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By Leon and kate
An earthquake is what happens when
two blocks of the earth suddenly slip past
one another. The surface where they slip
is called the fault or fault plane. The
location below the earth’s surface where
the earthquake starts is called the
hypocenter, and the location directly
above it on the surface of the earth is
called the epicenter.
What causes earthquakes?
Earthquakes are caused by an abrupt
shift of rock along a fracture in the
Earth.
Most earthquakes are caused by slow
movements inside the Earth that push
against the Earth's brittle, relatively thin
outer layer, causing the rocks to break
suddenly.
How are earthquakes recorded?
Earthquakes are recorded by instruments
called seismographs. The recording they
make is called a seismogram. The
seismograph has a base that sets firmly in
the ground, and a heavy weight that
hangs free. When an earthquake causes
the ground to shake, the base of the
seismograph shakes too, but the hanging
weight does not. Instead the spring or
string that it is hanging from absorbs all
the movement. The difference in position
between the shaking part of the
seismograph and the motionless part is
what is recorded.
Where do earthquakes occur?
Earthquakes occur all the time all over the
world, both along plate edges and along
faults. Most earthquakes occur along the
edge of the oceanic and continental plates.
The earth's crust (the outer layer of the
planet) is made up of several pieces,
called plates. The plates under the oceans
are called oceanic plates and the rest are
continental plates. The plates are moved
around by the motion of a deeper part of
the earth (the mantle) that lies underneath
the crust. These plates are always bumping
into each other, pulling away from each
other, or past each other. The plates usually
move at about the same speed that your
fingernails grow. Earthquakes usually occur
where two plates are running into each
other or sliding past each other.
http://video.nationalgeographic.com/vide
o/101-videos/earthquake-101
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