Laboratory Manual for Introductory Geology 4e

FIGURE 11.5 Travel-time curves show the relationship between distance traveled and relative seismic wave velocity.
35
R-wave
30
Seismic wave travel time (minutes)
25
20
15
S-wave
L-wave
10
P-wave
5
0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 9,000 10,000
Distance from epicenter (km)
Seismologists have measured velocities of P-, S-, L-, and R-waves in different rock
types and know that P-waves average approximately 6.3 kilometers per second at the
surface and S-waves about 3.6 km/s. A graph of these speeds, called a travel-time
diagram, is used to calculate the distance to an epicenter (FIG. 11.5). The black curves
in Figure 11.5 show how much time it takes (vertical axis) the four types of seismic
waves to travel the distances shown on the horizontal axis. To find the time it takes
a P-wave to reach a point 4,000 km from an epicenter, find the intersection of the
4,000-km vertical line with the P-wave travel-time curve. Draw a horizontal line from
the intersection to the time axis on the left and read the time required—7 minutes
in this example. Exercise 11.3 provides practice in reading travel-time curves, and
Exercise 11.4 takes you through the steps of locating an earthquake’s epicenter.
EXERCISE 11.3
Reading a Travel-Time Diagram
Name:
Course:
Refer to the travel-time diagram in Figure 11.5 to answer the following questions:
(a) How long does it take a P-wave to travel 5,000 km?
minutes
(b) How long does it take an S-wave to travel 5,000 km?
minutes
(c) How long does it take an L-wave to travel 5,000 km?
minutes
(d) How long does it take an R-wave to travel 5,000 km?
minutes
Section:
Date:
282 CHAPTER 11 EARTHQUAKES AND SEISMOLOGY