Finding the Epicenter
Name______________________________Per.____
This difference in arrival times between the P and S wave is
called the S-P interval. This time is used by earthquake
scientists to figure out how far the seismograph is from the
epicenter of the earthquake.
1. Find the P wave for the three seismograms on the next
page, and then the S wave. Now, take S – P, and write the
time in the table.

Station
S-P Interval
(seconds)
Eureka
NOTE- For Time you
count by 1’s, but for
Distance, count by 20
Elko
Las Vegas

2. Next, look at the graph. Find where the S-P interval for a
city touches or meets the diagonal line. Fill-in the distances
for Eureka, Elko, and Las Vegas in the table below.
3. On the map, find the city for each monitoring station, and use a compass or string to a draw
circle (or arc) of the proper radius around each one. Use the scale at the bottom of the page to
help you measure the radius-distance before you start drawing its circle.
Station
Distanceradius (km)
Eureka
Elko
Las Vegas
* you may want to use this 
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0
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200
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400
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600
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|
800
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IMPORTANT NOTE- For each line count by 2
Eureka, CA
Elko, NV
Las Vegas, NV
4. After you have drawn all three circles, put an X on the map at a point near to where all three of
the circles (or arcs) intersect or meet, to mark where you think the epicenter of the earthquake
should be.
5. Which city on the map is the closest to the epicenter of the earthquake?
6. Why is it not possible to find the epicenter of an earthquake with only 1 or 2 seismographs?
7. Why do you think there could be a problem in getting an exact point for the epicenter?
8. Now repeat the same process to find the
epicenter of a different earthquake, using the
same process. Three more seismograms are
located on the next page.
Station
S-P Interval
(seconds)
Distance (km)
Pusan
Tokyo
Akita
* you may want to use this 
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0
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200 400
600
800
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Pusan (South Korea)
Tokyo (Japan)
Akita (Japan)