Introduction

At low temperatures and pressures rock is a brittle material that will fail by fracture if the stresses become sufficiently large. Fractures are widely observed in surface rocks of all types. When a lateral displacement takes place on a fracture, the break is referred to as a fault. Surface faults occur on all scales. On the smallest scale the offset on a clean fracture may be only millimeters. On the largest scale the surface expression of a major fault is a broad zone of broken up rock known as a fault gouge; the width may be a kilometer or more, and the lateral displacement may be hundreds of kilometers.

Earthquakes are associated with displacements on many faults. Faults lock, and a displacement occurs when the stress across the fault builds up to a sufficient level to cause rupture of the fault. This is known as stick–slip behavior. When a fault sticks, elastic energy accumulates in the rocks around the fault because of displacements at a distance. When the stress on the fault reaches a critical value, the fault slips and an earthquake occurs. The elastic energy stored in the adjacent rock is partially dissipated as heat by friction on the fault and is partially radiated away as seismic energy. This is known as elastic rebound. Fault displacements associated with the largest earthquakes are of the order of 30 m.