Earthquakes and Earth’s Interior - The Changing Earth: Exploring Geology and Evolution - page 196

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Figure 8.8 Rayleigh and Love Seismic Surface Waves Surface waves travel along Earth's surface
or just below it.

Undisturbed material
for reference.

a

Undisturbed material

Rayleigh waves (R-waves) move
material in an elliptical path in
a plane oriented parallel to the
direction of wave movement.

b

Rayleigh wave (R-wave)

Rayleigh wave Love wave

Love wave (L-wave)

The arrival of R- and L-waves causes the
surface to undulate and shake from side to side.

Love waves (L-waves) move material back
and forth in a horizontal plane perpendicular to
the direction of wave movement.

c d

The motion of a Love wave (L-wave) is similar to that of
an S-wave, but the individual particles of the material move
only back and forth in a horizontal plane perpendicular to
the direction of wave travel (Figure 8.8c).

be determined ( Figure 8.10). Here is how it works. Subtract-

ing the arrival time of the fi rst P-wave from the arrival time of
the fi rst S-wave gives the P-S time interval for each seismic sta-
tion. Each of these time intervals is plotted on a time-distance
graph, and a line is drawn straight down to the distance axis
of the graph, thus giving the distance from the focus to each
seismic station (Figure 8.9b). Next, a circle whose radius equals
the distance shown on the time-distance graph from each of
the seismic stations is drawn on a map (Figure 8.10). The inter-
section of the three circles is the location of the earthquake's
epicenter. It should be obvious from Figure 8.10 that P-S time
intervals from at least three seismic stations are needed. If only
one were used, the epicenter could be at any location on the
circle drawn around that station, and using two stations would
give two possible locations for the epicenter.
Determining the focal depth of an earthquake is much
more difficult and considerably less precise than finding
its epicenter. The focal depth is usually found by making
computations based on several assumptions, comparing the
results with those obtained at other seismic stations, and then
recalculating and approximating the depth as closely as possi-
ble. Even so, the results are not highly accurate, but they do tell
us that most earthquakes, probably 75%, have foci no deeper
than 10 to 15 km and that a few are as deep as 680 km.

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LOCATING AN EARTHQUAKE
We mentioned that news articles commonly report an earth-
quake's epicenter, but just how is the location of an epicenter
determined? Once again, geologists rely on the study of seis-
mic waves. We know that P-waves travel faster than S-waves,
nearly twice as fast in all substances, so P-waves arrive at a
seismograph station fi rst, followed some time later by S-waves.
Both P- and S-waves travel directly from the focus to the seismo-
graph station through Earth's interior, but L- and R-waves
arrive last because they are the slowest, and they also travel the
longest route along the surface ( Figure 8.9a). However, only

the P- and S-waves need concern us here because they are the
ones important in fi nding an epicenter.
Seismologists, geologists who study seismology, have ac-
cumulated a tremendous amount of data over the years and
now know the average speeds of P- and S-waves for any specifi c
distance from their source. These P- and S-wave travel times
are published in time-distance graphs that illustrate the differ-
ence between the arrival times of the two waves as a function
of the distance between a seismograph and an earthquake's
focus (Figure 8.9b). That is, the farther the waves travel, the
greater the P-S time interval , which is simply the time differ-
ence between the arrivals of P- and S-waves (Figure 8.9a, b).
If the P-S time intervals are known from at least three
seismograph stations, then the epicenter of any earthquake can

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MEASURING THE STRENGTH
OF AN EARTHQUAKE
Following any earthquake that causes extensive dam-
age, fatalities, and injuries, graphic reports of the quake's