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. The most powerful of all the waves are the surface waves.
Surface waves travel on the earth's surface, and more slowly than any of the other waves.
The lava may flow out as a viscous liquid or it may explode from the vent as solid or liquid particles. Bending and cracking of the crust may release enough pressure to cause some of the hot mantle rock to melt, this is how magma is formed. Once formed, magma tends to rise as it is lighter than surrounding rocks. It may rise slowly and almost uninterrupted until it pours out of the surface or is blown out explosively as showers of red hot fragments. Volcanoes are not found everywhere.