CHAPTER 14 Earthquakes and Earth’s Interior
346
In addition, each unit of Richter
magnitude equates to roughly a 32-fold
energy increase. Thus, an earthquake with
a magnitude of 6.5 releases 32 times more
energy than one with a magnitude of 5.5,
and roughly 1000 times (
) more
energy than a 4.5-magnitude quake.
Furthermore, a major earthquake with a
magnitude of 8.5 releases millions of times
more energy than the smallest earthquakes
felt by humans (Figure 14.15).
Although the Richter scale has no
upper limit, the largest magnitude recorded
was 8.9. Great shocks such as these release
an amount of energy that is roughly equivalent to the detonation of 1 billion tons of
explosives. Conversely, earthquakes with
a Richter magnitude of less than 2.0 are
generally not felt by humans.
Richter’ s original goal was modest in
that he only attempted to rank shallow
earthquakes in southern California into
groups of large, medium, and small magnitude. Hence, Richter magnitude was
designed to classify relatively local earthquakes and is designated by the symbol
(M L )—where M is for magnitude and L is
for local.
The convenience of describing the size
of an earthquake by a single number that
can be calculated quickly from seismograms makes the Richter scale a powerful
tool. Further, unlike intensity scales that
can only be applied to populated areas of
the globe, Richter magnitudes can be
assigned to earthquakes in more remote
regions and even to events that occur in the
ocean basins. In time, seismologists modified Richter’ s work and developed new
Richter-like magnitude scales.
Despite its usefulness, the Richter scale
is not adequate for describing very large
earthquakes. For example, the 1906 San
Francisco earthquake and the 1964 Alaskan
earthquake had roughly the same Richter
magnitudes. However, based on the relative
32 * 32
size of the affected areas and the associated
tectonic changes, the Alaskan earthquake
released considerably more energy than the
San Francisco quake. As a result, the
Richter scale is said to be saturated for
major earthquakes because it cannot
distinguish among them.
MOMENT MAGNITUDE. In recent years,
seismologists have come to favor a newer
measure called moment magnitude (M W ),
which determines the strain energy
released along the entire fault surface.
Because moment magnitude estimates the
total energy released, it is better for measuring or describing very large earthquakes.
In light of this, seismologists have recalculated the magnitudes of older, strong
earthquakes using the moment magnitude
scale. For example, the 1964 Alaskan
earthquake was originally given a Richter
magnitude of 8.3, but a recent recalculation using the moment magnitude scale
resulted in an upgrade to 9.2. Similarly,
the 1906 San Francisco earthquake, which
had a Richter magnitude of 8.3, was
downgraded to a M W 7.9. The strongest
earthquake on record is the 1960 Chilean
subduction zone earthquake, with a
moment magnitude of 9.5.
Moment magnitude can be calculated
from geologic fieldwork by measuring the
average amount of slip on the fault, the area
of the fault surface that slipped, and the
strength of the faulted rock. The area of
the fault plane can be roughly calculated
by multiplying the surface-rupture length
by the depth of the aftershocks. This
method is most effective for determining
the magnitude of large earthquakes
generated along large faults in which the
ruptures reach the surface. Moment
magnitude can also be calculated using
data from seismograms.
C O N C E P T C H E C K 1 4 . 6
What information does the Modified
Mercalli Intensity Scale provide about an
earthquake?
An earthquake measuring 7.0 on the
Richter scale releases about ______ times
more energy than an earthquake with
a magnitude of 6.0.
Why is the moment magnitude scale
favored over the Richter scale?
3
2
1
Earthquake Belts
and Plate Boundaries
About 95 percent of the energy released by
earthquakes originates in the few relatively
narrow zones shown in FIGURE 14.16. The
zone of greatest seismic activity, called the
circum-Pacific belt, encompasses the coastal
regions of Chile, Central America, Indonesia, Japan, and Alaska, including the
Aleutian Islands (Figure 14.16). Most
earthquakes in the circum-Pacific belt
occur along convergent plate boundaries
where one plate slides at a low angle
beneath another. The zone of contact
between the subducting and overlying
plates forms a huge fault called a
megathrust, along which Earth’ s largest
earthquakes are generated. Because subduction zone earthquakes usually happen
beneath the ocean they may also generate
destructive waves called tsunami. For
example, the 2004 quake off the coast of
Sumatra produced a tsunami that claimed
an estimated 230,000 lives.
Another major concentration of strong
seismic activity, referred to as the AlpineHimalayan belt, runs through the mountainous regions that flank the Mediterranean
Sea and extends past the Himalayan Mountains (see Figure 14.16). Tectonic activity in
this region is mainly attributed to the collision of the African plate with Eurasia and
the collision of the Indian plate with southeast Asia. These plate interactions created
many faults that remain active. In addition,
numerous faults located away from these
plate boundaries have been reactivated as
India continues its northward advance into
Asia. For example, slippage on a complex
fault system in 2008 in the Sichuan
Province of China killed at least 70,000
people and left 1.5 million others homeless.
The “culprit” is the Indian subcontinent,
which shoves the Tibetan Plateau northeastward against the rocks of the Sichuan
Basin.
Figure 14.16 shows another continuous earthquake belt that extends for
thousands of kilometers through the
world’ s oceans. This zone coincides with
the oceanic ridge system, which is an area
of frequent but low-intensity seismic activity. As tensional forces pull the plates apart
during seafloor spreading, displacement
D I D Y O U K N O W ?
During the 1811–1812 New Madrid
earthquake, the ground subsided as much
as 15 feet and created Lake St. Francis
west of the Mississippi and enlarged
Reelfoot Lake to the east. Other regions
rose, creating temporary waterfalls in the
bed of the Mississippi River.
346
In addition, each unit of Richter
magnitude equates to roughly a 32-fold
energy increase. Thus, an earthquake with
a magnitude of 6.5 releases 32 times more
energy than one with a magnitude of 5.5,
and roughly 1000 times (
) more
energy than a 4.5-magnitude quake.
Furthermore, a major earthquake with a
magnitude of 8.5 releases millions of times
more energy than the smallest earthquakes
felt by humans (Figure 14.15).
Although the Richter scale has no
upper limit, the largest magnitude recorded
was 8.9. Great shocks such as these release
an amount of energy that is roughly equivalent to the detonation of 1 billion tons of
explosives. Conversely, earthquakes with
a Richter magnitude of less than 2.0 are
generally not felt by humans.
Richter’ s original goal was modest in
that he only attempted to rank shallow
earthquakes in southern California into
groups of large, medium, and small magnitude. Hence, Richter magnitude was
designed to classify relatively local earthquakes and is designated by the symbol
(M L )—where M is for magnitude and L is
for local.
The convenience of describing the size
of an earthquake by a single number that
can be calculated quickly from seismograms makes the Richter scale a powerful
tool. Further, unlike intensity scales that
can only be applied to populated areas of
the globe, Richter magnitudes can be
assigned to earthquakes in more remote
regions and even to events that occur in the
ocean basins. In time, seismologists modified Richter’ s work and developed new
Richter-like magnitude scales.
Despite its usefulness, the Richter scale
is not adequate for describing very large
earthquakes. For example, the 1906 San
Francisco earthquake and the 1964 Alaskan
earthquake had roughly the same Richter
magnitudes. However, based on the relative
32 * 32
size of the affected areas and the associated
tectonic changes, the Alaskan earthquake
released considerably more energy than the
San Francisco quake. As a result, the
Richter scale is said to be saturated for
major earthquakes because it cannot
distinguish among them.
MOMENT MAGNITUDE. In recent years,
seismologists have come to favor a newer
measure called moment magnitude (M W ),
which determines the strain energy
released along the entire fault surface.
Because moment magnitude estimates the
total energy released, it is better for measuring or describing very large earthquakes.
In light of this, seismologists have recalculated the magnitudes of older, strong
earthquakes using the moment magnitude
scale. For example, the 1964 Alaskan
earthquake was originally given a Richter
magnitude of 8.3, but a recent recalculation using the moment magnitude scale
resulted in an upgrade to 9.2. Similarly,
the 1906 San Francisco earthquake, which
had a Richter magnitude of 8.3, was
downgraded to a M W 7.9. The strongest
earthquake on record is the 1960 Chilean
subduction zone earthquake, with a
moment magnitude of 9.5.
Moment magnitude can be calculated
from geologic fieldwork by measuring the
average amount of slip on the fault, the area
of the fault surface that slipped, and the
strength of the faulted rock. The area of
the fault plane can be roughly calculated
by multiplying the surface-rupture length
by the depth of the aftershocks. This
method is most effective for determining
the magnitude of large earthquakes
generated along large faults in which the
ruptures reach the surface. Moment
magnitude can also be calculated using
data from seismograms.
C O N C E P T C H E C K 1 4 . 6
What information does the Modified
Mercalli Intensity Scale provide about an
earthquake?
An earthquake measuring 7.0 on the
Richter scale releases about ______ times
more energy than an earthquake with
a magnitude of 6.0.
Why is the moment magnitude scale
favored over the Richter scale?
3
2
1
Earthquake Belts
and Plate Boundaries
About 95 percent of the energy released by
earthquakes originates in the few relatively
narrow zones shown in FIGURE 14.16. The
zone of greatest seismic activity, called the
circum-Pacific belt, encompasses the coastal
regions of Chile, Central America, Indonesia, Japan, and Alaska, including the
Aleutian Islands (Figure 14.16). Most
earthquakes in the circum-Pacific belt
occur along convergent plate boundaries
where one plate slides at a low angle
beneath another. The zone of contact
between the subducting and overlying
plates forms a huge fault called a
megathrust, along which Earth’ s largest
earthquakes are generated. Because subduction zone earthquakes usually happen
beneath the ocean they may also generate
destructive waves called tsunami. For
example, the 2004 quake off the coast of
Sumatra produced a tsunami that claimed
an estimated 230,000 lives.
Another major concentration of strong
seismic activity, referred to as the AlpineHimalayan belt, runs through the mountainous regions that flank the Mediterranean
Sea and extends past the Himalayan Mountains (see Figure 14.16). Tectonic activity in
this region is mainly attributed to the collision of the African plate with Eurasia and
the collision of the Indian plate with southeast Asia. These plate interactions created
many faults that remain active. In addition,
numerous faults located away from these
plate boundaries have been reactivated as
India continues its northward advance into
Asia. For example, slippage on a complex
fault system in 2008 in the Sichuan
Province of China killed at least 70,000
people and left 1.5 million others homeless.
The “culprit” is the Indian subcontinent,
which shoves the Tibetan Plateau northeastward against the rocks of the Sichuan
Basin.
Figure 14.16 shows another continuous earthquake belt that extends for
thousands of kilometers through the
world’ s oceans. This zone coincides with
the oceanic ridge system, which is an area
of frequent but low-intensity seismic activity. As tensional forces pull the plates apart
during seafloor spreading, displacement
D I D Y O U K N O W ?
During the 1811–1812 New Madrid
earthquake, the ground subsided as much
as 15 feet and created Lake St. Francis
west of the Mississippi and enlarged
Reelfoot Lake to the east. Other regions
rose, creating temporary waterfalls in the
bed of the Mississippi River.