Lab- Locating the Epicenter
of an earthquake
Name: ___________________
Grade: ________
Introduction
You wake suddenly from a sound sleep. Are you dreaming or is the ground shaking? As a seismologist
for the U.S. Geological Survey, you know how to find out what happened. You drive to your office at the
Sacramento seismic station. Sure enough, the seismograph there detected seismic waves. Your job for the day
is to compare seismograph data with geologists at other seismic stations and find out where the epicenter of the
earthquake is located. Because aftershocks often occur along faults and earthquake zones, you need to prepare
a press release for people living near the epicenter, warning them that another earthquake could occur.
Problem
How do seismologists locate the epicenter of an earthquake?
Materials
ruler
pencil
drawing compass
Procedure
1. Interpret data: In addition to the seismograph record from your office, you have obtained
records from two other seismic stations. These records are shown in Figure 10.1. On the
Sacromento record, note the location of P, which represents the arrival time of the P wave at the
station. Now locate S, which indicates the arrival time of the S wave.
2. For the Sacramento data, use a ruler to draw a straight line from the start of the P wave down to
the time scale. Do the same for the S wave.
3. Use these two lines to determine the difference in arrival times for the P and S waves.
Record this data in Data Table 10.1.
4. Repeat Steps 2 thru 3 for the data from the other two seismic stations.
5. By comparing data collected during past earthquakes, seismologists have determined the
relationship between the arrival of P and S waves and the distance to an earthquake’s epicenter.
In Figure 10.2, you will find a graph seismologists have created showing that relationship.
On the “Distance in Arrival Time of P and S Waves” axis, each line is counting by ______________.
On the “Distance to Epicenter” axis, each line is counting by ________________.
Using the graph in Figure 10.2 and the differences in P and S arrival times you listed in
Data Table 10.1, determine each seismic station’s distance from the epicenter.
(HINT- Find your “Difference in Arrival Time” then draw a straight line to the curved black line,
then down to the “Distance” axis.)
Record the distance in Data Table 10.1
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6. Locate the three seismic stations on the map in Figure 10.3. Using the distance to the
epicenter for Sacramento from Data Table 10.1, the map scale, and the ruler…set the drawing
compass at a radius equal to the distance from Sacramento to the epicenter. Position the sharp
compass point on Sacramento and use the compass to draw a circle around Sacramento.
7. Repeat Step 6 for Tucson and Des Moines.
8. Note where the three circles intersect. This is the epicenter.
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Observations
1. Did the first two circles you drew determine a single, definite location? (1 pt)
At how many points did they intersect? (1 pt)
2. Describe where the three circles you drew intersected. (1 pt)
Analysis
1. Why was it important to use three geographically separate data sources in order to find the
epicenter? Why not one or two sources? (3 pts)
2. If your three circles had not intersected at one point, what could be two reasons why? Be specific.
Use examples from the lab itself. (2 pts)
What could you do to locate the epicenter and correct each situation? (2 pts)
3. On a seismograph reading, the height or “amplitude” of seismic waves is a measure of the
amount of the ground shaking. As the wave travels further, this amplitude decreases.
How might the amplitude of a wave from a powerful earthquake a long distance away compare to
that of a mild quake nearby? Explain your reasoning. (2 pts)
How could you then tell which quake was far away and which quake was closer? (2 pts)
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Conclusions- “Both Questions refer to the Earthquake you located in this lab!”
1. How would you expect the difference in the P and S wave arrival times for a seismic station in
New York (further east) to compare with the difference in wave arrival times for the station in
Des Moines? Explain your answer. (3 pts)
2. Write a one paragraph press release informing the public about the following:
- what took place (include size)
- where it took place
- how it most likely happened (fault type, stress, boundary type, any deformations, etc)
- and any impending dangers. (5 pts)
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