Earthquake Epicenters 1963-1998 (358,214 events) This file is in the public domain because it was solely created by NASA. Click on image to enlarge it.
An earthquake is caused by a sudden slip on a fault. The tectonic plates are always slowly moving, but they get stuck at their edges due to friction. When the stress on the edge overcomes the friction, there is an earthquake that releases energy in waves that travel through the earth's crust and cause the shaking that we feel. [From the NSGS website]