Once a fault has been formed its further motion is controlled by friction, which is a contact property rather than a bulk property. In the schizosphere the micromechanics of friction involve brittle fracture, but frictional behavior is fundamentally different from bulk brittle fracture. Here we examine this property in some detail and, in particular, discuss the stability of friction, which determines if fault motion is seismic or aseismic.

Theoretical concepts

Historical

Friction is the resistance to motion that occurs when a body is slid tangentially to a surface on which it contacts another body. It plays an important role in a great variety of processes. It is always present in machines in which there are moving parts, and a significant part of people's energy consumption is used in overcoming friction. Simple lubrication was also employed by the ancients to reduce friction. In engineering, therefore, considerable effort has been made to reduce friction. In other situations it is important to improve traction by making things less slippery. Because friction plays a role in everyday life, its basic properties are common knowledge and have been since ancient times. The wheel was an important invention primarily because it substitutes rolling friction for the much higher sliding friction. In spite of our familiarity with friction, its basic nature remained obscure until recent times.