Faults and Fractures

Faults and fractures are breaks in rock, ranging from hairline joints to crustal-scale faults along which earthquakes occur and along which blocks of the crust shift relative to one another.

Definition

A fracture is any break in rock across which cohesion is lost; a fault is a fracture or fracture zone along which there has been measurable displacement of the two sides, classified by the sense of that displacement.

Scope

This topic covers the brittle structures of the crust: joints and other fractures, and the three principal fault types — normal, reverse, and strike-slip — together with their relation to the stress field and to earthquakes. It treats the geometry and mechanics of brittle failure, complementing the ductile folds.

Core questions

• What distinguishes joints from faults?
• How do normal, reverse, and strike-slip faults relate to the orientation of the principal stresses?
• How does slip on faults generate earthquakes?

Key theories

Anderson's theory of faulting

Anderson related the three fault classes to the orientation of the principal stresses: a vertical maximum stress favors normal faults, a vertical minimum stress favors reverse faults, and a vertical intermediate stress favors strike-slip faults.

Mechanics of earthquakes and faulting

Earthquakes result from frictional stick-slip instability on faults, with elastic strain accumulating over time and releasing rapidly during slip, a framework that links fault mechanics to the seismic cycle.

Mechanisms

When stress in brittle crust exceeds rock strength, fractures form; joints open without shear, while faults accommodate shear displacement. The fault type that develops reflects which principal stress is vertical, following Anderson's framework. On established faults, friction allows stress to build elastically until it is released suddenly in an earthquake, after which strain begins to accumulate again.

Clinical relevance

Characterizing active faults is the foundation of seismic hazard assessment, while fracture networks control the permeability that governs groundwater flow, geothermal and petroleum production, and the integrity of engineered structures and waste repositories.

History

Anderson's 1951 dynamic theory linked fault types to stress orientation and remains a cornerstone of structural geology. Reid's elastic-rebound theory, developed after the 1906 San Francisco earthquake, and later work synthesized by Scholz, established the mechanical understanding of how faults store and release energy as earthquakes.

Key figures

• Ernest Masson Anderson
• Christopher Scholz
• Harry Fielding Reid

Seminal works

• anderson1951
• scholz2019

Frequently asked questions

What is the difference between a fault and a joint?

Both are fractures in rock, but a fault shows measurable displacement of the rock on either side, whereas a joint is a fracture across which there has been little or no shear movement.