Name_____________________
DETERMINING DISTANCE TO 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 earthquake 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, the Data Table and the ESRT calculate the following:
1. The arrival times for P and S-waves for each seismograph station (hr:min:sec).
2. Calculate the lag time (difference in arrival times) between P and S-waves.
3. Determine the distance (km) to the epicenter for each station.
4. Based on the distance to epicenter determine the P-wave travel time (min:sec) for each station.
5. Based on the P-wave travel time (min:sec) and the P-wave arrival time (hr:min:sec) calculate the Time
of Origin (hr:min:sec), when the earthquake occurred/when the P-Wave left the epicenter.
Seismograph
Station
Arrival time (clock time)
P-wave
S-wave
Lag Time
(min:sec)
Distance to
Epicenter
(km)
P-wave
Travel Time
(min:sec)
Time of Origin
(hr:min:sec)
CHICAGO
TAMPA
WINK
B: Use the calculated distance to epicenter above, the Epicenter Locations Map and a compass to answer
the following:
6. Using a compass and the scale at the bottom of the US map, extend the compass the desired distance
from your first city.
7. Place the compass on that city and draw a circle that represents its distance from the epicenter.
8. Do the same with the other two cities.
9. The point where the three circles intersect (or come really close) is the epicenter of the earthquake.
10. Draw a triangle in this region and label it Epicenter.
11. Answer the Questions in complete sentences.
SEISMOGRAMS
QUESTIONS:
1. Mark an X on the Epicenter Location Map to indicate the location of the epicenter.
2. What was the approximate location of the epicenter of this earthquake?
3. What information could you gather from just one seismograph?
4. Why are at least three seismographs needed to locate an epicenter?
5. How does the size of the circle you drew compare to the distance to the epicenter? Look at our data.
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).
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