A geological fault is a fracture or zone of fractures in the Earth’s crust where two blocks of rock have moved relative to each other. This movement, which can be rapid during an earthquake or slow through aseismic creep, separates a simple crack from a fault. Most faults exist deep underground. When they reach the surface, the visible features they create are dynamic and temporary, constantly being reshaped by environmental forces, making direct observation dependent on the recency of movement.
Defining Geological Faults
A fault represents a significant break in the rock layers, ranging in size from a few centimeters to hundreds of kilometers. The movement along the fault plane—the flat surface of the fracture—defines the three primary types of faults, which are classified based on the direction of displacement between the two blocks of rock on either side of the plane.
A normal fault occurs when the block of rock above the fault plane, known as the hanging wall, moves downward relative to the footwall below it. This faulting results from tensional stress, where the crust is being pulled apart, such as in rift valleys. Conversely, a reverse fault forms under compressional stress, where the hanging wall moves up and over the footwall, often leading to crustal shortening and mountain building.
The third major type is the strike-slip fault, characterized by predominantly horizontal, side-to-side motion. The two blocks slide past one another with little vertical displacement, and the fault plane is often nearly vertical. An observer standing on one side of a strike-slip fault would see the opposite block move either to the left or to the right.
Surface Visibility and Fault Scars
What an observer sees on the surface is not the deep, planar fault itself, but rather the fault trace or fault scarp—the topographic expression of the rupture. A fault scarp is a steep, step-like offset of the ground surface that forms when there is a significant vertical shift between the two blocks of rock. Scarps can vary widely in size, from small displacements of a few centimeters to towering cliffs several meters high, depending on the magnitude of the seismic event.
Surface visibility is uncommon and usually requires very recent movement because these features are highly susceptible to erosion. Weathering, mass wasting, and water runoff begin to degrade a scarp almost immediately, especially if the material is unconsolidated sediment. Over time, these forces wear down the steep slope, making the fault trace nearly indistinguishable or burying it under sediment.
Strike-slip faults, which may not create a prominent scarp, often create offset features. These include shifted streams, fences, roads, or rows of trees that have been laterally displaced across the fault line. The continuous grinding action of fault movement can also create linear valleys or trenches that follow the fault trace, which are often more apparent in aerial or satellite imagery than they are from the ground. Clear visibility of any fault feature is strongest immediately following a significant earthquake, where the sudden rupture creates a fresh, distinct break in the landscape.
Methods for Detecting Hidden Faults
Since most faults do not break the surface or their visible traces have been erased by erosion, geologists rely on indirect methods to map and study these hidden structures.
Seismic Reflection
Seismic reflection uses sound waves generated at the surface to map the subsurface rock layers. By analyzing how these waves travel through the Earth and bounce back, scientists can identify discontinuities or abrupt changes in the rock structure. This reveals the location and geometry of a buried fault plane.
Geodetic Monitoring
Geodetic monitoring provides another means of detection by measuring the incredibly slow, subtle movements of the Earth’s surface over time. Technologies like the Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar (InSAR) track ground deformation that indicates strain accumulation or slow creep along a fault that has not yet ruptured the surface. These tools can detect shifts of only millimeters or centimeters per year, offering insight into active zones with no visual surface expression.
Paleoseismology
Paleoseismology is a field dedicated to studying evidence of ancient earthquakes, often involving the physical excavation of trenches across suspected fault traces. By digging into the sediment, geologists view the layered stratigraphy and identify buried fault scarps or displaced layers that record past rupture events. This trenching allows for the dating of prehistoric earthquakes, providing a long-term record of a fault’s activity even when surface features have long since vanished.