Tsunami
Tsunami
You may have heard of tsunamis, the enormous waves that crash into the coastline and flood
entire towns, even cities. The 2011 Tōhoku tsunami, which slammed into Japan, caused
billions of dollars worth of damage. Thousands of lives were lost. It even caused the meltdown
of the Fukushima Daiichi Nuclear Power Plant.
But what exactly causes such waves to form in the first place?
The waves produced during the Tōhoku tsunami were, quite simply, huge. The tallest may
have reached a height of 133 feet, scientists believe. In certain areas, these waves traveled as
far as six miles inland, wiping out nearly everything in their path.
Waves of this size are usually caused by earthquakes. In the case of Tōhoku, the earthquake
was classified as an “undersea megathrust earthquake,” and it is considered the biggest
earthquake to ever have hit Japan. In fact, it was the fifth-largest earthquake ever measured,
since humans began tracking the size of earthquakes in 1900.
The earthquake caused the ocean floor to be thrust upwards by as much as 15 feet. When the
seabed rises, so does the ocean above it. And the more shallow the water is where the
earthquake occurs, the larger the tsunami will be. Unfortunately for the inhabitants of Japan’s
coastline, the earthquake happened just 45 miles off the coast.
Tsunamis behave differently than average ocean waves. For one thing, the type of waves you
see at the beach are caused by wind. They tend to have a wavelength—that is, the distance
between the crests of two consecutive waves—of around 330 feet. They also tend to average
about 6.6 feet in height.
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Tsunami
By contrast, deep ocean tsunamis typically have a wavelength of 120 miles. The amplitude of
the waves—or the distance from the top of the wave to the bottom—is usually around 3.3
feet.
This isn’t a very big wave at all. However, as the tsunami gets closer to shore, it grows
dramatically in size. This is because the sloping of the coastline causes the wave to be
compressed. While its speed and wavelength decreases, its amplitude increases. In turn, the
frequency of the waves, or the number of wave cycles per second, also increases.
One indication that a tsunami is on its way is that the water that normally covers the
shorelines begins to recede. Some of those who were unlucky enough to experience the
Tōhuku tsunami described the ocean receding out to sea, before coming back in the form of
some very big waves.
Not all tsunamis are formed by undersea earthquakes. They can be triggered by explosions
and underwater landslides. Picture a glacier cracking in half, and sending a chunk of ice miles
long plunging into the ocean. That will cause a gigantic amount of water to be displaced, and
can often result in a tsunami.
Similarly, a tsunami could be generated by a giant meteor splashing into the ocean from outer
space. Or a volcanic eruption in an underwater volcano. And yet the most common causes of
tsunamis remain underwater earthquakes.
The reason we don’t hear about too many tsunamis striking the Atlantic Ocean coastlines of
the US and Europe is that they more frequently happen in the Pacific. The floor of the Pacific
Ocean consists of oceanic plates and continental plates that constantly slide against one
another. On occasion, the plates get stuck, causing the upper plates to snap upward several
inches or even a few feet, pushing a whole section of the ocean up with it.
Again, a rise of few inches may not sound like much. But it can cause devastating waves. To
achieve equilibrium, the water above the crack in the plates spreads out. It starts moving
toward the shore very quickly, often at hundreds of miles per hour. And when it starts to reach
shallow land, that’s when it transforms into what we think of when we think of tsunamis.
Terrible disasters often result.
By definition, all waves transmit energy from one place to another. The energy of an
underwater earthquake, in other words, is often transmitted to land in the form of a series of
tsunamis. Like the surfers who ride them, waves carry energy forward.
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Tsunami
Ocean waves are only one of many kinds of waves. Another common wave form is the sound
wave. Sound waves travel through the air the way water waves travel through the ocean.
Similar to water waves, sound waves are created by vibrating objects, which cause the air
around them to vibrate as well. This vibrating air, in turn, causes the human eardrum to
vibrate. The brain interprets this vibration as sound.
The form of sound waves and water waves is the same. They exhibit the same consistent,
rippling pattern. You might imagine sound waves as tiny, invisible ocean waves crashing on the
shores of the human eardrum.
Then again, sound waves don’t only travel through the air. They can also move through water.
In fact, water is an excellent conductor of sound waves; the medium allows sound waves to
travel five times as fast as they would in the air. Sound waves generated under water can also
travel much longer distances.
Whales are particularly well known for using sound waves to communicate deep beneath the
surface of the ocean. Their “songs,” as we call their form of communication, can last for up to
30 minutes. Whales use sound waves as mating calls, or as a method of alerting other whales
to the existence of a nearby food supply. Some marine biologists believe that whales use
sound as a way to socialize in certain parts of the ocean. Each whale species has its own set of
sounds.
Unlike humans, whales receive sound waves through a section of their lower jaw, which then
transmits the message to the middle ear. There, however, the processing of sound waves
occurs as it does in humans: a sensitive membrane perceives the different vibration
frequencies.
It is important to remember that the amplitude of sound waves, not the frequency, is what
determines whether a sound is heard or not. Changes in amplitude create changes in volume.
This is why performing musicians play music through amplifiers on stage: so that the people in
the back rows can hear. Were bands somehow able to perform underwater they wouldn’t
need quite so many amplifiers.
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Questions: Tsunami
Name:
_____________ Date: _______________________
1. Which types of waves are described in the passage?
A
B
C
D
only water waves
water waves and light waves
water waves and sound waves
water waves, light waves, and sound waves
2. The author compares and contrasts sound waves and water waves. What is one thing
the author compares between sound waves and water waves?
A
B
C
D
The
The
The
The
color of sound and water waves.
cause of sound and water waves.
damage done by sound and water waves.
importance of sound and water waves.
3. Based on the passage, if an earthquake occurred at the bottom of the ocean, what
might happen as a result?
A
B
C
D
a
a
a
a
tsunami
volcanic eruption
hurricane
rise in ocean temperature
4. Based on information in the passage, what can be concluded about tsunamis?
A
B
C
D
The
The
The
The
closer a tsunami gets to shore, the faster it becomes.
closer a tsunami gets to shore, the smaller it becomes.
greater the wavelength of a tsunami, the taller it will be.
greater the amplitude of a tsunami, the taller it will be.
5. What is this passage mainly about?
A
B
C
D
the different ways sound can travel under water
the destruction caused by the Tōhouku tsunami
how sound waves and water waves form and behave
precautions humans should take against tsunamis if they live on a coast
1
®
© 2013 ReadWorks , Inc. All rights reserved.
Questions: Tsunami
6. Read the following sentences: “By definition, all waves transmit energy from one
place to another.”
What does the word transmit mean in the sentence above?
A
B
C
D
move
stop
strengthen
weaken
7. Choose the answer that best completes the sentence below.
Sound waves are similar to ocean waves in many ways; _______, they both travel
through water.
A
B
C
D
in conclusion
however
for example
particularly
8. What does amplitude tell us about a wave?
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
2
®
© 2013 ReadWorks , Inc. All rights reserved.
Questions: Tsunami
9. Describe how sound travels, starting with how a sound is created and ending with
how it reaches the human brain.
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
10. What are the possible results when the amplitude of ocean waves and sound waves
change?
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
3
®
© 2013 ReadWorks , Inc. All rights reserved.
Teacher Guide & Answers: Tsunami
Teacher Guide & Answers
Passage Reading Level: Lexile 980
1. Which types of waves are described in the passage?
A
B
C
D
only water waves
water waves and light waves
water waves and sound waves
water waves, light waves, and sound waves
2. The author compares and contrasts sound waves and water waves. What is one thing the author
compares between sound waves and water waves?
A
B
C
D
The color of sound and water waves.
The cause of sound and water waves.
The damage done by sound and water waves.
The importance of sound and water waves.
3. Based on the passage, if an earthquake occurred at the bottom of the ocean, what might happen as a
result?
A
B
C
D
a tsunami
a volcanic eruption
a hurricane
a rise in ocean temperature
4. Based on information in the passage, what can be concluded about tsunamis?
A
B
C
D
The closer a tsunami gets to shore, the faster it becomes.
The closer a tsunami gets to shore, the smaller it becomes.
The greater the wavelength of a tsunami, the taller it will be.
The greater the amplitude of a tsunami, the taller it will be.
5. What is this passage mainly about?
A
B
C
D
the different ways sound can travel under water
the destruction caused by the Tōhouku tsunami
how sound waves and water waves form and behave
precautions humans should take against tsunamis if they live on a coast
6. Read the following sentences: “By definition, all waves transmit energy from one place to another.”
What does the word transmit mean in the sentence above?
A
B
C
D
move
stop
strengthen
weaken
1
© 2013 ReadWorks®, Inc. All rights reserved.
Teacher Guide & Answers: Tsunami
7. Choose the answer that best completes the sentence below.
Sound waves are similar to ocean waves in many ways; _______, they both travel through water.
A
B
C
D
in conclusion
however
for example
particularly
8. What does amplitude tell us about a wave?
Suggested answer: Amplitude is the distance between the top and bottom of a wave.
9. Describe how sound travels, starting with how a sound is created and ending with how it reaches the
human brain.
Suggested answer: A sound wave is created by a vibrating object, which makes the air around it
vibrate. The vibrating air causes the human eardrum to vibrate. The brain interprets that vibration as
sound.
10. What are the possible results when the amplitude of ocean waves and sound waves change?
Suggested answer (answers may vary): A big increase of amplitude in an ocean wave could make it
destructive and dangerous when it hits land. A decrease in the amplitude of a sound wave could make the
sound too quiet to hear. An increase in the amplitude of a sound wave could make the sound loud enough
for people to notice it, or too loud for some people.
2
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