Earthquakes are generally caused by the sudden movement of rock along a fault line. Faults are breaks in the earth’s crust where rock on one side moves in a different direction than the rock on the other side. Since rock is heavy and has a lot of friction the movement is not smooth, so instead the rocks stick and tension builds up until there is a rupture and the tension is released. The movement and breakage of the rocks as the tension is released causes earthquakes.
There are three types of faults that cause earthquakes. The first is a normal dip-slip fault where one side moves down as the other moves up. The next is the reverse dip-slip fault where one side is forced to move down and under the other side. The third is the strike-slip fault, where the two sides slide past each other horizontally. The San Andreas Fault in California, one of the most famous, is a strike-slip fault.
Earthquakes can also be caused by volcanoes. The pressure of the magma under the earth’s surface puts pressure on the plates near the volcano as well. A violent volcanic eruption will disturb the plates and cause earthquakes near the volcano. This only occurs around volcanoes where there is pressure building, and generally not around volcanoes with free flowing lava as the magma is not building pressure under the surface.
