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Understanding earthquakes
Understanding earthquakes
By the end of the end of the lesson you should
be able to:
• Define the terms: earthquake, seismic wave, focus,
epicentre, seismometer, magnitude, Richter scale.
• Explain what causes an earthquake and why everything
shakes.
• Discover what the Richter scale is and how an
earthquake’s power can be measured.
• Give examples of damage caused by earthquakes
What are Earthquakes?
Convergent
Earthquakes are vibrations
caused by earth movements
at plate boundaries and at
major fault lines (cracks in
the earth’s surface).
Transform
They can occur at all
major plate margins but
the most severe
earthquakes are normally
found at Convergent and
Transforn boundaries.
Why do earthquakes happen?
1. The two plates at a plate boundary
cannot move past each other easily.
2.The two plates
lock together.
Friction causes
pressure to build up.
3. Suddenly the pressure
is released and the plates
jolt into a new position.
4. This causes seismic waves.
The vibrations they cause
are called an earthquake.
Anatomy of an Earthquake
An Earthquake is the shaking of the ground
caused by sudden motions along faults, or
fractures in the Earth’s crust.
Fault
A fracture in the rocks that
make up the Earth’s crust.
Epicentre
The point at the Earth’s
surface directly above
the focus.
Focus
The point within the Earth
where an Earthquake
rupture starts.
Plates
Massive rocks that
make up the outer
layer of the Earth’s
surface and whose
movement along faults
trigger earthquakes.
Seismic Waves
Waves that
transmit the
energy released by
an earthquake.
Forest
A
Town
B
Epicentre
Focus
An earthquake has occurred along this fault line. Match the
letter with the correct label.
How are earthquakes measured?
The Richter Scale measures the magnitude of a
tremor (how powerful it is) using an instrument
called a seismometer
The Richter Scale is measured
on a scale from 1 to 10.
It is a logarithmic scale which means
that a size ‘6’ on the Richter Scale is 10
times more powerful than a size ’5’ and
100 times more powerful than a size ‘4’.
(go up or down by a factor of 10)
1
2
3
4
5
6
7
8
9
10
Richter
Scale
The largest earthquake
ever recorded was in
Chile. It measured 9.5
on the Richter Scale.
The earthquake in
Italy (August 2016)
measured 6.2 on the
Richter Scale.
1
2
3
4
5
6
7
8
9
10
Richter
Scale
The Japan earthquake
(March 2011) measured
9.0 on the Richter
Scale.
How many times greater was the Japan earthquake?
Damage caused by Earthquakes
Show what you have learned
1.
Explain what causes an earthquake and why everything shakes.
2. Draw and label a cross section of a Convergent margin showing
the following information.
a.
b.
c.
The location of the fault line and the two tectonic plates.
Where the epicentre and focus of an earthquake are likely to occur.
Show the pattern of the seismic waves caused by an earthquake.
3. Define the terms epicentre, focus and seismic waves.
4. Explain what a seismometer is used for and how the magnitude
of an earthquake can be measured.
5. Give some examples of damage caused by earthquakes