An earthquake is caused by sudden, violent shifting of massive rocks called plates lying under the earth's surface. This movement of the plates releases stress that accumulates along geologic faults. A fault is a deep crack that marks the boundary between two of the plates. Earthquake belts occur along faults all around the world. Many run along coastal areas. The San Andreas Fault in central California is well-known for causing most severe earthquakes. The epicenter is the point on the surface where the earthquake is strongest. The Richter scale is used to measure the amount of energy that is released by the earthquake.