What is an earthquake? The simple definition of an earthquake is a sudden movement of the earth’s crust and upper mantle. To discover the more complex earthquake definition, let’s talk about the science behind and earthquake. The earth’s lithosphere, or crust, is separated into plates by fractures. These plates are moving very slowly, and at some points, are being pressed together. A great amount of pressure is created along these fault lines, and sudden release in that strain usually results in an earthquake. Thousands of earthquakes occur annually, although most of them are too small to be felt by humans.