A geological fault is a fracture or zone of fractures in the Earth’s crust where blocks of rock have moved relative to one another. This movement can range from slow, continuous creep to sudden, rapid slippage, which is the mechanism behind most earthquakes. Understanding these structures provides insight into the planet’s dynamics and the accumulation and release of strain energy in the brittle upper crust.
Defining the Geological Fault
A fault is defined as a planar fracture within a volume of rock across which there has been observable displacement. The surface along which this slip occurs is known as the fault plane, which can be oriented at any angle. The relative movement between the two sides is referred to as the displacement. To describe movement on a non-vertical fault plane, geologists use the terms hanging wall and footwall. The hanging wall is the block of rock that sits above the fault plane, while the footwall is the block located beneath it.
The Mechanics of Fault Formation
Faults form as a result of stress, which is the force applied to a rock mass, leading to deformation called strain. Rocks near the Earth’s surface respond to increasing stress by deforming elastically, storing energy. If the force continues to increase, the rock eventually reaches its breaking point, resulting in brittle deformation, which is the physical fracture of the rock mass.
The type of stress applied dictates the kind of fault that forms. Tensional stress involves forces that pull the crust apart, causing stretching. Compressional stress pushes rock masses together, leading to a shortening or thickening of the crust. Shear stress involves forces that cause rocks to slide horizontally past one another in opposite directions.
Classifying Fault Types by Movement
The three primary types of faults are classified based on the direction the hanging wall moves relative to the footwall, reflecting the specific stress regime that caused the break. Normal faults result from tensional stress. In a normal fault, the hanging wall block moves downward relative to the footwall block. This motion results in the extension of the crust and is commonly observed in rift valleys, such as the Basin and Range Province in the western United States.
Reverse faults are caused by compressional stress. Here, the hanging wall block moves upward relative to the footwall block, effectively pushing rock layers over one another. Reverse faults with a shallow dip angle (less than 45 degrees) are called thrust faults. This type of faulting is associated with mountain building events, causing crustal thickening, such as in the formation of the Rocky Mountains or the Himalayas.
Strike-slip faults involve purely horizontal movement along a nearly vertical fault plane. These faults are a product of shear stress, where the rock masses slide past each other rather than moving up or down. They are categorized as either right-lateral (dextral) or left-lateral (sinistral), depending on the direction of movement observed across the fault. The San Andreas Fault in California is a large-scale example of a right-lateral strike-slip fault.
Faults, Plate Tectonics, and Earthquakes
Faults are the localized expression of the global forces generated by plate tectonics, and they are particularly concentrated at plate boundaries. Divergent plate boundaries, where plates move away from each other, create tensional stress and are dominated by normal faults. Convergent boundaries, where plates collide, generate compressional stress and are characterized by reverse and thrust faults. Transform boundaries, where plates slide horizontally past one another, are defined by large strike-slip faults.
The movement along these faults is not constant; friction holds the rock blocks together, allowing stress and elastic energy to build up over time. When the accumulated stress finally exceeds the frictional resistance, the rock blocks suddenly slip, releasing the stored energy in the form of seismic waves, which causes an earthquake. The specific type of fault determines the mechanism of the earthquake and the pattern of ground shaking it produces.