A geological fault is a fracture in the Earth’s crust where rock masses on either side have moved relative to one another. These breaks can range from a few centimeters to hundreds of kilometers. Movement along these fractures is the primary mechanism responsible for most earthquakes, as built-up stress is suddenly released. Geologists classify these faults based on the direction of displacement and the type of tectonic stress that caused the break. Understanding these classifications is fundamental to interpreting the geological history of a region and assessing seismic risk.
The Fundamental Structure of a Fault
To understand the different types of faults, it is helpful to first define the basic structural elements involved in the movement. The fracture surface itself is called the fault plane, which can be angled from nearly horizontal to vertical. When the fault plane is inclined, geologists identify the two blocks of rock by their position relative to this plane.
The block of rock that rests above the fault plane is termed the hanging wall. Conversely, the block of rock positioned below the fault plane is known as the footwall. The relative movement of the hanging wall with respect to the footwall defines the three main fault types.
Normal Faults
Normal faults occur in regions where the Earth’s crust is being stretched, a process driven by tensional stress. This extensional force causes the crust to lengthen and thin out over a broad area. The defining characteristic of a normal fault is that the hanging wall block moves downward relative to the footwall block.
This movement effectively increases the horizontal distance between two points on opposite sides of the fault. Normal faults are common features at divergent plate boundaries, such as along rift valleys where plates are separating.
Reverse Faults
Reverse faults form in environments where the crust is being compressed, resulting from compressional stress. This stress occurs when rock blocks are pushed toward one another, causing the crust to shorten and thicken. The distinguishing feature of a reverse fault is the upward movement of the hanging wall block relative to the footwall block.
The hanging wall is thrust up and over the footwall, effectively stacking rock layers and decreasing the horizontal distance across the fault. Reverse faults are associated with convergent plate boundaries, where tectonic plates collide to form large mountain ranges. A specialized type of reverse fault, called a thrust fault, is one where the fault plane is inclined at a shallow angle, typically less than 45 degrees.
Strike-Slip Faults
Strike-slip faults represent movement where the blocks of rock slide past each other predominantly in a horizontal direction. This motion is caused by shear stress, which involves forces acting parallel to the fault plane but in opposite directions. The nearly vertical orientation of the fault plane means there is little to no vertical displacement of the hanging wall or footwall, differentiating them from normal and reverse faults.
The displacement is entirely lateral, parallel to the strike of the fault. These faults are classified based on the relative direction of movement as viewed from one side. If the block on the opposite side moves to the right, the fault is termed a right-lateral strike-slip fault, like the famous San Andreas Fault. Conversely, if the opposite block moves to the left, it is classified as a left-lateral strike-slip fault.