Name: ______________________________________________
Period: ___
Finding the Epicenter
Introduction:
Most earthquakes occur in remote areas away from human habitation, such as the middle of high mountain ranges or
the bottom of oceans. They may leave a scar on the land, but not always. So how do scientists know where an
earthquake occurs? In this activity you will learn how scientists use the p- and s-waves, along with a principle known as
“triangulation”, to locate the epicenter of an earthquake.
Materials:

Compass
Procedure:
1) Using the information from the seismograph below, record the p- and s-wave arrival times in the correct data
table. (Each little line in the seismograph is about 1.5 seconds.)
2) Subtract the p-wave arrival time from the s-wave arrival time to find the lag time. Record this in your data table
as well.
3) In order to find the distance, perform the following operation and record it in your table:
𝐿𝑎𝑔 𝑇𝑖𝑚𝑒
× 100 = 𝐷𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑓𝑟𝑜𝑚 𝐶𝑖𝑡𝑦
8
4) Using a compass, draw a circle around the station with a radius equal to the Distance from City.
5) Do all four stations for each earthquake. The epicenter is where at least three of the circles overlap. Star that
spot to indicate where the epicenter is.
Data:
165
165
Data Table:
City
P-wave Arrival Time
Earthquake 1
S-Wave Arrival Time
Lag Time
Distance from City
P-wave Arrival Time
Earthquake 2
S-Wave Arrival Time
Lag Time
Distance from City
Chicago
Kansas City
Santa Barbara
Seattle
City
Newport
Tucson
Rockville
McMinnville
Questions:
1) Which wave (p or s) arrives first?
2) What shape does an s-wave have?
What shape does a p-wave have?
3) Did the lag time get shorter or longer the further you got from the epicenter?
4) How could you prove that the epicenter was where you calculated it to be?
5) We did not talk about this in class, but what might the amplitude of a seismic wave be related to?
Earthquake Map #1:
Seattle
Chicago
Kansas
City
Santa
Barbara
0
500
1,000
1,500
2,000
2,500
3,000
Scale of Kilometers. Each box is 100 km.
Earthquake Map #2:
Newport
Rockville
McMinnville
Tucson