The Earth’s rigid outer shell, the lithosphere, is divided into massive tectonic plates that are constantly in slow motion against one another. These interactions generate immense forces, causing the crust to fracture and shift. The resulting fracture surfaces, where measurable displacement has occurred, are known as geological faults, representing the physical boundary where rock masses have moved past each other.
Defining a Geological Fault and its Formation
A geological fault is defined as a planar fracture within the Earth’s crust where the rock blocks on either side have experienced significant relative displacement along the fracture surface. The process begins when massive tectonic forces subject the rock to stress, which is the force applied over a given area. This constant application of force causes the rock body to accumulate strain, which is the resulting physical deformation.
If the stress exceeds the rock’s internal strength and capacity for elastic deformation, the rock body fails and fractures, leading to the formation of a fault. The surface along which this movement takes place is called the fault plane, which can be angled steeply or shallowly relative to the surface.
Geologists distinguish between the two rock masses on a non-vertical fault plane. The block of rock situated above the fault plane is called the hanging wall, while the block below the plane is termed the footwall.
Classifying Faults by Movement
Faults are broadly categorized based on the direction of movement between the hanging wall and the footwall relative to the angle of the fault plane. The movement is a direct consequence of the type of tectonic stress acting on the crust: tensional, compressional, or shear. The first two classifications, Normal and Reverse faults, involve primarily vertical motion and are collectively known as dip-slip faults because the movement is along the dip, or slope, of the fault plane.
Normal Faults
A Normal fault occurs in environments subjected to tensional stress, where the crust is being pulled apart or extended. In this scenario, the hanging wall block moves downward relative to the footwall block due to stretching and the pull of gravity. This type of faulting is commonly found at divergent plate boundaries and in areas of continental rifting, such as the Basin and Range Province in the Western United States.
Reverse Faults
Conversely, a Reverse fault forms when the crust is under compressional stress, meaning the rock masses are being pushed toward one another, causing crustal shortening. Here, the hanging wall block moves upward relative to the footwall block, effectively overriding it.
When a reverse fault has a shallow dip, typically less than 45 degrees, it is specifically designated as a Thrust fault. These compressional faults are characteristic of convergent plate boundaries, where they contribute to the formation of massive mountain ranges like the Himalayas and the Rocky Mountains.
Strike-Slip Faults
The third major classification, the Strike-Slip fault, involves movement that is almost entirely horizontal or lateral. This motion results from shear stress, where the rock blocks slide past each other in opposing directions.
Since the motion is parallel to the fault line, there is little to no vertical displacement of the hanging wall or footwall. The San Andreas Fault in California is a well-known example of a large Strike-Slip fault, accommodating the horizontal movement between two tectonic plates.
The Role of Faults in Causing Earthquakes
Faults are zones where massive amounts of elastic energy can be stored and then abruptly released. Active faults are typically “locked” for long periods, unable to slip smoothly due to friction and the immense pressure of the overlying rock. Tectonic forces continue to drive the plates, forcing the rock around the locked fault to deform gradually, accumulating strain energy.
This continuous process of strain accumulation and sudden release is explained by the Elastic Rebound Theory. The theory posits that the rock behaves elastically, bending and deforming under stress until the accumulated strain surpasses the frictional resistance holding the fault together.
At this point, the fault ruptures, and the strained rock suddenly snaps back toward its original, undeformed shape. This abrupt slip along the fault plane releases the stored energy in the form of seismic waves, which travel through the Earth and cause the ground shaking known as an earthquake.
The exact point within the Earth where the rupture first occurs is called the hypocenter, or focus. The epicenter is the location on the Earth’s surface directly above the hypocenter, which is often the place where the shaking is felt most intensely.