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Name ____________________________
Finding the Epicenter of an Earthquake
You are part of a student project working with the National Earthquake
Information Center. An earthquake has just occurred somewhere in the
United States, and several schools have recorded the event. Your job is to
analyze seismic waves detected at three different schools in order to figure
out the epicenter of the earthquake.
WHAT YOU’LL DO
Calculate the distance from surrounding seismometers to an earthquake’s epicenter.
Find the location of the earthquake’s epicenter.
Explain the relationship between seismic waves and the location of an earthquake’s
epicenter.
WHAT YOU’LL NEED
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calculator
compass, drawing
this lab
ruler
PROCEDURE
Preparing for Your Activity
1. You will examine seismograms showing two kinds of seismic waves: P waves and S
waves.
2. P waves have an average speed of 6.1 km/s. S waves have an average speed of 4.1 km/s.
a. How long does it take P waves to travel 100 km? Use the formula d= st where d
stands for distance, s stands for speed and t stands for time.
Givens:
Equation
Show your work:
Answer with Unit:
b. How long does it take S waves to travel 100 km? Use the same formula as you used
in 2a
Givens:
Equation
Show your work:
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Answer with Unit:
3. Because S waves travel more slowly than P waves, S waves will reach a seismograph
after P waves arrive. This difference in arrival times is known as the lag time.
4. Calculate the lag time that you would expect from a seismograph located exactly 100
km from the epicenter of an earthquake. (see #3 for a hint)
Measuring the Lag Time from Seismographic Records
5. The illustration [seismogram] shows the records produced by seismographs in three
cities following an earthquake.
6. Calculate the lag time for each of the following cities. Be sure to show your work and
include a unit!
Austin = _____________________________________________
Bismarck = ___________________________________________
Portland = ____________________________________________
7. Calculate the distance from each city to the epicenter of the earthquake using the
following formula:
d = (measured lag time ÷ lag time for 100 km)  100 km
SHOW ALL YOUR WORK!
Austin :
Bismark:
Portland:
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Procedure
1. Adjust the compass so that its width is equal to the distance from the epicenter to Austin.
2. Put the point of the compass on Austin, and draw a circle on your map. (Hint: It is easier to
move the paper under the compass than move the compass!)
How is the location of the epicenter related to the circle that you just made? _______________
_____________________________________________________________________________
3. Repeat the process from #1 using the distance from Portland to the epicenter. The epicenter
is one of the two sites where the circles intersect.
4. Repeat the process once more for Bismarck. At this point you should have 3 circles on the
map below.
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Analysis
1. Why is it necessary to use seismographs in three different locations to find the epicenter
of an earthquake? ________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
2. Would it be possible to use this method for locating an earthquake’s epicenter if
earthquakes produced only one kind of seismic wave? Explain your answer.
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
3. Someone tells you that the best way to determine the epicenter is to find a seismograph
where the P waves and S waves occur at the same time. What is wrong with this
reasoning?
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
4. Which city is closest to the epicenter? _____________________________________
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