Why do Earthquakes Happen ?
What causes earthquakes?
Just below Earth’s surface, the crust layer is divided up into several “plates”, which are always moving slightly. Between these plates are fault lines, which is where the plates come together. At these fault lines, the plates are constantly rubbing and sliding against each other, but sometimes due to irregularities and high friction, the plates become locked and stop moving.
The relative motion of the plates (meaning the plates are still trying to move) causes extreme pressure and stress to build up at the point where the plates are locked. (Potential energy greatly increases) Eventually, the energy and pressure becomes so great that it breaks through whatever was locking it, and all the built up potential energy is released as kinetic energy in the form of seismic waves and heat. (The heat is created from the friction of the plates rapidly sliding against each other.)
These different types of seismic energy waves, low frequency acoustic energy, are what cause all the damage done by earthquakes.
World Book Encyclopedia 2001