TEACHING CLUES AND CUES
The seismograms on
Master 3.3d are
seismograms that have
been simplified and
enlarged for classroom use. The
printed millimeter scale has also
been enlarged proportionately.
Do not use rulers for this
step or the
measurement will be
inaccurate. (Be sure
they make all their
measurements on the same wave
cycle, not the crest from one and the
trough from another. The diagram on
p.116 will be helpful.)
To measure the ampli-
tude, have students lay
a piece of paper across
the wave and mark the
Magnitude, Amplitude. Tell students that they are going to be working
with five seismograms, all recorded for one earthquake on September
2, 1992, with its epicenter at St. George, Utah. Explain that the
seismographs have been enlarged and simplified for their use but that
this has also changed the scale, so they must use the vertical scale for
all measurements. You will be demonstrating the steps to determine
magnitude by using the first seismogram on Master 3.3d. They will
record their data on the bottom of the third page of seismograms.
2. Project a transparency of Master 3.3d, page 1, and demonstrate how
to measure amplitude. (See vocabulary definition.) Using the scale on
the left side of the graph (the y axis) determine the greatest deflection
in millimeters above or below zero of the largest seismic wave. Record
this measurement in the amplitude column on the data table.
3. After all students have measured amplitude, start the next part of the
activity by asking them which earthquake wave travels fastest and
therefore should be the first wave recorded on the seismogram. Most
of them should be aware from previous lessons that it is the P wave.
On the transparency of Master 3.3d, page 1, point out the arrival of the
P and S waves. Ask students what the difference between the arrival of
the two waves is called (lag time (T
s
-T
p
). Ask students what should
happen to the lag time recorded at stations farther from the earthquake.
Offer a hint by comparing the progress of the waves to a race between
the family car and a race car. The longer the race is, the more of an
advantage the race car will have.
4. Using the projection of Master 3.3d, page 1, demonstrate how to
determine distance to the earthquake recorded by the first seismogram.
Using the scale at the top of the graph (the x axis), measure the
difference between P wave and S wave arrival times in seconds. Use
the formula
Distance = (T
s
-T
p
) x (8 km/sec) or (4.96 mi/sec)
to convert time to distance. Have students record this in their data
tables. Give students time to calculate the distances for the other four
seismograms, following the procedure you have modeled.
5. Project the transparency of Master 3.3e and demonstrate how to
find the magnitude of the earthquake. To do this, place the left end of
the ruler on the left scale at the distance calculated in step 4. Holding
the left edge of the ruler in place, move the right edge of the ruler to
the correct point for the base-to-peak amplitude. Read the Richter
magnitude on the center scale where the ruler crosses the graph and
record it in the data table. Have students repeat this process on their
own for the other stations. Average the magnitudes from the five
locations to determine the final magnitude for this earthquake, and
record it in the data table.
high and low peaks, then use the
scale on the y axis or the scale
printed at the bottom of the seismo-
gram to measure the amplitude or
distance.
A G U
/
F E M A
166
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E I S M I C
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L E U T H S
