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Tectonic Activity
• Earthquakes
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Introduction
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Causes of Earthquakes
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Measuring Earthquakes
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Effect of Earthquakes
An earthquake is the shaking and vibration of the Earth’s crust due to plate tectonics
(movement of plates)
Earthquakes can happen along any type of plate boundary. They also occur along faults
which are large cracks in the earth’s crust. Most faults are associated with large plate
boundaries where violent earthquakes usually occur.
Causes of Earthquakes
Earthquakes are caused when the tension is released from inside the crust. This happens
because plates do not move smoothly - sometimes they get stuck. When this happens a great
deal of pressure builds up. Eventually this pressure is released and an earthquake tends to
occur.
An earthquake starts deep below the earth’s surface at the focus. The focus is the point
inside the Earth's crust where the pressure is released. The epicentre of an earthquake is the
position on the earth’s surface directly above its focus.
The jerking movement caused by plates sticking then moving releases built-up pressure
inside the Earth's crust, in the form of seismic waves. The waves spread out from the focus.
The strongest waves are found near the centre of the earthquake. This means that the most
severe damage caused by an earthquake will happen close to the epicentre.
It is almost impossible to predict when they will occur. The effect of an earthquake depends
on the depth of an earthquake as well as its magnitude.
If the focus is very deep or the shockwaves have to travel through dense rock, the effect will
be less
Measuring Earthquakes
The power (magnitude) of an earthquake is measured on the Richter scale, using an
instrument called a seismometer.
The Richter scale is numbered 0-10 with 10 being the most powerful. The Richter scale is
logarithmic – an earthquake measuring 7 is 10 times more powerful than one measuring 6
and 100 times more powerful than one measuring 5. Up until 2 on the Richter Scale only
instruments will detect the earthquake. Earthquakes above 6 cause serious damage and
sometimes many deaths
The Mercalli scale measures the damage caused by an earthquake. The Mercalli scale goes
from I to XII e.g. “VI. Everyone feels movement. People have trouble walking. Objects fall
from shelves. Pictures fall off walls. Furniture moves. Plaster in walls might crack. Trees
and bushes shake. Damage is slight in poorly built buildings. No structural damage.”
Effects of Earthquakes
Primary effects occur immediately, and are all due to the shaking of the ground e.g.
buildings collapsing, destruction of roads and bridges.
Secondary effects happen afterwards, but can be even more devastating e.g. fire, tidal
waves and disease and landslides
Examples: Northwest Turkey, Izmit 1999
Primary effects:
Over 20,000 deaths
More than 100,000 buildings were destroyed
Over 100,000 families homeless.
Transport links destroyed
Landslides
Secondary effects:
Disease e.g. Cholera and typhoid (caused from polluted water)
Services unavailable e.g. water, electricity and sewage
Intense heat and dust (people who were made homeless had no shelter, thus were exposed)
Primary effects
Secondary effects
Social Impacts
Economic Impacts
(impact on people) (impact on
business in the
area)
Death,
Businesses &
Homes destroyed,
property destroyed,
Services e.g. water
disrupted,
Transport systems
damaged
Diseases
Looting,
Economic effect of
restoring
businesses
Environmental
Impacts (impact
on the landscape)
Landscape
destroyed,
Fires,
Tidal Waves,
Landslides