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Earthquakes
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Earthquakes occur every day
somewhere in the world. Most of the time people don't even notice they
happened. Earthquakes like the Landers Quake don't happen very often.
Basically an earthquake is trembling or shaking of the earth. Huge explosions
and volcanic eruptions can cause this but most earthquakes are causes by rocks
moving along a fault.
Energy is released along
the break. This energy causes the shaking of the earth. The point inside the
earth where the rock breaks or moves is called the focus of the earthquake.
The focus of most earthquakes is fairly shallow reaching only 100 kilometers
or so beneath the surface of the earth. Deep focus earthquakes, as deep as 700
kilometers usually occur at trench boundaries-where one plate is moving under
another
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Imagine for a
moment you are holding a plastic ruler. If you bend it far enough the ruler
will snap and both pieces are again straight. Much like the ruler, rocks in
the earth's crust that are under pressure bend, break, and snap back. A fault
is a break in rocks along which rocks have moved.