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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.