Download Exploring the Earth`s interior with seismic waves

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Exploring the Earth’s interior with seismic waves
T
he push-pull motions of sound waves in a solid
are called compressional waves to distinguish
them from the side-to-side motions of shear waves.
It’s harder to compress solids than to shear them,
so compressional waves always travel faster than
shear waves. Compressional waves are always the
P (primary) arrivals, and shear waves are the S
(secondary) arrivals. Another important property
of seismic waves is that their shear wave speeds
must be zero, because gases and liquids have no
resistance to shear. Shear waves cannot propagate
through any fluid: air, water, or the liquid iron in
Earth’s outer core.
From seismograms, geologists can calculate the
speed of a P or S wave by dividing the distance
traveled by the travel time. The measurements of
these wave speeds can then be used to infer which
materials the waves encountered along their paths.
For example, P and S waves travel about 17 percent
more rapidly through rock typical of oceanic crust
(gabbro) than through typical upper continental
crust (granite), and they travel about 33 percent
more rapidly through the upper mantle (peridotite).
Figure 1 In this experiment, two beams of laser light enter a bowl
of water from the top. Both beams are reflected from a mirror on
the bottom of the bowl. One is then reflected at the water-air interface and passes through the bowl to make a bright spot on the
table. Most of the energy from the other beam is bent downward
(refracted) as it passes from the water to the air, although a small
amount is reflected to form a second spot on the table. You can
also trace the paths of other beams reflected by the interfaces.
(Susan Schwartzenberg/The Exploratorium)
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The concepts of travel times and wave paths
sound simple enough, but complications arise
when waves pass through more than one type of
material. At the boundary between two different
materials, some of the waves bounce off (that is,
they are reflected) and others are transmitted into
the second material, just as light is partly reflected
and partly transmitted when it strikes a windowpane. The waves that cross the boundary between
two materials are bent, or refracted, as their velocity changes from that in the first material to that
in the second. Figure 1 shows a laser light beam
whose path bends as it goes from air into water,
much as a P or an S wave bends as it travels from
one material to another. By studying how fast seismic waves travel and how they are refracted and reflected at Earth’s internal boundaries, seismologists
have been able to measure the layering of Earth’s
crust, mantle, and core with great accuracy.
Paths of Seismic Waves in the Earth
If Earth were made of a single material with
constant properties from the surface to the center, P and S waves would travel from the focus
of an earthquake to a distant seismograph along
straight lines through the interior. When the first
global networks of seismographs were installed
about a century ago, however, seismologists discovered that the structure of Earth’s interior was
much more complicated.
The first observations of long-distance seismic waves showed that the paths of the P and S
waves curved upward through the mantle, as illustrated in figure 2. From the travel times and
the amount of upward bending, seismologists
were able to demonstrate that P waves traveled
much faster through rocks at great depths than
they did through rocks found on the surface. This
was hardly surprising, because rocks subjected
to great pressures in Earth’s interior are squeezed
into tighter crystal structures. The atoms in these
tighter structures are more resistant to further
compression, which causes P waves to travel
through them more quickly.
Seismologists were very surprised, however, by
what they found at progressively greater distances from an earthquake focus. After the P waves
and S waves had traveled beyond about 11 000 km
from the earthquake focus, they suddenly disappeared! Like airplane pilots and ship captains,
seismologists prefer to measure distances traveled on Earth’s surface in angular degrees, from
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0° at the earthquake focus to 180° at a point on
the opposite side of Earth. Each degree measures
111 km at the surface, so 11 000 km corresponds to
an angular distance of 103°. When they looked at
seismograms recorded beyond this distance, they
did not see the distinct P and S arrivals that were
so clear on seismograms recorded at shorter distances. Then, beyond about 16 000 km from the focus (143°), the P waves suddenly reappeared as big
arrivals, but they were much delayed compared
to their expected travel times. The S waves never
reappeared.
These observations were first put together in
1906 by the British seismologist R.D. Oldham, and
they provided the first evidence that Earth has a
liquid outer core. No S waves can travel through
the outer core because it is liquid, and there is
thus an S-wave shadow zone beyond 103° from
the earthquake focus (see figure 2 B). The propagation of P waves is more complicated (see figure 2
A). At 103°, their paths just miss the core, whereas
waves that would have traveled to greater distances encounter the core-mantle boundary. At the
core-mantle boundary, the P-wave speed drops by
almost a factor of two. Therefore, the waves are
refracted downward into the core and emerge at
greater distances after the delay caused by their
detour through the core. This refraction effect
forms a P-wave shadow zone at angular distances
between 103° and 143°.
Focus
Focus
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0
16
km
km
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11
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Inner core
P-wave
shadow
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P-wave
shadow
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103°
Outer core
103°
S-wave
shadow zone
Mantle
143°
Figure 2 (A), the pattern of P-wave paths through Earth’s interior.
The dashed blue lines show the progress of wave fronts through the
interior, at 2 minute intervals. Distances are measured in angular distance from the earthquake focus. The P-wave shadow zone extends
from 103° to 143°. P waves cannot reach the surface within this
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zone because of the way they are bent when they enter and leave the
core. (B), the pattern of S-wave paths through Earth’s interior. The
larger S-wave shadow zone extends from 103° to 180°. Although S
waves strike the core, they cannot travel through its fluid outer region and therefore never emerge beyond 105° from the focus.
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