Finding the Epicenter of an Earthquake
(student copy)
1. The graph on page 188 shows how the difference in arrival times
between P waves and S waves depends on the distance from the
epicenter of the earthquake. Find the difference in arrival time for
Denver on the y-axis of the graph. Follow this line across to the point
at which it crosses the curve. To find the distance to the epicenter,
read down from this point to the x-axis of the graph. Enter this distance
in the data table.
2. Repeat Step 1 for Houston and Chicago.
3. Use the scale to set your compass at a radius equal to the distance
from Denver to the earthquake epicenter that you recorded in your data
table.
4. Draw a circle on the map with the radius determined in Step 3 using
Denver as the center. (Hints: Draw your circles carefully. You may
need to draw some parts of the circles off the map.)
5. Repeat Steps 3 and 4 for Houston and Chicago.
Data Table
City
Difference in P and S
Wave Arrival Times
Denver, Colorado
2 min 40 sec
Houston, Texas
1 min 50 sec
Chicago, Illinois
1 min 10 sec
Distance to
Epicenter
(kilometers)
0
0
200
200
400
400
600
600
800
1,000
1,200
800 1,000 1,200 1,400 1,600 1,800 2,000
1,400 miles
kilometers
Seattle
San
Francisco
New York
Chicago
Salt Lake
City
Denver
Savannah
Houston
Miami
Analyze and Conclude
Answer the following questions on a separate sheet of paper.
1. Drawing Conclusions: Observe the three circles you have drawn. Where is the
earthquake's epicenter?
2. Measuring: Which city on the map is closest to the earthquake epicenter? How far, in
kilometers, is this city from the epicenter?
3. Inferring: In which of the three cities listed in the data table would seismographs detect the
earthquake first? Last?
4. Estimating: About how far from San Francisco is the epicenter that you found? What
would be the difference in arrival times of the P waves and S waves for a recording station
in San Francisco?
5. Interpreting Data: What happens to the difference in arrival times between P waves and S
waves as the distance from the earthquake increases?
6. Communicating: Review the procedure you followed in this lab and then answer the
following question. When you are trying to locate an epicenter, why is it necessary to know
the distance from the epicenter for at least three recording stations?
More to Explore
You have just located an earthquake's epicenter. Find this earthquake's location on the map of
Earthquake Risk in the United States in you textbook (pg 197). What is the risk of earthquakes
in the area of this quake? This area is not at a boundary of earth's lithospheric plates (pg 159).
Why is this area a risk for earthquakes?
Finding the Epicenter of an Earthquake
(teacher's copy with solutions and answers)
1. The graph on page 188 shows how the difference in arrival times
between P waves and S waves depends on the distance from the
epicenter of the earthquake. Find the difference in arrival time for
Denver on the y-axis of the graph. Follow this line across to the point
at which it crosses the curve. To find the distance to the epicenter,
read down from this point to the x-axis of the graph. Enter this distance
in the data table.
2. Repeat Step 1 for Houston and Chicago.
3. Use the scale to set your compass at a radius equal to the distance
from Denver to the earthquake epicenter that you recorded in your data
table.
4. Draw a circle on the map with the radius determined in Step 3 using
Denver as the center. (Hints: Draw your circles carefully. You may
need to draw some parts of the circles off the map.)
5. Repeat Steps 3 and 4 for Houston and Chicago.
Data Table
City
Difference in P and S
Wave Arrival Times
Distance to
Epicenter
(kilometers)
Denver, Colorado
2 min 40 sec
1,600
Houston, Texas
1 min 50 sec
1,000
Chicago, Illinois
1 min 10 sec
600
0
0
200
200
400
400
600
600
800
1,000
1,200
800 1,000 1,200 1,400 1,600 1,800 2,000
1,400 miles
kilometers
Seattle
San
Francisco
New York
Chicago
Salt Lake
City
Denver
Savannah
Houston
Miami
Analyze and Conclude
Answer the following questions on a separate sheet of paper.
1. Drawing Conclusions: Observe the three circles you have drawn. Where is the
earthquake's epicenter?
The epicenter is located on the border of western Tennessee and Kentucky, close to the border and
"boot" of Missouri.
2. Measuring: Which city on the map is closest to the earthquake epicenter? How far, in
kilometers, is this city from the epicenter?
Chicago is closest to the epicenter, approximately 600 kilometers away..
3. Inferring: In which of the three cities listed in the data table would seismographs detect the
earthquake first? Last?
The first of the three cities to detect the earthquake would be Chicago; the last city to detect the
earthquake would be Denver.
4. Estimating: About how far from San Francisco is the epicenter that you found? What
would be the difference in arrival times of the P waves and S waves for a recording station
in San Francisco?
San Francisco is approximately 2,900 kilometers from the epicenter. The difference in arrival times
would be approximately 4 minutes 25 seconds.
5. Interpreting Data: What happens to the difference in arrival times between P waves and S
waves as the distance from the earthquake increases?
The difference in the arrival times increases as the distance from the epicenter increases.
6. Communicating: Review the procedure you followed in this lab and then answer the
following question. When you are trying to locate an epicenter, why is it necessary to know
the distance from the epicenter for at least three recording stations?
The earthquake epicenter can be anywhere on the circumference of the circle drawn for the first city.
The earthquake detected by the second city, combined with the detection by the first city, narrows the
epicenter to the two common points shared by each circumference. The detection by the third city has
only one of the two points in common and identifies the epicenter.
More to Explore
You have just located an earthquake's epicenter. Find this earthquake's location on the map of
Earthquake Risk in the United States in you textbook (pg 197). What is the risk of earthquakes
in the area of this quake? This area is not at a boundary of earth's lithospheric plates (pg 159).
Why is this area a risk for earthquakes?
The earthquake occurred in an area of moderate risk. (This area is known as the New Madrid fault.)
Scientists believe that this area of the United States has faults, or fractures, in the earth's lithosphere
that never turned into a complete lithospheric plate. You can think of these as partial, or incomplete,
"cracks".
Difference in Arrival Time of P and S waves (min)
9
8
7
6
5
4
3
2
1
1000
2000
3000
4000
5000
Distance to Epicenter (km)
6000
7000
Difference in Arrival Time of P and S waves (min)
9
8
7
6
5
4
3
2
1
1000
2000
3000
4000
5000
Distance to Epicenter (km)
6000
7000