The Earth’s Interior
Name:
Activity 4: A Mantle and a Core
Lab Section:
Date:
In the Java applet, the graph on the left shows seismic velocity as a function of depth in
the Earth. Earth is divided into two shells, a mantle and a core. Blue lines show P-wave
velocities. A white line shows S-wave velocities, which are initially set to zero. Click and
drag in the graph to change velocity. Click with the right mouse button to change
simultaneously the surface and bottom velocities for each shell (Macintosh users: hold
down the apple key while clicking). Click Calculate Now to run your model.
1. Set the P-wave velocity equal to 10.5 km/s in both the mantle and core. Leave the Swave velocity set to zero. Run the model to see the paths taken by P waves. These
paths should be straight lines that radiate outward from the epicenter.
a. Does this model produce a shadow zone?
b. Do P waves change direction when they enter the core?
2. Set the P-wave velocity equal to 9 km/s in the mantle and 12 km/s in the core. Leave
the S-wave velocity set to zero. Run the model again.
a. Does this model produce a shadow zone?
b. Do P waves entering the core bend up toward the surface or down toward the center?
3. Set the P-wave velocity equal to 12 km/s in the mantle and 9 km/s in the core. Leave
the S-wave velocity set to zero. Run the model again.
a. Does this model produce a shadow zone?
b. Do P-wave entering the core bend up towards the surface or down toward the center?
4. Considering your results from these three models, does the existence of a shadow zone
for P waves require velocity differences between the core and mantle? If so, what
difference is required?
5. Direct S waves are blocked by the core. What does this observation imply about S-wave
velocities in the outer part of the core?
Copyright Houghton Mifflin Company. All Rights Reserved.
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The Earth’s Interior
Activity 4: A Mantle and a Core
6. As a final seismology exercise, create a velocity model that matches the observed travel
times for P and S waves. Set the P-wave velocity at the top of the mantle to 8.2 km/s
and the S-wave velocity to 4.5 km/s. Set the S-wave velocity in the core to 0 km/s.
Keep the P-wave velocity uniform in the core. Record your results in the table below.
Mantle
At top
P wave
8.2 km/s
S wave
4.5 km/s
Copyright Houghton Mifflin Company. All Rights Reserved.
Core
At bottom
Uniform
0 km/s
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