Brittle tectonics may be considered on two timescales, in which earthquakes are the short timescale phenomena and faulting is the long timescale process. Faults grow and develop by the cumulative action of earthquakes, and the faults therefore contain the history of past seismicity. In this chapter we discuss the mechanics of faults, which are treated as quasi-static cracks with friction. We begin with a discussion of the elementary theory of faulting, followed by a more modern treatment of the formation and growth of faults and a description of the rocks and structures formed by faulting. Here we rely more heavily on geological observations than elsewhere. We summarize with a discussion of the strength and rheology of faults, finishing with the topic of heterogeneity and its role in faulting, which continues a subtheme to be found throughout this book.

Mechanical framework

Anderson's theory of faulting

In his seminal paper of 1905 and in his memoir of 1951, E. M. Anderson developed the modern mechanical concepts of the origin of faults and emphasized their important role in tectonics. His key contribution was to recognize that faults result from brittle fracture and to apply the Coulomb criterion to this problem. This led him to expect that faults should sometimes form conjugate sets, with planes inclined at acute angles on either side of the maximum principal stress and which include the intermediate principal stress direction (Section 1.1.4, Equation (1.32)).