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Waves

Plate Tectonics - Plates sliding past each other.

There is a famous example of this where the Pacific Plate moves north
relative to the North American Plate.
Most of this boundary is under the sea,
but part of it is on land on the West Coast of North America.
This boundary is called the San Andreas Fault,
and the city of San Francisco is built right on top of it.

The plates do not slide smoothly past each other.
They get stuck at the boundary (there is very high friction)
and then very large forces build up as they keep trying to slide.
Eventually the forces are great enough to overcome the friction
and one plate moves rapidly against the other for a few seconds.
This is an earthquake,
and the shock can send out waves through the Earth's surface
which are strong enough to cause buildings to collapse.
It is not possible for scientists to predict accurately when an earthquake
will happen. The enormous forces can cause the plates to slide past
each other at any time. It is also not possible to predict when a volcano
will erupt for the same reason.

Much of San Francisco was destroyed by an earthquake in 1906.
Many of the wooden buildings were lost in the fire which followed.
The earthquake remains the biggest to be recorded anywhere in the USA
measuring 8·1 on the Richter scale. San Francisco has experienced
other earthquakes since and will surely have more in the future,
but now the buildings are made to be earthquake resistant,
and can suffer a good deal of shaking before they fall down.

Needless to say, earthquakes claim the most casualties in poor countries
where bad housing and overcrowding
make the people who have to live there very vulnerable.

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Copyright © 2008 Dr. Colin France. All Rights Reserved.