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What are earthquakes and how
do they occur?
Chapter 5: Lesson 3:
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What is an earthquake?It is the sudden movement of earth’s crust.
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When layers of rock that are
stuck together suddenly slip an
earthquake occurs.
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When two plates slide past each
other, they create a transform fault,
like the San Andreas fault.
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Slipping along a fault begins under the earth’s surface.
(80 km – 644 km deep)
The place where the slipping begins is called the focus.
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Waves of energy ripple outwards from the focus.
When the waves reach the surface they form the epicenter.
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The epicenter is the point from where the waves of the earthquake
spread.(People start feeling the
earthquake)
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Earthquakes occur at plate boundaries
(like volcanoes)
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Seismometers
Instruments used to measure the waves produces by an earthquake.
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There are 3 types of seismic waves
P waves(Primary waves)
S waves(Secondary
waves)
Lg waves(Surface waves)
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P waves- Move back and forth- Fastest- Travels through solid and liquid
layers of earth.
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S waves- Moves up and down.- Travel half as fast as P waves.- Travels through solid layers of
earth only.
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Lg waves- Travels as ripples across earth’s
surface.- Slowest.- Causes most damage.
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Lg waves
P waves
S waves
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Locating earthquakes
Seismometer station 1
Earthquake epicenter can
happen any where along
circumference
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Seismometer station 1
Seismometer station 2
Seismometer station 3
The point where the 3 circles meet is the
epicenter
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MagnitudeThe measure of the amount of
energy released by an earthquake.
Richter scale Mercalli scale
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Richter scale
Measures the magnitude at the epicenter
(From weakest to strongest)
The Richter scale starts from 1 and each whole number has 30 times more energy than the number before it.
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Mercalli scale
Measures what people felt and what happens to things
in a place
On the mercalli scale earthquakes are rated from I – XII(XII is the most severe)
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Mercalli scale
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The earthquake in Pakistan on October 8, 2005 was rated XII
the mercalli scale.
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Aftershock
Earthquakes with lesser magnitude.
(Felt after the main earthquake)
Earthquakes with larger magnitudes have many aftershocks with large magnitudes.
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When earthquakes occur on the ocean floor waves of
water are pushed upwards in all directions.Deep in the sea the
energy is spread through the water so the waves are
not tall.
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In shallow waters the waves are very
tall causing tsunamis.
When the waves reach shallow water the energy is spread through less water.