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Shadow Zone Activity
What Earthquake Waves Tell Us about the Earth’s Interior
By: Donald G. Korba
Birchmount Park Collegiate Institute
Scarborough, Ontario, Canada
Objective: To determine graphically the boundary between the mantle and outer core and the
boundary between the inner and outer core. To relate earthquake waves to the earth’s interior
boundaries.
Background: When an earthquake occurs, vibrations called seismic waves are produced.
These waves travel outward from an earthquake’s focus, its point of origin, in all directions.
Two kinds of seismic waves travel through the Earth: P waves and S waves. The third kind,
surface waves, result when P and S waves reach the surface and travel outward along the
surface from an earthquake’s epicenter, the point on the surface directly above its focus. By
studying P and S waves, scientists have been able to develop a model of the Earth’s interior.
P waves can travel through solids, liquids, and gases, and they move faster through dense
material than through less dense material. Studies of earthquake waves have shown that P
wave velocity increases or decreases at certain depths inside the Earth. A change in velocity
suggests the presence of a boundary between materials of different densities.
The outer layer of the Earth is known as the crust. As P waves travel from an epicenter
through the crust, they suddenly increase in speed at a depth of about 35 km. The change in
speed indicates they are entering a material of higher density than the crust. This layer is
called the mantle. At a depth of about 2,900 km, P waves sharply decrease in speed,
indicating they are entering the core, a material of lower density than the mantle. At about
5,100 km, P waves speed up, signifying the presence of a denser material. Scientists
determined that the core can be divided into an outer core and an inner core, with the inner
core having a denser material.
S waves can travel through solid material only, not through liquids. Like that of P waves, their
speed changes according to the density of the material through which they are traveling,
speeding up in dense material and slowing down in less dense material.
Scientists have found that S waves cannot be detected on the side of the Earth opposite an
earthquake; therefore, S waves were not able to travel through the Earth’s outer core. Since S
waves cannot travel through liquids, and P wave velocity suggested that the outer core was
less dense than the inner core, scientists think that the outer core may be somewhat fluid and
the inner core solid.
Shadow zone: P and S waves are not detected at seismograph stations that are at an angular
distance greater than 103 degrees from the epicenter of an earthquake. P waves are detected
at an angular distance of about 143 degrees or more. These findings indicate a zone on the
Earth’s surface between 103 degrees and 143 degrees from an epicenter in which no P or S
waves can be detected. This region is called an earthquake’s shadow zone. Its existence can
Shadow Zone Activity
be explained by the way seismic waves bend as they travel through the Earth and by the
inability of S waves to travel through liquids.
When seismic waves travel through material that changes density gradually, they change
direction gradually. When they travel from one material to another material of different density,
they change direction sharply. As P and S waves travel through the mantle, they may bend
slightly because of slightly different densities. When P waves enter the boundary between the
mantle and the core, the difference in densities causes them to bend sharply. They are
reflected away from the core and reach the surface at about 103 degrees from an epicenter, or
they are bent inward and travel through the core. When they leave the core, they are bent
sharply again and reach the surface at about 143 degrees from an epicenter. When S waves
reach the boundary between the mantle and the core, they are reflected back because they
cannot travel through the dense fluid in the outer core. They reach the surface at about 143
degrees from an epicenter. The result is the shadow zone between 103 degrees and 143
degrees from an epicenter where no P and S waves are detected.
Procedure:
1. Circles 1 and 2 on the worksheet represent cross sections of the
Earth.Each dot shows the focus of an earthquake. On circle 1, draw
and number (in any order) a dot on the surface of the Earth above
each focus to show the epicenter of each earthquake.
2. Using a protractor, with the vertex at the Earth’s center, mark off
an angle of 103 degrees on the circle starting 0 degrees at epicenter
1. Repeat on the other side of epicenter 1. Then draw a line joining
each 103 degree mark to epicenter 1 (Figure 1). You should draw
only the solid lines.
3. Repeat step 2 for each of the 15 other epicenters.
4. On circle 2, repeat steps 1, 2, and 3, marking an angle of 143 degrees instead of 103
degrees.
Shadow Zone Activity
Physical Science
Name _________________________
Period _____ Due Date ___________
Pre-Lab Questions: Answer the following questions using COMPLETE SENTENCES.
1. What is the purpose of this lab?
2. What are seismic waves?
3. What are the two types of seismic waves?
4. What can be determined by studying these seismic waves?
5. P waves can travel through what type(s) of materials?
6. What does a change in velocity of P waves suggest to scientists?
7. Draw a picture of the Earth according to what P waves tell us. (Include the crust, mantle
and two types of core.)
8. S waves can travel through what type(s) of materials?
9. What additional information does the travel of S waves tell us?
10. What is an earthquake’s shadow zone?
11. What occurs when P or S waves travel through different densities of material?
Shadow Zone Activity
Shadow Zone Activity
Physical Science
Name _________________________
Period _____ Due Date ___________
Post-Lab Questions: Answer the following questions using COMPLETE SENTENCES.
1. Describe your graph of S waves (circle 1).
2. On your graph of S waves (circle 1) you have outlined a boundary area. Circle which
boundary this best represents.
a. oceanic crust and upper mantle
b. continental crust and upper mantle
c. upper mantle and mantle
d. mantle and outer core
e. outer core and inner core
3. If the S waves stop at this boundary (from question 2), what do we know about the make-up
of the middle?
4. On your graph of P waves (circle 2) you have outlined a boundary area. Circle which
boundary this best represents.
a. oceanic crust and upper mantle
b. continental crust and upper mantle
c. upper mantle and mantle
d. mantle and outer core
e. outer core and inner core
5. What is the inner layer of the Earth – solid, liquid or gas? If the P waves don’t go straight
through it, what happens to them? (Hint: Reread the last background paragraph).