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“Locating the Epicenter of an Earthquake”
Introduction:
Triangulation is one method scientists use to locate the epicenter of an earthquake.
The epicenter is the location at the surface of the Earth that represents the focus inside
the Earth where the earthquake originates on a fault line.
Triangulation requires seismic information from three different recording stations. Seismic
recording stations are located all over the world. Scientists time how long it takes
seismic waves to reach each recording station. Seismic waves will arrive at recording
stations that are close to the epicenter before they reach stations farther away from the
epicenter. This information is converted into a length measurement to find the distance
from the station to the earthquake. Recall that a radius is the distance halfway across a
circle (from its center to its edge!). Using the recording station as the center, circles are
now drawn with the radius equaling the distance from the station to the epicenter. The
epicenter lies somewhere along this circle, but we need more information to find out its
exact location.
Earthquakes can occur at any point on the circle, however, when three separate circles are
drawn, the circles will intersect at one point, which represents the epicenter of the
earthquake.
Earthquake Search
Wherever an earthquake occurs, two types of shock waves spread out in all directions.
Each type of wave travels at a different speed. The earthquake waves that travel fastest
are known as P, or primary, waves. The waves that travel more slowly are called S, or
secondary, waves. After an earthquake occurs, P waves reach places sooner than S waves
do.
Both types of waves start out from the earthquake's focus. To detect an earthquake,
seismologists use an instrument called a seismograph. By using this instrument, scientists
figure out when the first P and S waves from the earthquake arrive at the seismograph's
location.
The difference between the arrival times of P and S waves is used to find the location
of the earthquake's epicenter. This information must be obtained from at least three
locations.
Now it is your turn to determine where an earthquake's epicenter is located. To do this,
you will need a drawing compass and the map that follows.
Carefully study the graph on this page. It shows the relationship between the difference
in arrival times of P and S waves and the distance to the epicenter of an earthquake. The
time is in minutes. The distance is in kilometers. Use the graph to answer the questions
that follow.
Problem
How can an earthquake be located?
Materials (per student)
Drawing compass with pencil
The accompanying graph
The accompanying map of the United States
Procedure
1. Carefully observe Figure 1, which shows a comparison difference in arrival time between
P and S waves and distance to the epicenter of an earthquake. Note the two quantities are
directly related: that is, the greater the difference in arrival time, the greater the
distance to the epicenter.
2. Before going further in this investigation you will need to become familiar with the
graph. Use the graph to answer questions 1 through 4 in Observations.
3. Now that you know how to read the graph, see if you can put it to use. Assume that an
earthquake has occurred and that the times of arrival of the P and S waves from it have
been detected and recorded by seismographs located at the three cities listed in the Data
Table. Note that the difference in P and S wave arrival times has been included in the
table.
4. Using Figure 1, determine each city's distance from the earthquake epicenter. Enter your
figures in the Data Table.
5. Use the map scale to set your compass at a radius equal to the distance from Denver
to the earthquake epicenter.
6. Draw a circle with the radius determined in step 5, using Denver as the center. Draw the
circle on the map in Figure 2.
7. Repeat steps 5 and 6 for Houston and Miami.
8. If you have worked carefully, the three circles should intersect at one point. This point
marks the epicenter of the earthquake.
OBSERVATION
1. If the difference in arrival times of P and S waves at a certain location is 3
minutes, how far from that location is the earthquake's epicenter?
a. 430 km
b. 1400 km
c. 1800 km
d. 2100 km
2. If a recording station is 4600 km from an earthquake's epicenter, what is the
difference in arrival times of P and S waves?
3. If a seismograph shows that a P wave arrives 7 min 20 sec before the S
wave, how far is it to the earthquake’s epicenter?
4. If the seismograph is located 2200 km from the earthquake epicenter, how
great will be the difference in arrival time between P and S waves at the
station?
Analysis and Conclusion
Now suppose an earthquake has occurred somewhere in the United States. It’s P and S
waves are recorded by seismographs located at the three cities shown in the Data
Table.
DATA TABLE
City
Difference in Arrival
Times of P and S Waves
Denver
2 min 25 sec
Houston
4 min 10 sec
Miami
5 min 40 sec
Distance in Km from
epicenter
Place the map on your desk. Use the scale on the map to set the compass points apart
at a distance equal to the distance from Denver to the earthquake's epicenter. Place
the sharp point of the compass in Denver. Draw a circle with the pencil point. Repeat,
but this time set the compass for the distance from Houston to the earthquake's
epicenter. Repeat, but this time set the compass for the distance from Miami to the
earthquake's epicenter.
1. Which city on the map is closest to the earthquake's epicenter?
How far is this city from the epicenter?
Where is the epicenter of the earthquake located?
2. Of the scientists at the three cities in the Data Table, which would have
become aware of the earthquake first?
Second?
Third?
3. Why is it necessary to have data from at least three recording stations to be able to
locate the epicenter?
4. Suppose the epicenter of this earthquake is located in San Francisco. What will be
the difference in arrival times of P and S waves in New York City?
5. As the distance between an observer and an earthquake decreases (as you
get closer to the earthquake epicenter), what happens to the difference in
arrival times of the P and S waves?
a. increase
b. decrease, or
c. remain the same?
Explain your answer:
6. What could happen to the Earth’s surface when vibrations from an
earthquake travel through the crust?
7. What relationship do you think exists between the amount of energy an
earthquake contains and the amount of damage it will do?
8. Is it possible for seismologist to know for sure an earthquake will not occur
in a particular area? Explain why or why not:
9. Will it ever be possible to prevent earthquakes? Explain why or why not: