Activity
LOCATING AN EARTHQUAKE EPICENTER
Try this activity to see how to plot the distance of several seismograph stations from epicenters of two earthquakes
and how to use that data to interpret where the earthquake epicenters are located.
Problem:
How can finding the distance of several seismograph stations from an earthquake be used to find the epicenter?
Hypothesis: _________________________________________________________________________
____________________________________________________________________________________
Materials:
The S and P Wave Time –Distance Graph; a world map; a drawing compass.
Procedure:
1. Determine the difference between the arrival times of the S and P waves from the data table below.
2. Use the Time-Distance Graph to determine the distance in kilometers from the earthquake to the
seismograph station. Example: The difference in the arrival times in Paris for earthquake B is 12 minutes.
On the chart, the P and S wave curves are separated 12 minutes vertically at 12,250 km.
3. Using the map scale and the distances to each earthquake as radii, place one end of a compass at each seismograph
station location. Use the compass to draw a circle with the radius equal to the distance to the earthquake’s epicenter.
4. Identify the epicenter after drawing at least three circles for each earthquake.
Data table:
Location of
Seismograph
(1) New York
(2) Seattle
(3) Rio de Janiero
(4) Paris
(5) Tokyo
Wave
Wave Arrival Times
Earthquake A Earthquake B
Distance (km)
A
B
P
S
Diff
2:24:05 PM
2:28:55 PM
__________
1:19:00 PM
1:24:40 PM
__________
____
____
P
S
Diff
2:24:40 PM
3:30:00 PM
__________
1:14:37 PM
1:16:52 PM
__________
____
____
P
S
Diff
2:29:00 PM
2:38:05 PM
__________
1:23:00 PM
1:33:30 PM
__________
____
10,500
P
S
Diff
2:30:15 PM
1:24:05 PM
1:36:05 PM
__________
____
12,250
____
____
P
S
Diff
__________
__________
__________
PROCEDURE: Locating an earthquake epicenter using a Time-Distance graph and S- & P- wave arrival times
1.
Determine the lag time interval between the P-waves and S-waves using the seismogram arrival time
data from the three seismic locations. Which seismic station is the closest to the earthquake epicenter?
2.
To determine the distance from each seismic station to the epicenter, use the following steps:
a. Examine the seismogram data and approximate, in minutes/seconds, the time
interval (lag time) between the P-wave and the S-wave arrivals. (to the nearest 10 sec).
b. Locate the time scale on the Time-Distance Graph along the left side. Place the edge of a piece
of scratch paper along this scale. On the paper mark where the zero point is and the time lag
obtained from the arrival time interval.
c. Slide the paper to the right on the graph until the two markings fit
between the P- and S-wave curves and is parallel to the time axis. (Be precise!)
d. The distance from the epicenter to the first location can be determined
by reading the value on the horizontal axis of distances directly below the edge of the scratch paper.
e. Repeat the process for the other two locations and use these three distances to draw
three circles on a world map to triangulate the location of the earthquake
.
S and P Wave Time-Distance Graph