Earthquakes occur due to the movement of tectonic plates in the Earth's crust. Plates constantly shift and move, building energy. The release of that energy is an earthquake.
The Earth's crust is not a single, solid mass of rock; it is composed of many distinct sections or tectonic plates. These plates are in constant motion upon the upper layers of the Earth's mantle. As they move, the edges of the plates rub together, putting strain on the rocks. The stress builds until it releases along the plate's weakest point--a fault.
Faults release this energy in three distinct ways. A strike-slip event occurs when faults slip sideways, either right or left. Normal events (the most common) involve downward motion at the fault. Normal events mean that the Earth's crust is stretching. Upward movement at a fault may be either a reverse or thrust event. In a reverse event, faults move at an angle greater than 45 degrees, while a thrust event involves movement at an angle less than 45 degrees. In both reverse and thrust events, the Earth's crust is compressed.
Some areas of the world are more prone to earthquakes than others. Regions that lie along the edges of plates tend to experience more and stronger earthquakes than regions that do not. This information allows for the creation of seismic hazard maps that pinpoint areas most at risk for severe earthquakes.