chapter
83
Earthquakes and Volcanoes
1
section ●
Earthquakes
Before You Read
Have you ever experienced an earthquake or perhaps seen
an earthquake in a movie or on TV? On the lines below,
describe an earthquake.
Read to Learn
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
What causes earthquakes?
Have you ever bent a stick? If so, you probably noticed
that it changes shape while you are bending it and springs
back when you stop. But if you don’t stop bending the stick,
it changes permanently. If its elastic limit is passed, the stick
may break. As it breaks, you can feel vibrations in the stick.
What You’ll Learn
■
how energy from an
earthquake reaches
points on Earth’s surface
■ about primary,
secondary, and surface
waves produced by
earthquakes
■ how to recognize
earthquake hazards and
prepare for them
Underline As you read this
section, underline the key terms
and main ideas in each
paragraph to help you
understand new information.
What is elastic rebound?
Rocks are like other solid materials. If enough force pulls
or pushes on them, they will change shape. They may even
break. After breaking, the ends of the broken pieces may
snap back. This snapping back is called elastic rebound.
Inside Earth, pushing and pulling forces cause rocks to
change shape slowly over time. As they are strained, potential
energy builds up in them. This energy is released suddenly
when the rocks finally break or move. The breaking and the
movement that follows causes vibrations that move through
rock. If they are strong enough, the vibrations are felt as
earthquakes. An earthquake is a movement of the ground
that occurs when rocks inside Earth pass their elastic limit,
break suddenly, and experience elastic rebound.
A Organize Information
●
Construct a three-tab Foldable
as shown. Under the tabs, write
information about faults.
Reading Essentials
101
Types of Faults
Normal fault
Reverse fault
1.
Determine Trace over
the arrows in each figure.
What do the arrows show?
What is a fault?
When part of a rock breaks, rocks on either side move as
a result of elastic rebound. The surface where rocks break
and move is called a fault. Rocks can break in different
ways, depending on the forces that cause the break. The
figures above show three different faults, a normal fault, a
reverse fault, and a strike-slip fault.
What are the types of faults?
Sometimes rocks are pulled apart because of tension
forces. This is what occurs in a normal fault, as shown in the
normal fault figure. Normal faults form where tension forces
pull rocks apart, and the rock above the fault moves down.
Sometimes rocks are pushed together, or compressed.
Reverse faults are caused by compression. This is shown in
the reverse fault figure. When the two rocks push together,
rock above the fault is pushed up.
Sometimes sections of rock move past one another in
opposite directions along Earth’s surface as shown in the
strike-slip fault figure. This is called shearing. Strike-slip
faults are caused by shear forces.
102
Earthquakes and Volcanoes
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Picture This
Strike-slip fault
Making Waves
Do you recall the last time you shouted for a friend to
save you a seat on the bus? When you called out, energy
traveled through the air in the form of sound waves. These
sound waves were released by your vocal cords and were
affected by your tongue and mouth. The sound waves
traveled from your mouth outward through the air to your
friend. Your friend identified the familiar sound of your
voice as belonging to you. Sound is a form of energy that
travels through the air in waves.
Earthquakes also release energy in waves. Earthquake
waves move through material inside Earth and along
Earth’s surface. Waves from an earthquake are called
seismic waves.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
How does energy from an earthquake reach
Earth’s surface?
Movement along a fault releases energy causing
vibrations. When this energy is released, it moves away from
the fault in the form of seismic waves. The point deep
inside Earth where energy is released causing an earthquake
is a focus. Some of the energy from the earthquake travels
straight up to Earth’s surface where it can be felt. The
epicenter is the point on Earth’s surface directly above the
earthquake focus. This release of energy can be seen in the
figure below.
2.
Identify What are waves
from an earthquake called?
Picture This
3.
Label Add the labels
epicenter and seismic wave
to the figure.
1.
Fault
Focus
2.
Reading Essentials
103
How do seismic waves travel?
When seismic waves leave the focus of an earthquake,
some travel through Earth’s interior, and others travel along
the surface.
Primary Waves The seismic waves that travel fastest
through rock material are primary waves, or P-waves.
Primary waves cause the material to move from side to side,
in the same direction that the wave is moving.
Secondary waves Other seismic waves that travel through
Earth’s interior are called secondary waves. Secondary waves,
or S-waves, do not move as fast as P-waves. As they move
through rock material, they cause the material to vibrate at
right angles to the direction of the wave.
Determine What two
types of seismic waves
travel through Earth’s
interior?
Learning from Earthquakes
On your way to lunch, suppose you walk twice as fast as
your friend does. What would happen to the distance
between the two of you? The distance between you would
become greater the farther you walk, and you would arrive
first. When scientists study seismic waves, they apply the
same principle. Scientists look at the different speeds of
seismic waves and at the different arrival times to figure out
the exact location of an earthquake’s epicenter.
How are earthquakes measured?
5.
Describe What do
seismologists do?
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Earthquakes and Volcanoes
Scientists who study earthquakes are called seismologists.
They use instruments called seismographs to record seismic
waves. One type of seismograph has a drum that holds a roll
of paper on a frame. A pendulum with an attached pen hangs
over the frame. When seismic waves reach the station, the
drum vibrates. The pen on the pendulum traces a record of
the vibration. The height of the lines traced on the paper
measures the magnitude of the earthquake. Magnitude is
the measure of energy released by an earthquake.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
4.
Surface waves Seismic waves that travel along Earth’s
surface are called surface waves. They are the largest and
slowest type of seismic wave. They cause more damage than
other types of seismic waves. Surface waves move in different
ways. When some surface waves travel over Earth’s surface,
they may move the rock and soil in a backward rolling motion.
They move across land like waves of water. Some surface
waves shake or sway the rock and soil from side to side.
Travel time (min)
16
15
14
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12
11
10
9
8
7
6
5
4
3
2
1
0
Picture This
6.
First S wave
ve
ur
c
ve
wa
S- First P wave
5 min
Interpret Look at the
seismograph reading. How
far away was the
earthquake epicenter from
this location? How many
minutes apart did the
waves arrive?
ve
cur
e
v
a
P-w
1,000 2,000 3,000 4,000
Distance to epicenter (km)
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
How is an epicenter located?
The epicenter of an earthquake is the point on the surface
of Earth directly above the focus. Far away from the epicenter,
the P-waves and S-waves arrive at different times. But close
to the epicenter, the waves arrive at almost the same time.
This difference in arrival time is shown in the graph above.
Once scientists know the P-wave and S-wave arrival times
for at least three seismograph stations, they can figure out
the location of an earthquake’s epicenter. They draw circles
on a map. Each circle shows the distance from a
seismograph station to the earthquake. The point where
three or more circles intersect is the location of the
epicenter. Look at the map below. The three seismographic
stations are in Tokyo, Berkeley, and Anchorage. The
epicenter is the point where the three circles intersect.
Picture This
7.
Interpret On the map,
which seismograph station
is closest to the epicenter of
the earthquake?
B
Reading Essentials
105
How strong are earthquakes?
Some earthquakes are not felt on the surface of Earth.
People do not even know these small earthquakes are
happening. Larger earthquakes, on the other hand, can
cause major damage.
What is the Richter scale?
Applying Math
8.
Calculate How much
more energy is released
from an earthquake with a
magnitude of 8 than from
an earthquake with a
magnitude of 6? Show your
work in the box.
Richter (RIHK tur) magnitude is based on the
measurements of heights of seismic waves as they are
recorded on seismographs. Scientists use this information to
determine the Richter magnitude of an earthquake. Richter
magnitude describes how much energy an earthquake
releases. Very weak earthquakes have low magnitudes like
1.0 on the Richter scale. Strong earthquakes have high
magnitudes in the range of 6 to 7. For every increase of 1.0
on the Richter scale, an earthquake actually releases 32 times
more energy. This means that an earthquake with a
magnitude of 7.5 releases 32 times more energy than an
earthquake with a magnitude of 6.5.
Another way to measure earthquakes is by the modified
Mercalli intensity scale. This scale measures the intensity of an
earthquake. Intensity is a measure of the amount of damage
to structures and to rocks and soil in a specific area. The
amount of damage depends on how strong the earthquake is,
kinds of structures in an area, distance from the earthquake’s
epicenter, and the nature of the surface material.
The Mercalli scale uses Roman numerals I through XII.
An earthquake with an intensity of I would be felt by few
people. An intensity-VI earthquake would be felt by
everyone. An intensity-XII earthquake would cause major
damage to Earth’s surface and to human-built structures.
What are tsunamis?
9.
Identify What are
powerful seismic sea waves
called?
106
Earthquakes and Volcanoes
When an earthquake occurs on the ocean floor, powerful
waves are produced. These waves travel outward from the
earthquake in all directions. A powerful seismic sea wave is
called a tsunami. Tsunamis traveling in open ocean water
are low and fast moving. But tsunamis change as they
approach land. The speed of the tsunami slows and the
height of the wave increases. Huge tsunami waves can be up
to 30 m high. These huge waves can cause large amounts of
damage along coastal areas.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
How is earthquake intensity measured?
Earthquake Safety
You have read about the destruction earthquakes can
cause. If the area you are in has had earthquakes before,
chances are it will again. You can prepare for that.
What measures can you take to be safer?
There are things you can do to make your home safe. Put
heavy things on lower shelves so they won’t fall on you.
Make sure your gas hot-water heater and appliances are well
secured. There are now sensors that can be placed on gas
lines. The sensors shut off the gas when the vibrations of an
earthquake are felt. This helps prevent fires.
During an earthquake, stay away from windows and
anything that could fall on you. Sometimes the safest thing
is to get outside. But then you must watch for fallen power
lines and fire hazards.
10.
Explain What is one
thing you can do to make
your home safer during an
earthquake?
11.
Describe one thing that
can be done to make
buildings and structures
more seismic-safe.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
How do people build seismic-safe structures?
Seismic-safe structures are buildings that can stand up
against the vibrations caused by most earthquakes. It is not
possible to prevent earthquakes, but structures can be built
that withstand them. California has many regulations for
making sure that buildings are seismic-safe. Many high-rise
office buildings now stand on huge steel-and-rubber
supports. This can help buildings to ride out the vibrations
of an earthquake. Underground water and gas pipes are
replaced with pipes that will bend during an earthquake.
This can help prevent broken gas lines and therefore prevent
damage from fires. Highways and bridges are built with
spiral rods in the concrete columns. These spiral rods
reinforce the structure, and keep it from collapsing.
Can earthquakes be predicted?
Right now, it is impossible to predict the exact time and
place an earthquake will occur. However, scientists do know
which locations are more likely to have earthquakes. In
these places, they monitor the movement along faults to see
when an earthquake might occur. They also watch the
groundwater levels and electrical properties in rocks. It is
possible to predict that an earthquake of a certain
magnitude will probably occur in a certain location within
the next 30 to 100 years. But it is difficult to predict an
earthquake more exactly than that.
Reading Essentials
107
After You Read
Mini Glossary
earthquake: movement of the ground that occurs when
rocks inside Earth pass their elastic limit, break suddenly,
and experience rebound
epicenter: point on Earth’s surface directly above the
earthquake’s focus
fault: fracture that occurs where rocks break which results in
movement of opposing sides
focus: point deep inside Earth where energy is released,
causing an earthquake
magnitude: measure of energy released by an earthquake
seismic-safe: describes the ability of structures to stand up
against vibrations caused by an earthquake
seismic waves: earthquake waves including primary waves,
secondary waves, and surface waves
seismograph: instrument used to record seismic waves
tsunami: powerful seismic sea wave that begins over an
ocean-floor earthquake
1. Review the terms and their definitions in the Mini Glossary. Write two sentences
explaining how earthquakes are measured. Use at least three glossary words.
____________________________ waves
___________________ waves
• P-waves
• Travel fastest through rock
___________________ waves
• S-waves
• Travel slower through rock
___________________ waves
• Slowest waves
• Largest waves
• Cause most destruction
• Travel on Earth’s surface
3. You underlined key terms and main ideas in each paragraph in this section. Did this
strategy help you learn more about earthquakes? Would you use it again?
End of
Section
108
Earthquakes and Volcanoes
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2. Fill in the boxes with the correct terms.