The various earthquake waves are created the moment rocks begin to break, and emanate from the
focus outward.
Due to the differences in the methods of propagation, they travel at different
speeds, and thus reach seismographs at different times.
The length of time it takes for one of these
waves to reach the seismograph is called its
“
travel time
”.
Figure 9.40 is an example of a
seismograph recording of an earthquake plotted on a P and S wave travel time chart.
In this
example, the earthquake occurred some 250 km from the recording station.
The first wave of the
earthquake was recorded about 38 minutes after the earthquake.
Because the S wave is slower, it
was recorded at the station 70 minutes after the earthquake.
FIGURE 9.39
The Propagation of Love Waves
The brown curved block moves back and forth with
a horizontal motion between the light blue parallel planes.
Illustration by Stan Celestian
TIME IN MINUTES
DISTANCE IN KILOMETERS TO THE EPICENTER
10
20
30
40
50
60
70
80
90
100
110
100
200
300
400
500
600
700
800
EPICENTER
TIME ZERO
and
DISTANCE
ZERO
FIGURE 9.40
The Relationship Between the Travel Times of the P and S waves.
Illustration by Stan Celestian
THE PROPAGATION OF LOVE WAVES

However, in the vast majority of cases, the distance to the earthquake is unknown.
Knowing the
differences in the P and S wave travel times, the distance can be ascertained using this chart.
For
example, if the P and S wave are separated by 20 minutes, the distance to the earthquake is about
160 km.
This is how it is determined using this chart:
On the chart of Figure 9.41, a vertical black
line is drawn in the left column, just above the blue box labeled EPICENTER.
That vertical black line
starts at zero minutes and ends at 20 minutes.
That 20 minute vertical line is then placed between
the S wave line and the P wave line so that it accurately fits the space separating the S wave line
and the P wave line (as shown by the yellow arrow).
The position on the graph, where the P wave
line and the S wave line are
separated by 20 minutes, corresponds to a distance to the epicenter of
160 km.
What would the distance to the epicenter be if the travel time difference were 47 minutes?
Create a
vertical line that is 47 minutes long in the TIME IN MINUTES part of the chart.
Then move that line
to the chart where that still vertical line fits accurately between the P and S wave lines.
(The answer
is found next to Figure 9.45, Preliminary Earthquake
Report, in
red.)
One seismograph can then be used to determine the
distance to an earthquake.
If an earthquake is
located 160 km from a seismograph, a circle with a
160 km radius is drawn on a map.
The epicenter is
located somewhere on that circle.
Figure 9.42 is a
map of the southwestern United States.
On the map
is a red
circle with a radius of 160 km centered on
Phoenix,
Arizona.
From
just
the
seismograph
information from Phoenix, all that can be determined
is that the epicenter lies somewhere on the perimeter
of that red circle.
In order to find the exact epicenter,
a process of triangulation is required.
The

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