Earthquakes are usually caused when rock underground suddenly breaks
along a fault. This sudden release of energy causes the seismic waves
that make the ground shake. When two blocks of rock or two plates are
rubbing against each other, they stick a little. They don't just slide
smoothly; the rocks catch on each other. The rocks are still pushing
against each other, but not moving. After a while, the rocks break
because of all the pressure that's built up. When the rocks break, the
earthquake occurs. During the earthquake and afterward, the plates or
blocks of rock start moving, and they continue to move until they get
stuck again. The spot underground where the rock breaks is called the focus of the earthquake. The place right above the focus (on top of the ground) is called the epicenter of the earthquake.
Earthquakes occur all the time all over the world, both along plate edges and along faults.
Along Plate EdgesMost 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.
Earthquakes can also occur far from the edges of plates, along faults. Faults are cracks in the earth where sections of a plate (or two plates) are moving in different directions. Faults are caused by all that bumping and sliding the plates do. They are more common near the edges of the plates.
Types of FaultsNormal faults are the cracks where one block of rock is sliding downward and away from another block of rock.
Reverse faults are cracks formed where one plate is pushing into another plate. They also occur where a plate is folding up because it's being compressed by another plate pushing against it.
Strike-slip faults are the cracks between two plates that are sliding past each other.
Hope this helps :)
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