Name: _________________________________
Pd. ______
Date: ______________
Examining Seismograms
The three basic types of seismic wave generated by an earthquake at its focus are P-waves, Swaves, and Surface waves. P and S-waves are body waves and travel through the interior of
the earth. P=waves have the greatest velocity and reach the seismic station first. S-waves
arrive at the seismic station after the P-waves. The amount of time that passes between the Pwave arrival and the S-wave arrival is important in helping seismologists determine the
epicenter of the earthquake.
Today we will be using a travel-time chart, where the vertical separation between the P and S
curves is equal to the difference in the arrival times between the P-wave and S-wave. To
accurately locate an earthquake epicenter, records from three seismograms are necessary
Figure 1: A typical Seismogram.
(Earlier)
Time
(Later)
1. Which of the two waves arrives at the seismic station first? ______________________
2. From the seismogram in Figure 1. What is the difference in arrival times between the
first P-wave and the first S-wave? _________________
Figure 2: Seismograms from three different stations.
(Earlier)
Time
(Later)
3. Using the Seismograms in Figure 2, Fill in Table 1.
Table 1. Seismogram Data Table.
Station
P-Wave Start Time
S-Wave Start Time
Lag Time (S-P wave)
Station A: St. Louis,
Missouri
Station B: Houston,
Texas
Station C: Los Angeles,
California
4. Use Figure 3 to determine the time in minutes or distance in km of the various P-waves
and S-waves.
a. How long does it take the P-wave to reach 4000 km: ____________ minutes
b. How long does it take the S- wave to reach 4000 km: ____________ minutes
c. What is the lag-time of these two waves at 4000 km: ____________ minutes
(S-wave – P-wave) = Lag-time
d. How long does it take the P-wave to reach 9000 km: _____________ minutes
e. How long does it take the S-wave to reach 9000 km: _____________ minutes
f. What is the lag-time of these two waves at 9000 km: _____________ minutes
(S-wave – P-wave) = Lag-time
g. How far has the P-wave traveled at 4 minutes: ________________ km
h. How far has the S-wave traveled at 4 minutes: ________________ km
i. How far has the P-wave traveled at 13 minutes: ________________ km
j. How are has the S-wave travels at 13 minutes: _________________ km
Figure 3: Travel Time Graph
7 minutes
4000 km