Faulting refers to the process where rock masses within the Earth’s crust fracture and shift relative to one another. This movement creates a fault, which is a break in the rock along which significant displacement has occurred. These breaks are a response to immense tectonic forces acting deep within the planet, shaping large-scale geographical features and causing most tectonic earthquakes.
The Process of Rock Fracture
Faulting begins with stress, the force applied to a rock unit over a given area. This applied force causes the rock to change shape, a resulting deformation known as strain. Rocks experience three primary types of directed stress: tensional, compressional, and shear, each leading to a different type of fault.
Tensional stress pulls the rock apart, stretching and thinning it. Compressional stress pushes the rock masses together, causing thickening. Shear stress involves forces sliding past each other in opposite, parallel directions. As stress builds, the rock initially undergoes elastic deformation, meaning it can return to its original shape if the stress is released.
If stress continues to increase and exceeds the rock’s internal strength, the rock reaches its elastic limit. The rock then fails in a process called brittle fracture, rather than deforming plastically. This sudden, permanent break and the resulting displacement of rock blocks along the fracture surface defines geological faulting.
Classifying Faults
Geologists classify faults based on the direction of movement of the rock blocks relative to the fault plane (the surface of the fracture). To describe this movement, scientists use the terms “hanging wall” (the block positioned above the fault plane) and “footwall” (the block located beneath it).
Normal faults occur in regions dominated by tensional stress, where the crust is being pulled apart. The hanging wall block moves downward relative to the footwall block. This type of faulting is associated with crustal extension, commonly found at divergent plate boundaries where new crust is being formed.
Reverse faults result from compressional stress, which pushes rock masses together and causes crustal shortening. The hanging wall block moves upward relative to the footwall block. A specific type, known as a thrust fault, occurs when the fault plane has a relatively shallow angle (typically less than 45 degrees). These faults are characteristic features of convergent plate boundaries where one plate is being forced over another.
Strike-slip faults are caused by shear stress, resulting in movement that is predominantly horizontal, or parallel to the fault plane. Two blocks slide past one another. They are classified as right-lateral or left-lateral based on the direction the opposite block appears to move when viewed across the fault. The San Andreas Fault in California is a well-known example of a large, active right-lateral strike-slip fault.
Landforms Shaped by Faulting
Extensive normal faulting over geological time creates distinctive landforms, particularly in areas undergoing crustal extension. When the crust is stretched, a series of parallel normal faults form paired structures known as horsts and grabens. A graben is a block of crust that has dropped downward relative to the blocks on either side.
Conversely, a horst is the uplifted block that remains elevated between two adjacent grabens. The Basin and Range Province in the western United States is characterized by this alternating topography of uplifted mountain ranges (horsts) and down-dropped valleys (grabens).
When a graben forms on a continental scale, it creates a massive linear depression known as a rift valley. The East African Rift Valley is the largest active example, where tensional forces are slowly pulling the continent apart, resulting in a system of down-faulted valleys. These features demonstrate the long-term geographical consequences of faulting. The resulting fault-block mountains, created by the tilting and uplift of these blocks, represent the enduring impact of tectonic forces on the Earth’s surface.
The Connection to Earthquakes
Faulting is the direct mechanical cause of most tectonic earthquakes, representing the sudden release of accumulated energy. Active faults are not constantly slipping; instead, they often become locked in certain sections due to friction and irregularities along the fault plane, known as asperities. Even when locked, tectonic forces continue to push the surrounding rock, causing it to slowly bend and store elastic strain energy.
This process is explained by the elastic rebound theory, which describes how strain energy builds up until it overcomes the fault’s frictional resistance. When the rock’s strength is exceeded, the fault suddenly slips, and the rock blocks rapidly snap back toward their unstrained shapes. This rapid movement generates seismic waves that travel outward through the Earth, experienced as an earthquake.
The location where the initial rupture begins on the fault plane is called the hypocenter, or focus, of the earthquake. The epicenter is the point on the Earth’s surface directly above the hypocenter. The fault itself is the pre-existing zone of weakness, and the sudden movement along this structure is the mechanism that generates the seismic event.