An Earthquake is a sudden tremor or movement of the earth's crust, which originates naturally at or below the surface. There are two real causes of earthquakes. First, they can be linked to explosive volcanic eruptions; they are in actually really common in areas of volcanic activity where they either proceed or ago with eruptions. Secondly, they can be triggered by Tectonic activity associated with plate margins and faults. The majority of earthquakes world wide are of this type.  An earthquake can be likened to the effect observed when a stone is thrown into water. After the stone hits the water a series of concentric waves will move outwards from the center. The same events occur in an earthquake. There is a sudden movement within the crust or mantle, and concentric shock waves move out from that point. Geologists and Geographers call the origin of the earthquake the focus. Since this is often deep below the surface and difficult to map, the location of the earthquake is often referred to as the point on the Earth surface directly above the focus. This point is called the epicenter. The strength, or magnitude, of the shock waves determines the extent of the damage caused. Two main scales exist for defining the strength, the Mercalli Scale and the Richter Scale. Earthquakes are three dimensional events, the waves move outwards from the focus, but can travel in both the horizontal and vertical plains. This produces three different types of waves which have their own distinct characteristics and can only move through certain layers within the Earth. Lets take a look at these three forms of shock waves.