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Natural Disasters
Information Booklet
Year 4/5 F
Torrens Valley Christian School 2011
Earthquake
Tsunami
Tornado
What is an earthquake?
Earthquakes are the shaking, rolling or sudden shock of the earth’s
surface. They are the Earth's natural means of releasing stress. More
than a million earthquakes rattle the world each year.
The epicentre of an earthquake is the point on the Earth's surface
directly above the source of the earthquake. The source, also known
as the focus, can be as deep as 700 kilometres. Smaller earthquakes
occur much more frequently than large ones and most cause little or
no damage. A very large earthquake can be followed by a series of
smaller events called aftershocks during a period of adjustment which
may last for several months.
The size of earthquakes is determined by measuring the amplitude of
the seismic waves recorded on a seismograph. A formula is applied
to these which converts them to a magnitude scale, a measure of the
energy released by the earthquake.
Magnitude
Description of effect
less than 3.4 Usually felt by only a few people near the epicentre.
Felt by people who are indoors and some outdoors; vibrations
3.5 - 4.2
similar to a passing truck.
Felt by many people; windows rattle, dishes disturbed, standing
4.3 - 4.8
cars rock.
Felt by everyone; dishes break and doors swing, unstable objects
4.9 - 5.4
overturn.
Some damage to buildings; plaster cracks, bricks fall, chimneys
5.5 - 6.1
damaged.
Much building damage; houses move on their foundations,
6.2 - 6.9
chimneys fall, furniture moves.
Serious damage to buildings; bridges twist, walls fracture, many
7.0 - 7.3
masonry buildings collapse.
7.4 - 7.9
Causes great damage; most buildings collapse.
greater than Causes extensive damage; waves seen on the ground surface,
8.0
objects thrown into the air.
What causes an earthquake?
There are about 20 plates along the surface of the earth that move
continuously and slowly past each other. When the plates squeeze or
stretch, huge rocks form at their edges and the rocks shift with great
force, causing an earthquake. Think of it this way: Imagine holding a
pencil horizontally. If you were to apply a force to both ends of the
pencil by pushing down on them, you would see the pencil bend.
After enough force was applied, the pencil would break in the middle,
releasing the stress you have put on it. The Earth's crust acts in the
same way. As the plates move they put forces on themselves and
each other. When the force is large enough, the crust is forced to
break. When the break occurs, the stress is released as energy which
moves through the Earth in the form of waves, which we feel and call
an earthquake.
The theory of plate tectonics is a interesting story of continents
drifting from place to place breaking apart, colliding, and grinding
against each other. The plate tectonic theory is supported by a wide
range of evidence that considers the earth's crust and upper mantle
to be composed of several large, thin, relatively rigid plates that move
relative to one another. The plates are all moving in different
directions and at different speeds. Sometimes the plates crash
together, pull apart or sideswipe each other. When this happens, it
commonly results in earthquakes.
Most earthquakes occur on the edge of plates, especially where one
plate is forced under another such as happens off Sumatra or past
another as occurs in California. Some regions have more
earthquakes than others with 80 per cent of all recorded earthquakes
taking place around the edge of the Pacific Plate, in New Zealand,
Papua New Guinea, Japan, Canada, USA and South America.
Earthquakes also can cause a tsunami, or a series of waves which
can cross an ocean and cause extensive damage to coastal regions.
In areas where there are steep slopes, vibrations resulting from
earthquakes may cause landslides.
What is a Tsunami?
A tsunami is a large ocean wave usually caused by an underwater
earthquake or a volcanic explosion. Most tsunamis, about 80 percent,
happen within the Pacific Ocean’s “Ring of Fire,” a geologically active
area where tectonic shifts make volcanoes and earthquakes
common. Tsunami (pronounced soo-nahm-ee) is from the Japanese
word for harbour wave.
Tsunami waves travel in all directions from the area of disturbance,
much like the ripples that happen after throwing a rock. In deep
ocean, tsunami waves may appear only a foot or so high. But as they
approach shoreline and enter shallower water they slow down and
begin to grow in energy and height. Waves can sometimes reach
heights of over 100 feet (30.5 metres) when they reach land. These
walls of water can cause widespread destruction when they crash
ashore.
Tsunamis race across the sea at up to 500 miles (805 kilometres) an
hour, which is about as fast as an airplane. At that pace they can
cross the entire expanse of the Pacific Ocean in less than a day.
Their long wavelengths mean they lose very little energy along the
way.
A tsunami’s trough, the low point beneath the wave’s crest, often
reaches shore first. When it does, it produces a vacuum effect that
sucks coastal water seaward and exposes harbor and sea floors.
This retreating of sea water is an important warning sign of a tsunami,
because the wave’s crest and its enormous volume of water typically
hit shore five minutes or so later. Recognizing this phenomenon can
save lives. The best defense against any tsunami is early warning
that allows people to seek higher ground.
Tsunami Wave
Diagram (showing
the trough)
A tsunami is usually composed of a series of waves, called a wave
train, so its destructive force may be compounded as successive
waves reach shore. People experiencing a tsunami should remember
that the danger may not have passed with the first wave and should
await official word that it is safe to return to vulnerable locations.
Tsunamis are NOT the same as tidal waves. Tidal waves are caused
by the forces of the moon, sun, and planets upon the tides, as well as
the wind as it moves over the water. With typical waves, water flows
in circles, but with a tsunami, water flows straight. This is why
tsunamis cause so much damage!
What are tornadoes?
Tornadoes are vertical funnels of rapidly spinning air. Their winds
may top 250 miles (400 kilometres) an hour and can clear-cut a
pathway a mile (1.6 kilometres) wide and 50 miles (80 kilometres)
long. They can destroy large buildings, uproot trees and hurl vehicles
hundreds of yards.
Tornadoes are born in thunderstorms and are often accompanied by
hail. Giant, persistent thunderstorms called supercells create the
most destructive tornadoes.
These violent storms occur around the world, but the United States is
a major hotspot with about a thousand tornadoes every year.
"Tornado Alley," a region that includes eastern South Dakota,
Nebraska, Kansas, Oklahoma, northern Texas, and eastern
Colorado, is home to the most powerful and destructive of these
storms. U.S. tornadoes cause 80 deaths and more than 1,500 injuries
per year.
America’s Tornado
Alley (shaded area
on map).
Tornadoes move at speeds of about 10 to 20 miles (16 to 32
kilometers) per hour, although they've been clocked in bursts up to 70
miles (113 kilometers) per hour. Most don't get very far though. They
rarely travel more than about six miles (ten kilometers) in their short
lifetimes.
Tornadoes are classified as weak, strong, or violent storms.
Weak Tornadoes
Strong Tornadoes
Violent Tornadoes






69% of all
tornadoes
Less than 5% of
tornado deaths
Lifetime 1-10+
minutes
Winds less than
110 mph



29% of all
tornadoes
Nearly 30% of all
tornado deaths
May last 20
minutes or longer
Winds 110-205
mph



Only 2% of all
tornadoes
70% of all
tornado deaths
Lifetime can
exceed 1 hour
Lifetime can
exceed 1 hour
Today the average warning time for a tornado alert is 13 minutes.
Tornadoes can also be identified by warning signs that include a
dark, greenish sky, large hail, and a powerful train-like roar.
How tornadoes form
A tornado begins as a column of warm humid air rising quickly in a
thunderstorm. While scientists are not completely sure exactly how it
happens, this warm air interacts with air that is cooler and moving
much faster and causes the column of air to begin to rotate
horizontally.
When this column of air gets caught up in the updraft from the storm,
the updraft tightens the spin and it speeds up. Try to imagine that this
is like an ice skater whose spin becomes faster as they pull their
arms close to their body. This creates a funnel cloud.
The rain and hail in the storm cause this funnel to touch down
vertically towards the ground, creating a tornado.
Tornadoes' distinctive funnel clouds are actually transparent. They
become visible when water droplets pulled from a storm's moist air
condense or when dust and debris are taken up. Funnels typically
grow about 660 feet (200 metres) wide.