What is an earthquake?

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Inside planet Earth there are many layers. The outermost layer or crust is made up of large pieces of earth called plates. These plates fit together kind of like a puzzle, except the pieces are always moving around very slowly. As they move and press against each other, they don't fit as well together and they build up tension or friction. When the pressure between the moving plates gets too great, it releases what are called seismic waves or an earthquake.   

Where the edges of different plates (or puzzle pieces) meet together is called a fault or fault line. It is at these fault lines where the built up pressure from plate movement causes those big pieces of rocky earth to shift way below the surface of the earth...or an earthquake! But remember, earthquakes don't just happen anywhere- they occur where there are fault lines. When they do happen, a quake can be so small that we don't even notice. Other times, the release of energy is so great that is causes the ground to shake a whole lot, which can do lots of damage to buildings, roads and more. 

Fault Facts!

• Did you know that earthquakes can happen as deep as 400 miles under the earth's surface?! However, most occur much closer up- at less than 50 miles.
  
  
• The San Andreas Fault runs almost the entire length of California! That's why so many earthquakes occur in our sunny state out west. However, it is actually made up of individual smaller faults that join together.  
  
  
• Slow moving! The rate of movement along the San Andreas Fault is only 2 inches per year. Our hair grows faster than that!
  
  
• Earthquakes usually last less than a minute.