Name___________________________________ Lab #_____________
LOCATING AN EPICENTER
INTRODUCTION: In your previous labs you learned that Earthquakes commonly occur near plate
boundaries. When there is movement along a fault (commonly at plate boundaries) an earthquakes
occurs. At a plate boundary, large pieces of the Earths crust are moving in different directions, over time
pressure builds up at these locations. After a given amount of time (10’s of years to 1000’s of years) the
pressure becomes too great and a break occurs in the crust. The crust quickly jumps a few inches or as
much as 10 feet. This sudden movement along the fault generates shock waves which travel through the
Earth. Different types of shock waves are generated, they are called P waves, S waves, and L waves.
These waves are radiated in every direction away from the focus. The focus is the point in the Earth
(below ground) where the actual movements take place.
Earthquakes occur every 30 seconds, day after day, yet most are too small to be felt by humans. Scientists
use instruments called seismographs to monitor movement in the crust. Thousands of seismographs are
located all over the planet.
OBJECTIVE: In this lab you will use seismograms (readings from a seismograph) from 3 distant
stations to locate the epicenter of an earthquake.
VOCABULARY:
fault:
epicenter:
focus:
focal depth:
P-wave:
S-wave:
seismograph:
seismogram:
PROCEDURE A:
Using the three seismograms provided and the ESRT calculate and record the following:
1. The arrival times for P and S-waves.
2. The difference in the arrival time between P and S-waves.
3. The distance (in km) of the epicenter from each city (use info in #2 and tick mark technique)
4. Now that you know the distance to the epicenter, how long will it take a P-Wave to travel that distance.
5. Now you have the time the P-Wave arrived at the station (#1) and you have the time it takes a P-Wave
to travel that distance, you can calculate when the earthquake occurred or when the P-Wave left the
epicenter.
6. Now after you have the distance from each city to the epicenter, look at the map of the US.
7. Using a compass and the scale at the bottom of the US map, extend the compass the desired distance
from you first city.
8. Then go to that city and draw a circle that represents the distance that city is from the epicenter.
9. Do the same with the other two cities.
10. The point where the three circles intersect (or come real close) is the epicenter of the earthquake.
11. Draw a triangle in this region and label it Epicenter.
12. Answer the questions in complete sentences.
DISCUSSION QUESTIONS: (Answer in complete sentences)
1. What was the approximate location of the epicenter of this earthquake?
2. What information could you gather from just one seismograph?
3. What information could you gather from two seismographs? (think of you map with 2 circles)
4. Why are at least three seismographs needed to locate an epicenter?
5. How does the size of the circle you drew and the distance to the epicenter compare?
6. Why does the time between the arrival of the P-wave and S-wave become greater and greater as you get
farther away from the epicenter? After explaining this, draw a graph to show the relationship.
CONCLUSION: Describe, step by step, how the epicenter of an earthquake can be located. Basically you
should write out what you did in this lab to locate the epicenter. (this should be several sentences)