All earthquakes may be classified as one of three types, tectonic, volcanic or artificial. Tectonic earthquakes are caused by movements of the plates which make up the crust of the earth. Infrequently, these types of earthquakes can occur in the center of a plate when pressure builds and ruptures the plate. One such earthquake, which occurred around 1811, was powerful enough to change the course of the Mississippi River. Of the three types, tectonic earthquakes are the most devastating and account for almost half of the world's destructive earthquakes and as much as three-quarters of the earth's seismic energy.
Volcanic earthquakes are generally much smaller and less intense than tectonic ones.
They are caused when magma flows upward and fills the chambers beneath a volcano. The sides and tops of the volcano swell and tilt with pressure and result in numerous small earthquakes. Volcanic earthquakes are seldom destructive, but have particular interest because they are often the forerunner for volcanic eruptions. The eruption of Mount Saint Helens in Washington in 1980 is only one example. Sometimes, earthquakes occur as a result of human actions such as when atomic explosives are detonated underground or when new reservoirs are filled.