Name: _________________________________
Interpreting Seismograms and Time-Travel Graphs
Time (sec)
Background Information:
Scientists use seismograms and time-travel graphs to locate earthquake epicenters. An
earthquake releases energy that travels through the earth in the form of waves. Primary and secondary
waves travel at different speeds. Both kinds of waves start out at the focus of the earthquake at the
same time. However, they reach distant seismograph stations at different times. The graph below
shows how this works.
Distance (km)
Using the graph above, answer the following questions:
1. Suppose a seismograph station is 3,000 km from the focus of an earthquake. How long, in
seconds, does it take the P wave to arrive? _________________________________
2. How long does it take the S wave to arrive? ________________________________
3. The difference in arrival time for P and S waves is called the lag time. What is the lag time for
this earthquake? [S wave – P wave = lag time] ______________________________
How to find distance when you know the lag time:
Suppose you know the lag time between the arrival of the P and S waves at a location. You can
then use the graph to find out how far the place is from the epicenter. Use the following
instructions:
 Take a sheet of paper and hold it against the vertical axis (y axis) of the graph.
 On the edge of the paper, make two marks – one at zero, and the other
at the amount of lag time. (Ex: if the lag time is 30 seconds, there
would be one mark at zero and one mark at 30 seconds)
 Slide the paper to the right, keeping the paper straight up-and-down as
you move it.
 Find a place on the graph where the distance between the two curves
(P wave and S wave) is the same distance as the marks on your paper.
 Follow that point straight down to the x-axis to find the distance to the
epicenter for the specific lag time.
4. At your location, you record a lag time of 180 seconds between the P and S waves. How far are
you from the epicenter? ____________________________
a. When did the P wave arrive? ____________________
b. When did the S wave arrive? ____________________
5. The geologists at Penn State recorded a lag time of 140 seconds between the P and S waves for
an earthquake. How far is Penn State from the epicenter? __________________________
On November 9, 1980, an earthquake struck the West Coast of North America. The seismograms below
recorded the number of seconds after 2:27PM, Pacific Time, that the P and S waves reached 3 different
cities.
For each city:
 Determine the arrival time, in seconds, for the P and S waves, by drawing a vertical line straight
down from the first peak of each wave. Record these time in the table below.
 To figure out the lag time, subtract the P wave arrival time from the S wave arrival time. Record
your answer in seconds
 Use the lag time and the travel-time graph on the front page to determine the distance of each
city from the earthquake’s epicenter. [use the instructions on the other paper if you need help!]
Seismograph
Station
Arrival Time after 2:27 in seconds
P wave
S wave
Lag Time (sec)
(S wave - P wave)
Distance (km)
Phoenix, AZ
Denver, CO
Seattle, WA
Extra Practice when finished (fill in the table):
Lag Time
Distance
P Wave arrival time
S wave arrival time
60 seconds
4,400 km
180 seconds
620 seconds