7. Project the transparency of Master 3.3b, Wattsville Map, and have
students take turns giving you the values to draw on the transparency.
Then draw in the isoseismals according to their interpretation of the
data.
C. Conclusion
Ask students to find the area where the damage was most intense and
color this area red. Then build a class discussion around these
questions:
Q
Where do you think the epicenter of the earthquake is located?
(Most students will think that the area with the most damage is the
epicenter. In certain cases this is a false assumption. Damage may
be related to siting or construction practices. An example of this is
the 1985 Mexico City earthquake in which the fault was over 350
km away.)
Q
Why did some buildings near the epicenter withstand the shocks
better than others? (perhaps because their structures were sounder
or the soil under them was more firm)
Q
What are some advantages of the Mercalli rating procedure? (In
ideal conditions, it can be done quickly, and it doesn’t require any
instruments. It describes the impact in human terms: on human
beings and their structures.)
Q
What are some disadvantages to this procedure? (It requires human
observers, so it could not assign an intensity to an earthquake in the
middle of an ocean or in a deserted place. It also requires the
exchange of information over a fairly wide area. To arrive at
Mercalli ratings today, an official survey is conducted through the
U.S. mail. It is not as precise or objective as instrument readings;
different people might describe the same situation differently.)
Now place the answer key transparency on the overhead so it overlays
student data. If there is a discrepancy between the isoseismals shown
on the master and those students produced, they will see one of the
drawbacks to this system of measurement. Ask students for
suggestions to solve this problem.
ADA P T A T I ON S AND E X T E N S I ON S
1. Write a modern-day version of the 1931 Mercalli Intensity Scale.
2. Go to the library and search newspapers for qualitative information
on an earthquake in your area. Construct an isoseismal map for the
earthquake.
A G U
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F E M A
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E I S M I C
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