CHAPTER 14 Earthquakes and Earth’s Interior
358
C H A P T E R
F O U R T E E N
Earthquakes and Earth’ s Interior
in Review
Earthquakes are vibrations of Earth produced by the rapid release
of energy from rocks that rupture because they have been subjected to
stresses that exceed their strength. This energy, which takes the form
of seismic waves, radiates in all directions from the earthquake’ s
source, called the focus. The movements that produce most large
earthquakes occur along large fractures, called faults, that are usually
associated with plate boundaries.
Along a fault, rocks store energy as they are bent. As slippage occurs
at the weakest point (the focus), displacement will exert stress farther
along a fault, where additional slippage will occur until most of the
built-up strain is released. An earthquake occurs as the rock elastically
returns to its original shape. The “springing back” of the rock is termed
elastic rebound. Small earthquakes, called foreshocks, often precede a
major earthquake. The adjustments that follow a major earthquake
often generate smaller earthquakes called aftershocks.
Two main types of seismic waves are generated during an earthquake: (1) surface waves, which travel along the outer layer of Earth,
and (2) body waves, which travel through Earth’ s interior. Body waves
are further divided into primary (P) waves, which push (squeeze) and
pull (stretch) rocks in the direction the wave is traveling, and
secondary (S) waves, which “shake” the particles in rock at right angles
to their direction of travel. P waves can travel through solids, liquids,
and gases. Fluids (gases and liquids) will not transmit S waves. In any
solid material, P waves travel about 1.7 times faster than S waves.
The location on Earth’ s surface directly above the focus of an earthquake is the epicenter. Using the difference in arrival times between P
and S waves, the distance separating a recording station from the
earthquake epicenter can be determined. When the distances are
known from three or more seismic stations, the epicenter can be
located using a method called triangulation.
Seismologists use two fundamentally different measures to describe
the size of an earthquake—intensity and magnitude. Intensity is a
measure of the degree of ground shaking at a given locale based on
the observed effects. The Modified Mercalli Intensity Scale is divided
into 12 levels of severity based on observed effects such as people
awakened from sleep, furniture moving, plaster cracking and falling,
and finally—total destruction. Magnitude is calculated from seismic
records and estimates the amount of energy released at the source of
an earthquake. Using the Richter scale, the magnitude of an earthquake is estimated by measuring the amplitude (maximum displacement) of the largest seismic wave recorded. A logarithmic scale is
used to express magnitude, in which a tenfold increase in ground
shaking corresponds to an increase of 1 on the magnitude scale.
Moment magnitude is currently used to estimate the size of moderate
and large earthquakes. It can be calculated using the amount of slip
on the fault surface, the area of the fault surface, and the strength of
the faulted rock.
A close correlation exists between earthquake epicenters and plate
boundaries. The greatest energy is released by earthquakes along the
margin of the Pacific Ocean, known as the circum-Pacific belt, and the
mountainous regions that flank the Mediterranean Sea and continue
past the Himalayan complex. Another zone of comparatively weak
seismicity runs through the world’ s oceans along the oceanic ridge
system.
The primary factors that determine the amount of destruction
accompanying an earthquake are the magnitude of the earthquake and
the proximity of the quake to a populated area. Structural damage
attributable to ground shaking depends on several factors, including
(1) the intensity and (2) the duration of ground shaking, (3) the nature
of the material upon which the structure rests, and (4) the design of the
structure. Secondary effects of earthquakes include landslides, ground
subsidence, fire, and tsunami damage.
Substantial research to predict earthquakes is underway in Japan,
the United States, China, and Russia—countries where earthquake
risk is high. No reliable method of short-range prediction has yet
been devised. Long-range forecasts are based on the premise that
earthquakes are repetitive or cyclical. Seismologists study the history
of earthquakes for patterns so their occurrences may be predicted.
Long-range forecasts are important because they provide information
used to develop the Uniform Building Code and to assist in land-use
planning.
As indicated by the behavior of P and S waves as they travel
through Earth, the four major zones of Earth’ s interior are: (1) crust
(the very thin outer layer, 5 to 40 kilometers); (2) mantle (a rocky
layer located below the crust with a thickness of 2900 kilometers);
(3) outer core (a layer about 2270 kilometers thick, which exhibits the
characteristics of a mobile liquid); and (4) inner core (a solid metallic
sphere with a radius of about 1216 kilometers).
Key Terms
aftershocks (p. 338)
asthenosphere (p. 357)
body waves (p. 341)
core (p. 357)
crust (p. 356)
earthquakes (p. 336)
elastic rebound (p. 338)
epicenter (p. 336)
faults (p. 336)
focus (p. 336)
foreshocks (p. 339)
inner core (p. 357)
intensity (p. 343)
liquefaction (p. 348)
lithosphere (p. 357)
magnitude (p. 343)
mantle (p. 357)
Modified Mercalli Intensity
Scale (p. 343)
moment magnitude (p. 346)
outer core (p. 357)
primary (P) waves (p. 341)
Richter scale (p. 345)
secondary (S) waves (p. 341)
seismic gaps (p. 354)
seismic sea waves (p. 350)
seismic waves (p. 336)
seismograms (p. 340)
seismographs (p. 339)
seismology (p. 339)
surface waves (p. 341)
tsunami (p. 350)
358
C H A P T E R
F O U R T E E N
Earthquakes and Earth’ s Interior
in Review
Earthquakes are vibrations of Earth produced by the rapid release
of energy from rocks that rupture because they have been subjected to
stresses that exceed their strength. This energy, which takes the form
of seismic waves, radiates in all directions from the earthquake’ s
source, called the focus. The movements that produce most large
earthquakes occur along large fractures, called faults, that are usually
associated with plate boundaries.
Along a fault, rocks store energy as they are bent. As slippage occurs
at the weakest point (the focus), displacement will exert stress farther
along a fault, where additional slippage will occur until most of the
built-up strain is released. An earthquake occurs as the rock elastically
returns to its original shape. The “springing back” of the rock is termed
elastic rebound. Small earthquakes, called foreshocks, often precede a
major earthquake. The adjustments that follow a major earthquake
often generate smaller earthquakes called aftershocks.
Two main types of seismic waves are generated during an earthquake: (1) surface waves, which travel along the outer layer of Earth,
and (2) body waves, which travel through Earth’ s interior. Body waves
are further divided into primary (P) waves, which push (squeeze) and
pull (stretch) rocks in the direction the wave is traveling, and
secondary (S) waves, which “shake” the particles in rock at right angles
to their direction of travel. P waves can travel through solids, liquids,
and gases. Fluids (gases and liquids) will not transmit S waves. In any
solid material, P waves travel about 1.7 times faster than S waves.
The location on Earth’ s surface directly above the focus of an earthquake is the epicenter. Using the difference in arrival times between P
and S waves, the distance separating a recording station from the
earthquake epicenter can be determined. When the distances are
known from three or more seismic stations, the epicenter can be
located using a method called triangulation.
Seismologists use two fundamentally different measures to describe
the size of an earthquake—intensity and magnitude. Intensity is a
measure of the degree of ground shaking at a given locale based on
the observed effects. The Modified Mercalli Intensity Scale is divided
into 12 levels of severity based on observed effects such as people
awakened from sleep, furniture moving, plaster cracking and falling,
and finally—total destruction. Magnitude is calculated from seismic
records and estimates the amount of energy released at the source of
an earthquake. Using the Richter scale, the magnitude of an earthquake is estimated by measuring the amplitude (maximum displacement) of the largest seismic wave recorded. A logarithmic scale is
used to express magnitude, in which a tenfold increase in ground
shaking corresponds to an increase of 1 on the magnitude scale.
Moment magnitude is currently used to estimate the size of moderate
and large earthquakes. It can be calculated using the amount of slip
on the fault surface, the area of the fault surface, and the strength of
the faulted rock.
A close correlation exists between earthquake epicenters and plate
boundaries. The greatest energy is released by earthquakes along the
margin of the Pacific Ocean, known as the circum-Pacific belt, and the
mountainous regions that flank the Mediterranean Sea and continue
past the Himalayan complex. Another zone of comparatively weak
seismicity runs through the world’ s oceans along the oceanic ridge
system.
The primary factors that determine the amount of destruction
accompanying an earthquake are the magnitude of the earthquake and
the proximity of the quake to a populated area. Structural damage
attributable to ground shaking depends on several factors, including
(1) the intensity and (2) the duration of ground shaking, (3) the nature
of the material upon which the structure rests, and (4) the design of the
structure. Secondary effects of earthquakes include landslides, ground
subsidence, fire, and tsunami damage.
Substantial research to predict earthquakes is underway in Japan,
the United States, China, and Russia—countries where earthquake
risk is high. No reliable method of short-range prediction has yet
been devised. Long-range forecasts are based on the premise that
earthquakes are repetitive or cyclical. Seismologists study the history
of earthquakes for patterns so their occurrences may be predicted.
Long-range forecasts are important because they provide information
used to develop the Uniform Building Code and to assist in land-use
planning.
As indicated by the behavior of P and S waves as they travel
through Earth, the four major zones of Earth’ s interior are: (1) crust
(the very thin outer layer, 5 to 40 kilometers); (2) mantle (a rocky
layer located below the crust with a thickness of 2900 kilometers);
(3) outer core (a layer about 2270 kilometers thick, which exhibits the
characteristics of a mobile liquid); and (4) inner core (a solid metallic
sphere with a radius of about 1216 kilometers).
Key Terms
aftershocks (p. 338)
asthenosphere (p. 357)
body waves (p. 341)
core (p. 357)
crust (p. 356)
earthquakes (p. 336)
elastic rebound (p. 338)
epicenter (p. 336)
faults (p. 336)
focus (p. 336)
foreshocks (p. 339)
inner core (p. 357)
intensity (p. 343)
liquefaction (p. 348)
lithosphere (p. 357)
magnitude (p. 343)
mantle (p. 357)
Modified Mercalli Intensity
Scale (p. 343)
moment magnitude (p. 346)
outer core (p. 357)
primary (P) waves (p. 341)
Richter scale (p. 345)
secondary (S) waves (p. 341)
seismic gaps (p. 354)
seismic sea waves (p. 350)
seismic waves (p. 336)
seismograms (p. 340)
seismographs (p. 339)
seismology (p. 339)
surface waves (p. 341)
tsunami (p. 350)