Four of the most common natural disasters that occur in the world today are earthquakes, volcano eruptions, tornadoes, and hurricanes or typhoons. Each disaster leaves a trail of death and destruction wherever they appear. An Earthquake is the shaking of the ground which occurs when masses of rock change position below the earth's surface. The shifting masses send out shock waves that are strong enough in some cases to wreck buildings, roads and bridges. They may even alter the surface of the earth by thrusting up cliffs through the earths surface.
Earthquakes go on almost continuously.
It is estimated that close to one million earthquakes occur each year. The shock, or seismic waves caused by the shifting masses of rock move out in all directions and in large earthquakes can be felt up to one thousand miles away from the center of the earthquake. The strength of an earthquake is measured on the Richter scale with zero being the mildest of shocks and nine being an earthquake which produces catastrophic damage. It is possible to prevent major earthquakes by drilling holes in danger areas and pumping water into them for short periods however they are generally impossible to stop. There are two classes of seismic waves, interior waves and surface waves.