Name ____________________
Due Monday Feb 4
GEOL 106 Writing Assignment 3: Earthquakes
1) On the block diagram, label the following
a) fault
b) focus of an earthquake
c) epicenter of an earthquake
d) fault scarp
e) wave fronts emanating
from an earthquake
2) Explain the elastic rebound theory in your own words. The time-series of sketches may be 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? In what order do they
arrive at seismic stations?
5) Explain the movement of P and S wave fronts through a body of rock, referring to Fig. 4.8 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. 4.8 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.
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