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An earthquake is formed when faulting and folding of the earth happens. There are great blocks of rock that are usually tightly against one another at a fault, or crack in the earth. After many years of being pushed towards or against one another, the rocks sometimes have a sudden movement and assume a new position. Even if they only move an inch or two, the vibrations of their movements can cause the earth to shake.
There are two main areas of the earth where earthquakes tend to take place. One makes a circle around the Pacific Ocean going from the tip of Chile in South America and right up the coast of North America and around to Japan, the Philippine Islands and New Zealand.
The other is in the Mediterranean Sea area from Northern Africa to Asia Minor.
These areas area called earthquake belts.
Earthquakes that happen underwater are called tsunamis. These can cause huge tidal waves to occur.
Earthquakes are recorded on instruments called seismographs. The USGS keeps track of information on earthquakes and informs the public about earthquakes when they happen.