VOCABULARY
Epicenter:
the point on
Earth’s surface directly
above the focus of an
earthquake.
ACTIVITY THREE
F
IND THE
E
PICENTER
: D
ECODING
S
EISMOGRAMS
RA T I ONA L E
Students can find the location of an earthquake by triangulation if they
know the distances from at least three seismograph stations.
F OCU S QU E S T I ON
How do seismologists use seismograms to locate the epicenter of an
earthquake?
OB J E C T I V E S
Students will:
1. Calculate the distance from an earthquake to a seismograph station.
2. Use five calculated distances to triangulate the location of the
earthquake’s epicenter.
MA T E R I A L S
Q
Student copies of Master 3.3f, Several Seismographs
Q
Student copies of Master 3.3g, Sample Seismograms
Q
Student copies of Master 3.3h, Map of Station locations
Q
Transparency made from one page of Master 3.3g, Sample
Seismograms
Focus (pl. foci):
the point within the
Earth that is the origin of an
earthquake, where stored energy is
first released as wave energy.
Seismogram:
the record of
earthquake ground motion recorded
by a seismograph.
Seismograph:
an instrument that
records vibrations of the Earth,
especially earthquakes.
Triangulation:
using data from three
or more known points to locate an
unknown point, in this case the
epicenter of an earthquake.
Q
Overhead projector
Q
Transparency made from Master 3.3h, Map of Station Locations
Q
Student copies of Master 3.3i, Time/Distance Reference Table
Q
Drawing compasses
Q
Metric rulers with millimeter scales
P ROC E DUR E
Teacher Preparation
Make one copy of each of the masters (3.3f through 3.3i) for every
two students in your class.
A. Introduction
Ask students chosen at random to explain the difference between an
earthquake’s focus and its epicenter and between a seismograph and a
seismogram. Review these distinctions if necessary.
Distribute copies of Master 3.3f, Several Seismographs, then project a
transparency of Master 3.3f and describe their operation.
B. Lesson Development
1. Divide the class into pairs of students. Distribute a set of
seismograms (Master 3.3g, 3 pages), one map (Master 3.3h), and
copies of the Time/Distance Reference Table (Master 3.3i) to each
A G U
/
F E M A
168
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E I S M I C
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L E U T H S
