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Chapter 7
Earthquakes
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Section 1 What Are Earthquakes?
Section 2 Earthquake Measurement
Section 3 Earthquakes and Society
Concept Map
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Chapter 7
Section 1 What Are Earthquakes?
Bellringer
Describe what happens during an earthquake. What do
you think causes earthquakes?
Write your answers in your Science Journal.
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Chapter 7
Section 1 What Are Earthquakes?
What You Will Learn
• Earthquakes are ground motions that result from the
release of energy when blocks of rock move.
• Most earthquakes occur along tectonic plate
boundaries because the movement of tectonic plates
causes stress in Earth’s crust.
• Earthquake energy travels through rock as seismic
waves.
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Chapter 7
Section 1 What Are Earthquakes?
Where Earthquakes Happen
• Earthquakes are movements or shaking of the
ground that happen when blocks of rock move
suddenly and release energy. The transfer of this
energy causes the ground to shake.
• Most earthquakes take place near the boundaries of
tectonic plates. However, some earthquakes have
occurred far from tectonic plate boundaries.
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Chapter 7
Section 1 What Are Earthquakes?
Where Earthquakes Happen, continued
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Chapter 7
Section 1 What Are Earthquakes?
Where Earthquakes Happen, continued
• Tectonic plates move in different directions and at
different speeds.
• Two plates can push toward or pull away from each
other, or slip past each other horizontally.
• These movements break Earth’s crust into a series of
faults.
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Chapter 7
Section 1 What Are Earthquakes?
Faults at Tectonic Plate Boundaries
• A fault is a break in Earth’s crust along which blocks
of rock slide relative to each other.
• Specific types of plate motion take place at different
tectonic boundaries.
• Each type of motion creates a particular kind of fault.
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Chapter 7
Section 1 What Are Earthquakes?
Earthquakes at Divergent Boundaries
• At divergent tectonic plate boundaries, two tectonic
plates pull away from each other.
• Tension causes the lithosphere to break into a series
of fault blocks.
• Some of these block drop down and form a series of
normal faults.
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Chapter 7
Section 1 What Are Earthquakes?
Earthquakes at Divergent Boundaries,
continued
• A mid-ocean ridge is a
divergent boundary.
• Ocean lithosphere is thin
and weak.
• Therefore, earthquakes
that happen along
normal faults are
shallow.
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Chapter 7
Section 1 What Are Earthquakes?
Earthquakes at Convergent Boundaries
• At convergent tectonic plate boundaries, two
tectonic plates collide with one another.
• When plates collide, two things may happen:
• Both plates can crumple up to form
mountains.
• Or one plate can move underneath the other
and sink into the mantle.
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Chapter 7
Section 1 What Are Earthquakes?
Earthquakes at Convergent Boundaries,
continued
• The process of one plate moving under another is
called subduction.
• When plates collide, the two plates are
compressed.
• Compression causes the lithosphere to break into
a series of fault blocks.
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Chapter 7
Section 1 What Are Earthquakes?
Earthquakes at Convergent Boundaries,
continued
• Fault blocks are thrust over one another as the
plate moves.
• The blocks form a series of reverse faults.
Earthquakes happen along reverse faults.
• Two types of earthquakes occur at subduction
zones.
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Chapter 7
Section 1 What Are Earthquakes?
Earthquakes at Convergent Boundaries,
continued
• Earthquakes at depths
of less than 50 km
occur between the two
plates.
• Earthquakes at depths
up to 700 km occur
inside the down-going
plate.
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Chapter 7
Section 1 What Are Earthquakes?
Earthquakes at Transform Boundaries
• At transform boundaries, two plates move past one
another horizontally.
• The rocks on both sides of the fault are sheared or
broken as they grind past each other in opposite
directions.
• The shear stress causes the rock to break into
blocks that form a series of strike-slip faults.
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Chapter 7
Section 1 What Are Earthquakes?
Earthquakes at Transform Boundaries,
continued
• Most transform boundaries exist between plates
made of oceanic lithosphere.
• Some transform boundaries occur between plates
made of continental lithosphere.
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Chapter 7
Section 1 What Are Earthquakes?
Earthquakes at Transform Boundaries,
continued
• Rocks deep below
Earth’s surface tend to
react to shear stress by
folding, not breaking.
• As a result, earthquakes
along strike-slip faults
occur at depths of less
than 50 km.
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Chapter 7
Earthquakes
Plate Motion and Earthquake Characteristics
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Chapter 7
Section 1 What Are Earthquakes?
Fault Zones
• Places along plate boundaries where large
numbers of interconnected faults are located are
called fault zones.
• Faults in fault zones can occur at different depths,
have different lengths, and cut through the
lithosphere in different directions.
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Chapter 7
Section 1 What Are Earthquakes?
Fault Zones, continued
• The San Andreas fault
zone is located along a
transform boundary.
• The fault zone is
primarily a strike-slip, or
transform, fault system.
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Chapter 7
Section 1 What Are Earthquakes?
Why Earthquakes Happen
• As tectonic plates move, stress on rocks causes
the rocks to deform.
• Rocks can deform in a plastic manner, like clay
being molded.
• Folded rocks are a result of plastic deformation.
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Chapter 7
Section 1 What Are Earthquakes?
Why Earthquakes Happen, continued
• Rocks can deform in an elastic manner, like a
rubber band being stretched.
• Elastic deformation leads to earthquakes.
• Just like a rubber band breaking, rock that deforms
in an elastic manner slips and releases energy.
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Chapter 7
Section 1 What Are Earthquakes?
Why Earthquakes Happen, continued
• The sudden return of elastically deformed rock to
its original shape is called elastic rebound.
• Elastic rebound happens when stress on rock
along a fault becomes so great that the rock
breaks.
• Rocks on either side of the fault jerk past each
other and release energy.
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Chapter 7
Section 1 What Are Earthquakes?
Why Earthquakes Happen, continued
• Energy travels through rock as seismic waves.
• Seismic waves cause the ground to move.
• The strength of an earthquake is related to the
amount of energy that is released during elastic
rebound.
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Chapter 7
Section 1 What Are Earthquakes?
Why Earthquakes Happen, continued
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Chapter 7
Section 1 What Are Earthquakes?
Earthquake Waves
• Earthquake waves are the physical result of the
movement of energy through Earth as seismic
waves.
• Seismic waves that travel through Earth’s interior
are called body waves.
• Seismic waves that travel along Earth’s surface are
called surface waves.
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Chapter 7
Section 1 What Are Earthquakes?
Earthquake Waves, continued
• There are two types of body waves: P waves and
S waves.
• P waves, or pressure waves, are the fastest
seismic waves.
• P waves are also called primary waves, because
they are always detected first after an earthquake.
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Chapter 7
Section 1 What Are Earthquakes?
Earthquake Waves, continued
• P waves can travel through solids, liquids, and
gases.
• P waves squeeze and stretch rock as they travel
forward.
• S waves, or shear waves, are the second-fastest
seismic waves.
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Chapter 7
Section 1 What Are Earthquakes?
Earthquake Waves, continued
• S waves are also known as secondary waves,
because they arrive later than P waves after an
earthquake.
• S waves cannot travel through parts of Earth that
are completely liquid.
• S waves shear rock horizontally from side to side.
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Chapter 7
Earthquakes
Body Waves: S Waves and P Waves
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Chapter 7
Section 1 What Are Earthquakes?
Earthquake Waves, continued
• Surface waves travel more slowly than body
waves. They produce motion only near the top of
Earth’s crust.
• Because their energy focuses on the surface,
surface waves tend to cause the most damage.
• Surface waves can travel in an up-and-down or
back-and-forth motion.
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Chapter 7
Section 1 What Are Earthquakes?
Earthquake Waves, continued
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Chapter 7
Section 2 Earthquake Measurement
Bellringer
How do major earthquakes and minor earthquakes
affect the area in which they happen?
What causes the difference in their effects?
Write your answers in your Science Journal.
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Chapter 7
Section 2 Earthquake Measurement
What You Will Learn
• To find an earthquake’s epicenter, you must
triangulate by using data from three or more
seismometers.
• Magnitude is a measure of an earthquake’s strength.
• The intensity of an earthquake depends on four major
factors.
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Chapter 7
Section 2 Earthquake Measurement
Studying Earthquakes
• Scientists use instruments called seismometers, or
seismographs, to record seismic waves.
• Seismometers record the vibrations of P waves, S
waves, and surface waves.
• Seismometers also record the time it takes for waves
to arrive at a seismometer station.
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Chapter 7
Section 2 Earthquake Measurement
Studying Earthquakes, continued
• Seismometers create a tracing of earthquake motion
called a seismogram.
• Seismograms help to locate the earthquakes
epicenter, the point on Earth’s surface directly above
the earthquake’s starting point.
• The earthquake’s starting point inside Earth is called
the focus.
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Chapter 7
Section 2 Earthquake Measurement
Studying Earthquakes, continued
• The earthquake’s
starting point inside
Earth is called the
focus.
• The epicenter is directly
above the focus.
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Chapter 7
Section 2 Earthquake Measurement
Studying Earthquakes, continued
• The lag time between the arrival of P waves and S
waves tells scientists how far the waves have
traveled.
• Scientists draw a circle around a seismometer station
that has a radius equal to the distance the waves
have traveled.
• Scientists draw circles around three seismometer
stations and find the point of intersection.
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Chapter 7
Section 2 Earthquake Measurement
Studying Earthquakes, continued
• The point at which all
circles intersect is the
epicenter.
• This process of locating
the epicenter is called
triangulation.
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Chapter 7
Earthquakes
S-P Time Method: Finding an Epicenter
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Chapter 7
Section 2 Earthquake Measurement
Earthquake Magnitude
• Magnitude is the measure on an earthquake’s
strength.
• The greater the magnitude, the stronger the
earthquake.
• In the past, the Richter scale was used to describe
earthquake strength. Now, scientists use the
magnitude moment scale.
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Chapter 7
Section 2 Earthquake Measurement
Earthquake Magnitude, continued
• The Richter Scale measures ground motion from an
earthquake and adjusts for distance to find an
earthquake’s magnitude.
• Richter-scale values range from 0-9.
• Each increase of one number represents a tenfold
increase in strength.
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Chapter 7
Section 2 Earthquake Measurement
Earthquake Magnitude, continued
• The magnitude moment scale is a more accurate
measure of earthquake strength.
• Magnitude moment (Mw) represents the:
• size of the area of the fault that moves
• average distance moved by fault blocks, and
• rigidity of rocks in the fault zone.
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Chapter 7
Section 2 Earthquake Measurement
Earthquake Intensity
• An earthquake’s intensity is the effect of the
earthquake on people.
• The Modified Mercalli scale describes earthquake
intensity.
• Intensity ranges from barely noticeable to total
destruction of an area.
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Chapter 7
Section 2 Earthquake Measurement
Earthquake Intensity, continued
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Chapter 7
Section 2 Earthquake Measurement
Earthquake Intensity, continued
• Earthquake intensity maps show the level of intensity
expected in different areas that experience the same
earthquake.
• Data from past earthquakes are used to create
earthquake intensity maps.
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Chapter 7
Section 2 Earthquake Measurement
The Effects of Earthquakes
• Effects of earthquakes can vary over a wide area.
• Effects depend on the size of the earthquake.
• Effects also depend on three other factors: distance
from the epicenter, local geology, and type of
construction in the area.
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Chapter 7
Section 2 Earthquake Measurement
The Effects of Earthquakes, continued
Distance from the Epicenter
• The total energy in a seismic wave stays relatively
constant as the wave travels.
• Seismic waves grow increasingly larger as they move
away from the epicenter.
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Chapter 7
Section 2 Earthquake Measurement
The Effects of Earthquakes, continued
• As seismic waves grow larger, the amount of energy
at any one point decreases.
• Therefore, an earthquake is less destructive to areas
that are farther away from the epicenter.
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Chapter 7
Section 2 Earthquake Measurement
The Effects of Earthquakes, continued
Local Geology
• The amount of damage caused by an earthquake
depends on the material through which seismic
waves travel.
• Seismic waves are particularly dangerous when they
travel through water-saturated soil or sediment.
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Chapter 7
Section 2 Earthquake Measurement
The Effects of Earthquakes, continued
• When seismic waves
shake water-saturated
sediment or soils,
sediment grains lose
contact with each other
and are surrounded by
water.
• This process is called
liquefaction.
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Chapter 7
Section 2 Earthquake Measurement
The Effects of Earthquakes, continued
• Liquefaction can intensify ground shaking.
• Liquefaction can also cause the ground to settle,
which can cause structures to tilt or collapse.
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Chapter 7
Section 2 Earthquake Measurement
The Effects of Earthquakes, continued
Earthquake-Resistant Construction
• Brick and concrete structures are easily damaged by
earthquakes.
• Wood and steel structures are more flexible and less
likely to be damaged.
• Shorter buildings, on strong, anchored foundations
are also less likely to be damaged.
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Chapter 7
Section 3 Earthquakes and Society
Bellringer
If you have experienced an earthquake, write a short
paragraph that describes how you felt and what you
did to protect yourself.
If you have not experienced an earthquake, write a
paragraph that describes what you think you would
do and how you think you would feel during a
moderate earthquake.
Write your answers in your Science Journal.
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Chapter 7
Section 3 Earthquakes and Society
What You Will Learn
• The magnitude of earthquakes may be related to how
frequently earthquakes happen.
• Earthquakes and tsunamis can affect human
societies.
• Homes, buildings, and bridges can be strengthened
to decrease earthquake damage.
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Chapter 7
Section 3 Earthquakes and Society
Earthquake Hazard
• Earthquake hazard is a measure of how likely an
area is to have damaging earthquakes in the future.
• Earthquake-hazard level is determined by past
seismic activity.
• California has a very high earthquake-hazard level.
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Chapter 7
Section 3 Earthquakes and Society
Earthquake Hazard, continued
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Chapter 7
Section 3 Earthquakes and Society
Earthquake Forecasting
• Forecasting earthquakes is difficult.
• Scientists study earthquakes to discover patterns in
earthquake strength and frequency.
• The relationship between earthquake strength and
frequency is based on the amount of energy released
during earthquakes.
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Chapter 7
Section 3 Earthquakes and Society
Earthquake Forecasting, continued
• Stronger earthquakes
are much rarer than
weaker earthquakes.
• Millions of small
earthquakes release the
same amount of energy
as one large
earthquake does.
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Chapter 7
Section 3 Earthquakes and Society
Earthquake Forecasting, continued
• The gap hypothesis is a way to forecast earthquake
location, strength, and frequency.
• The gap hypothesis states that sections of active
faults that have had relatively few recent earthquakes
are likely to be the sites of strong earthquakes in the
future.
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Chapter 7
Section 3 Earthquakes and Society
Earthquake Forecasting, continued
• The areas along an active fault where relatively few
earthquakes have happened are called seismic
gaps.
• Stress has a long time to build at seismic gaps.
• When a fault breaks at a seismic gap, the sudden
release of stress can cause a large-magnitude
earthquake.
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Chapter 7
Section 3 Earthquakes and Society
Earthquake Forecasting, continued
• In 1988, scientists
predicted a > 6.5
magnitude earthquake
would happen within 30
years in a seismic gap
near Santa Cruz.
• The 1989 Loma Prieta
earthquake of
magnitude 6.9
happened in the gap.
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Chapter 7
Earthquakes
Gap Hypothesis and Seismic Gaps
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Chapter 7
Section 3 Earthquakes and Society
Reducing Earthquake Damage
• Much of the loss of human life during earthquakes is
caused by buildings that collapse.
• Retrofitting can make older buildings more
earthquake-resistant.
• Common ways to retrofit include strengthening
structures with steel and fastening the building to its
foundation.
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Chapter 7
Section 3 Earthquakes and Society
Reducing Earthquake
Damage, continued
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Chapter 7
Section 3 Earthquakes and Society
Are You Prepared for an Earthquake?
• You can plan ahead to protect yourself and your
property from earthquake damage.
• Safeguard your home by putting heavy objects on
low shelves.
• Ask your parents about having your home
strengthened.
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Chapter 7
Section 3 Earthquakes and Society
Are You Prepared for an Earthquake?,
continued
• Find places that are safe within each room of your
home and outside your home.
• Make a plan to meet with others in a safe place after
the earthquake.
• Store water, nonperishable food, a fire extinguisher,
flashlight with batteries, radio, medicines, and a first
aid kit in a safe place.
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Chapter 7
Section 3 Earthquakes and Society
Are You Prepared for an Earthquake?,
continued
• If an earthquake happens when you are indoors, stay
indoors until the earthquake stops.
• Crouch or lie face down under a table or desk in the
center of the room.
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Chapter 7
Section 3 Earthquakes and Society
Are You Prepared for an Earthquake?,
continued
• If you are outside, stay outside.
• Lie down away from buildings, power lines, or trees
and cover your head with your hands.
• If you are in a car, stop the car and remain inside.
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Chapter 7
Section 3 Earthquakes and Society
Are You Prepared for an Earthquake?,
continued
• After the earthquake, stay calm and get your
bearings as quickly as possible.
• Identify immediate hazards, such as downed power
lines, broken glass, or fire hazards.
• Stay out of damaged buildings.
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Chapter 7
Section 3 Earthquakes and Society
Are You Prepared for an Earthquake?,
continued
• Return home only when someone in authority says it
is safe.
• Remember that there may be aftershocks, which may
cause more damage.
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Chapter 7
Section 3 Earthquakes and Society
Tsunamis
• Earthquakes on the ocean floor can generate
tsunamis.
• A tsunami is an extremely long wave that can travel
across the ocean at speeds of up to 800 km/h.
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Chapter 7
Section 3 Earthquakes and Society
Tsunamis, continued
• Tsunamis most often form when an earthquake
causes a vertical movement of the sea floor, which
displaces an enormous volume of water.
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Chapter 7
Section 3 Earthquakes and Society
Tsunamis, continued
• In the open ocean, tsunami waves can seem very
small.
• As tsunami waves enter shallow water along a
coastline, the energy of the waves is compressed.
• The waves get rapidly taller. By the time they reach
shore, waves can be taller than 30 m.
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Chapter 7
Section 3 Earthquakes and Society
Tsunamis, continued
• Tsunamis can cause damage and loss of life by
smashing into and washing away anything in their
path.
• Almost 150 tsunamis happened worldwide during the
20th century.
• In 2004, an undersea earthquake of magnitude 9.3
caused a tsunami that killed more than 280,000
people and left 1.25 million people homeless.
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Chapter 7
Section 3 Earthquakes and Society
Tsunamis, continued
• Tsunamis are monitored by most of the nations that
border the Pacific Ocean.
• These nations provide seismic and tide data to the
Pacific Tsunami Warning Center (PTWC) in Hawaii.
• If a tsunami has been generated by an undersea
earthquake, the center sends a bulletin to warn
officials in threatened areas.
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Chapter 7
Earthquakes
Concept Map
Use the terms below to complete the concept map
on the next slide.
seismometer
earthquakes
body waves
seismic waves
surface waves
S waves
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Chapter 7
Earthquakes
Concept Map
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Chapter 7
Earthquakes
Concept Map
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