needed:
Meter Stick
Masking Tape / Marker (flags or anything that can be used to mark on the ground)
Stopwatches (3)
Large compass (can be cheaply constructed with a measurement marked string
attached to a pencil)
Poster sized pieces of paper (Blackboard and Chalk could work too)
Calculator (at least 1)
Pencils (Colored Pencils or Markers if available)
Area to work in – approximately 6m x 6m
Vocabulary:
Seismic Waves
Epicenter
P-wave
S-wave
Triangulation
Lesson Preparation:
- Write the following on poster paper for introduction/review and as a reference for the
activity.
Earthquakes
Seismic Waves
P – Waves
Primary Waves
Arrive First
Compression Waves – Liquid and Solid
S – Waves
Secondary Waves
Arrive Second
Shear Waves - Solid
- Draw the following tables and graphs on large poster paper for use in the
demonstration:
Table 1: Travel Time for Walk and Slow Walk Times
Table 1
Distance
(m)
0m
2m
4m
6m
Slow Walk
Time
0
Walk
Time
0
Slow Walk - Walk
Time
0
Table 2: Earthquake Data for 3 Stations
Table 2
Station
1
2
3
Slow Walk - Walk
Time
Distance to
Epicenter
Graph 1: Travel Time Graph for Walk/Slow Walk Times v Distance
(Corresponding to Table 1 Data)
Graph 1
Time (s)
0m
Distance (m)
6m
Graph 2: Graph of Station Locations – To be used with Table 2 Data
Graph 2
6m
Distance
(m)
Distance (m)
6m
Introduction / Description:
Review and introduce concepts for the experiment, using the prepared poster as a
reference (and keep the poster up for the rest of the class).
Anatomy of an Earthquake:
What is an earthquake? The parts?
What do we measure?
Why? What information do we want?
What other events could generate seismic waves, like an earthquake?
Define the 2 kinds of seismic waves:
P waves
S waves
Hypothesis (can write on the blackboard) what would happen/be recorded at a seismic
station if an earthquake happened.
Hypothesis how we could get the information (that we mentioned above) from the
recorded data.
Activity:
Explain to students that the following activity will give them a chance to see how P and S
waves are actually used by earth scientists.
Demonstration: S-P Wave Triangulation Simulation
Description: Simulation of how S and P waves are used to locate the epicenter of an
earthquake through the use of triangulation.
- Students will model how earthquake waves travel at different speeds
- Students will construct and utilize a distance and time graph to represent the P
and S waves
- Students will use the constructed graph to locate the epicenter of a simulated
earthquake
Part 1: Travel Times of P and S waves
-
Explain that the first part of the demonstration is to establish the different
speeds that P and S waves travel at by simulating P-waves with walking, and
S waves with slow walking.
-
Mark a 6m long straight line with meter stick. Put marks at 0m, 2m, 4m, and
6m. Have 3 students with stop watches stand at 2m, 4m, and 6m.
-
Put up Table 1 on the wall. Designate one student to write on the table, and
one to do calculations (simple subtraction, division) with a calculator. Two
students with calculators could also be used to check each other, or to speed
the calculations.
-
Pick 3 students to be S waves (slow walk speed). Establish a cadence speed –
for example, by walking heel – toe. Have each student walk the 6m line, with
the timer students recording how long it took to pass each mark. Have the
recorder put the data in Table 1.
-
Make sure times are roughly similar. More students could be used to include
more people, and increase accuracy.
-
Pick 3 more students to be P waves (walk speed). Do the same as with S
waves. Establish a cadence (normal walking pace), have each student walk the
6m line with the times recorded in Table 1.
-
The calculator student will subtract Slow Walk – Walk times for 2m, 4m, and
6m, and the recorder will record the time differences in Table 1. Then
calculate and record average times, and time difference for each interval for
both walk and slow walk.
-
As a class, help a (different student) recorder plot the data for the trials on
Graph 1. Distance on the x-axis (0, 2m, 4m, 6m). Three (either color or
symbol coded) lines corresponding to average time for walk (P waves), for
slow walk (S waves), and time difference. Fit a straight line.
Questions for the class at this time:
1) What does any point on the graph (point to one) represent?
2) Do we see the expected behavior of waves represented (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 speed for P and S waves in m/s.
4) Compare with real speeds of P and S waves in earth’s crust and upper mantle.
a. P: 6000-8000 m/s
b. S: 3500-4500 m/s
Part 2: Locating an Earthquake
-
Mark a 6m x 6m square (does not need to be exact), and place seismic stations
(a student with a stopwatch and a student with a paper and pencil). We’re
trying to visualize triangulation, so 3 stations may be best, but more can be
included depending on class size. Also, stations (for calculation/visual
convenience) should be placed at 3 of the corners.
-
Have a student pick a place in the grid to represent the earthquakes epicenter –
mark with tape. Measure its position on the grid, but don’t tell the students
yet!
-
Show the student Graph 2 and Table 2. Mark where the seismic stations are on
Graph 2, and explain the relationship between the floor and the graph.
-
Pick a student recorder for Table 2. Explain the roll of the recorders at each
station (write down the arrival time of the different waves, as reported by the
stopwatch student at that station). The data from each station is then reported
to Table 2 recorder.
-
Pick 6 students to represent waves from the earthquake center. 3 students will
be P waves and walk like before, and 3 will be S waves and slow walk as
before. Have the 6 students stand at the epicenter. Explain that 2 students, 1 P
and 1 S wave, will walk toward each station at the appropriate speed.
-
Do a “dry run” without recording data – say “go” and have the 6 students
move towards the stations. Students should be able to see how P waves arrive
first, and then the S waves. The students should also be able to observe how
waves move out in a widening circle from the epicenter.
o Students (of the same wave type) could even start holding hands, so
that as they move out, they would be forced to stretch their arms and
eventually let go – simulating the expanding wave front.
-
Then do a real trail. The stopwatch student measure the time BETWEEN P
and S wave arrival times. Hit “start” when P arrives, and “stop” when S
arrives. The student recorder for each station will write the time down.
Question: Thinking about how earthquakes happen in the real world, how does that
explain why we record the difference in time, rather than when the waves started to
move?
-
Repeat trial; make sure everything got recorded correctly.
-
Return to classroom/desks. Table 2 and Graph 2 should be at the front of the
classroom.
-
As a class, utilize the Graph 1 and Table 1 “calibration” to use the time
recorded in table 2 to calculate the Distance from Epicenter to each Station,
and record in Table 2.
Question: What can we tell about the epicenter of the earthquake based on our data?
-
Explain how with only one station, we only know the distance from the station
to the earthquake, but no direction. Explain and show on the graph how we
can think of this as having a “circle” of possible epicenters, equidistance from
the station.
-
Utilize the compass to draw the appropriate circles around each station (the
radius of which is the calculated epicenter – to – station distance).
-
Ask the class to point out where the epicenter must be (the junction between
all three circles).
-
Now, find where the actual epicenter in was (measured on the floor) and
compare. It should be close, but not perfect.
Conclusion:
Discuss the similarities/differences of our simulation with the real world:
How big was our error in locating the epicenter? What factors may have
contributed to it? How could we improve the experiment? What factors in the real world
would contribute to error – how could they be reduced?
How are using people walking/slow walking similar to the behavior of P and S
waves, how are they different?
We calibrated by walking and timing – how would scientists determine the speeds
of real S and P waves?
Scientists are also interested in the magnitude of an earthquake. Given what we
know about the relationship between time and distance, what is the likely relationship
between registered magnitude and distance?
Actual earthquakes originate below the surface, at the foci. What would the effect
of non-zero depth have on our simulation and the travel times?
Seismic waves are also used to determine the interior of the earth, and
earthquakes over vast distances – how would having to account for the different layers
(solid, liquid) change the calibration (i.e. affect the arrival times of P and S waves)? How
would having to include the spherical nature of the earth change the experiment?
Assessment / Evaluation:
Ask the students/students in groups to come up with their own explanation to the
following in order to demonstrate an understanding of how triangulation works:
Would this method work if an epicenter occurred outside the triangle created by
the stations? What might that look like, compared to our graph 2? (Invite them to draw on
the board).
Why did we use 3 stations? What would happen if we only had 2 or 1? What
might a graph like that look like?
What are some other real-world systems that use triangulation?
Resources:
Braile, L.W. and S. J. Braile. “Walk - Run Activity --An S and P Wave Travel Time
Simulation (“S minus P” Earthquake Location Method)” June, 2000.
http://web.ics.purdue.edu/~braile/edumod/walkrun/walkrun.htm. Accessed Oct.
22, 2010