Earthquakesexternal image mountainbar.GIF.

When stresses inside the Earth make rocks slide and break quickly past each other along fault lines, earthquakes are formed. The quick movement of these plates causes the ground to shake which is what happens during an earthquake. This motion occurs 40 miles below the surface, or at the focus. Body waves branch out from the focus, which causes tremors miles away. There are three types of surface waves. The primary waves take only twenty minutes to reach there destination. That is called compressional. Transverse are the secondary waves.They move at a slower pace and take twice as long to reach the same place as primary waves. Movement is faster in dense, rigid rock and slower in lighter liquid materials like the atmosphere. Differences in the state of matter from layer to layer affect the behavior of waves. Waves are also influenced by the kind of boundary the plates lie on.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.