'Earthquakes' Geology Worksheet Template
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2GEOL 103 Writing Assignment 8 Key: Earthquakes
Name _______________________
Lab section: Monday or Tuesday (circle one)
To be counted, must be turned in by Monday December 2.
1) Sketch a block diagram (you don’t need different rock layers) that
illustrates the following
fault
focus of an earthquake
epicenter of an earthquake
fault scarp
wave fronts emanating from an earthquake
2) Explain the elastic rebound theory in your own words. Use a series of
sketches if you find that helpful.
As stress builds up, rocks begin to bend elastically to accommodate
the stress. When the stress becomes too great, the two blocks of
rock move past each other along a fault, releasing the stress as
seismic energy, and the rocks “spring back” elastically, and the
bending (folding) is not recorded in the rocks.
3) Explain the physical operations in a simple seismograph. What parts move (and why do they move) and what does not
move? (Modern seismographs use more electronic components instead, but the principles are the same.) Use the following
components: earth, box (holding the seismograph), spring or lever holding a mass, mass (attached to a pen), recording paper.
When an earthquake occurs, the Earth, the box holding the seismograph, the spring or lever, and the recording paper
all move together. Due to its inertia and because it is mounted on a spring or lever, the mass (attached to a pen) does
not move relative to the other components, thus the system records the movement of the Earth where the
seismograph is located. Other answers are possible, depending on the perspective of the observer.
4) What are the name of the three types of seismic waves that can pass through rock following an earthquake?
P-waves (primary), S-waves (shear waves), surface waves, in that order. See Fig 13.5.
5) Explain the movement of P and S wave fronts through a body of rock, referring to Figure 13.5 in your text. What does each
type of wave do to the rock it passes through? How do the velocities of each wave differ from each other? What types of
materials do S waves not travel through.
See Fig. 13.5 and associated text and captions. P-wave velocity > S-wave velocity >
surface wave velocity. S-waves do not travel through liquids or gases because these substances do not have
sufficient shear strength.
6) What are the differences between the Modified Mercalli Intensity Scale and the Richter Scales? What do each measure?
An intensity scale measures damage to human-related objects and indicates whether and how much people can
sense an earthquake. The Richter scale is more quantifiable, and is based upon how much the ground displacement
occurs a certain distance from an earthquake; recorded by analyzing data from several seismographs. Richter
magnitude is linearly related to the energy released by an earthquake.
7) Explain in your own words what taking the logarithm does to numbers that vary widely from each other.
Taking the logarithm of a group of widely varying numbers compresses the numbers so they can be more easily
compared as illustrated in the table below.
x
x as a power of ten
log(x)
0
1
10
0
1
10
10
1
2
100
10
2
3
1000
10
3
4
10000
10
4
5
100000
10
5
6
1000000
10
6
8) What are the relative differences in ground motion and energy released when comparing Richter magnitude 5 and 7
earthquakes?
At the same distance from a seismograph, a magnitude 5 earthquake would cause 100 times less ground movement
than a magnitude 7 earthquake. This is because the Richter scale is a log scale, with a factor of ten difference
between each Richter unit (10 x 10 = 100 times more ground movement). There is about30 times more energy
associated with each Richter unit, so 30 x 30 = 900 times less energy associated with the magnitude 5 quake.
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