An earthquake is the sudden, violent shaking of the Earth’s surface, resulting from a rapid release of energy in the Earth’s crust. This event is caused by a geological feature known as a fault. Faults are weak spots where immense forces within the Earth are concentrated, and their movement is the direct source of nearly all seismic activity.
Defining a Geological Fault
A fault is a fracture or zone of fractures that separates two blocks of rock, where significant displacement has occurred. The surface along which the movement happens is called the fault plane, which can be nearly vertical or slope at an angle. The two rock masses on either side of the fault plane are termed the hanging wall and the footwall.
The footwall is the block of rock positioned beneath the fault plane. Conversely, the hanging wall is the block of rock that rests above the fault plane. Geologists use the relative movement of these two walls to classify the type of fault.
The Mechanism of Earthquake Generation
A fault causes an earthquake through a cycle of stress buildup and sudden release, a process explained by the Elastic Rebound Theory. Tectonic plates are constantly moving, but friction along a fault line often locks the two blocks of rock in place. As the plates continue to move, immense stress accumulates in the rocks on either side of the locked fault.
This stress causes the rock to slowly bend and deform, storing energy known as elastic strain. The rock deforms without breaking until the internal stress exceeds the rock’s strength or the frictional resistance holding the fault locked. When this breaking point is reached, the fault ruptures, and the two blocks of rock suddenly slip past each other.
This rapid slip releases the stored elastic energy as seismic waves that radiate outward, causing the ground shaking of an earthquake. Following the rupture, the rocks on both sides of the fault snap back toward their original shape, which is the “elastic rebound” that relieves the strain. The process then begins again as continuous tectonic plate movement accumulates new stress along the fault.
Classifying Fault Movement
Faults are classified by the direction of movement of the hanging wall relative to the footwall, determined by the type of tectonic force applied. The motion is categorized into three main types: normal, reverse, and strike-slip faults. Normal faults occur where the crust is pulled apart by tensional forces, extending the crust. In a normal fault, the hanging wall block moves downward relative to the footwall block.
Reverse faults are created by compressional forces that push the crust together, resulting in crustal shortening. In this case, the hanging wall block moves upward and over the footwall block. A specific type of reverse fault with a very shallow dip is called a thrust fault, which is common in mountain-building regions.
The third type is the strike-slip fault, caused by shearing forces that slide the blocks horizontally past one another. The fault plane is usually nearly vertical, meaning there is little vertical movement, and the terms hanging wall and footwall are less relevant. These faults are described as either right-lateral or left-lateral, depending on the direction of movement of the block opposite the observer.
Fault Location and Tectonic Setting
Most major faults occur along the boundaries where the Earth’s tectonic plates meet and interact. The type of plate boundary directly corresponds to the type of fault movement found there. Divergent boundaries, where plates pull apart, are associated with normal faults due to the tensional forces involved.
Convergent boundaries, where plates collide, generate compressional forces that result in reverse and thrust faults. Transform boundaries, where plates slide horizontally past each other, are defined by strike-slip faults, such as the San Andreas Fault in California. These plate-boundary faults are responsible for the vast majority of the world’s seismic activity.
Less common are significant faults that exist within the interior of tectonic plates, far away from any boundary, known as intraplate faults. These faults can still produce large earthquakes, like those in the New Madrid seismic zone in the central United States. Intraplate earthquakes are caused by plate boundary forces transmitted into the plate interior, often reacting along pre-existing zones of weakness in the crust.