A geological fault is a fracture, or a zone of fractures, within the Earth’s crust where two blocks of rock have moved relative to each other. This displacement generates most earthquakes, which are sudden releases of energy built up along these zones of weakness. Earthquakes represent the momentary slip of the crust, transforming accumulated strain into seismic waves that radiate outward. The study of these fault structures is central to understanding the forces that shape our planet’s surface.
Defining the Fault Structure
A fault is distinguished from a simple crack, or joint, by the presence of measurable displacement between the rock masses on either side of the fracture surface. A joint is a break where no movement has occurred, while a fault involves significant relative motion along a distinct surface known as the fault plane. This plane can be nearly horizontal, vertical, or inclined at any angle.
The rock masses adjacent to an inclined fault plane are defined by their position relative to this plane. The block of rock situated above the fault plane is called the hanging wall. Conversely, the rock mass lying beneath the inclined fault plane is designated the footwall. The angle at which the fault plane slopes downward from the horizontal is known as the dip, which is used to classify the type of fault.
The Mechanics of Earthquake Generation
The process by which faults generate earthquakes is explained by the elastic rebound theory, first proposed after the 1906 San Francisco earthquake. Tectonic forces from the continuous movement of the Earth’s plates constantly apply stress to the rocks across a fault boundary. Because fault surfaces are locked by friction, the rocks do not immediately slip but instead begin to bend and deform, storing potential energy.
This deformation causes the rock to store potential energy in the form of elastic strain, accumulating over periods that can span decades to centuries. As stress builds, the strain eventually exceeds the frictional strength holding the fault together, known as the rock’s elastic limit. When this strength is overcome, the locked fault suddenly ruptures, and the strained rock snaps back to its original shape in a rapid process called elastic rebound.
This sudden release of stored energy travels outward from the rupture point as seismic waves, which are felt as an earthquake. The specific point within the Earth where the initial rupture occurs is called the hypocenter, or focus. Directly above the hypocenter on the Earth’s surface is the epicenter. The rupture propagates outward from the hypocenter at speeds up to several kilometers per second until the energy is dissipated.
Classifying Fault Movement
Faults are categorized based on the direction of relative movement between the hanging wall and the footwall, reflecting the type of tectonic stress acting on the crust.
In a normal fault, tensional forces pull the crust apart, causing the hanging wall block to move down relative to the footwall. These faults are associated with regions where the crust is being stretched, such as rift valleys or divergent plate boundaries.
The opposite movement defines a reverse fault, where compressional forces push the crust together. This results in the hanging wall block moving up and over the footwall. When the angle of the fault plane is shallow (less than 45 degrees), a reverse fault is called a thrust fault. These features are common in collision zones where tectonic plates converge, causing crustal shortening and mountain building.
The third category is the strike-slip fault, where movement is predominantly horizontal, with the two blocks sliding past each other laterally. These faults are caused by shearing forces and tend to have a nearly vertical fault plane. Movement is described as either right-lateral or left-lateral, depending on the apparent direction of movement from an observer’s perspective. The San Andreas Fault is an example of a right-lateral strike-slip fault.