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
Name _________________________
Physical Science 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
Name _________________________
Physical Science 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).