A geological fault is a fracture or zone of fractures in the Earth’s crust where the rocks on either side have moved relative to each other. These movements are the result of immense stresses built up by the slow, continuous motion of tectonic plates across the planet’s surface. When the stress overcomes the friction holding the rocks together, the sudden slip releases energy, which we experience as an earthquake. Geologists use the term “active fault” to distinguish those that are likely to move again from those that are essentially dormant.
Defining an Active Fault
The classification of a fault as “active” is based on a specific, recent geological timeline, reflecting its potential to generate future earthquakes. The most widely accepted definition for engineering and regulatory purposes holds that an active fault is one that has shown evidence of movement during the Holocene epoch, which spans the last approximately 11,700 years. This timeframe is considered relevant because it represents the most current regional tectonic stress regime.
Faults that have moved within the broader Quaternary period (the last 2.58 million years) but not within the Holocene are often deemed “potentially active” or “Late Quaternary active.” A fault that has moved recently is expected to accumulate stress and rupture again, whereas a truly inactive fault is no longer being driven by the current tectonic forces.
The Mechanics of Fault Movement
Fault movement is controlled by the type of stress applied to the rock mass, and geologists categorize these movements into three primary types. A normal fault occurs in areas where the crust is being pulled apart (tensional stress). In this scenario, the block of rock above the inclined fault plane, called the hanging wall, moves downward relative to the block below it, causing the crust to extend.
The second type is a reverse fault (or a thrust fault if the angle is shallow), caused by compressional stress. Here, the crust is being pushed together, and the hanging wall moves up and over the footwall, resulting in a shortening of the crust. The third major category is the strike-slip fault, where the movement is predominantly horizontal and parallel to the fault plane, driven by shearing forces. The San Andreas Fault is a well-known example, where the adjacent blocks slide past each other rather than moving up or down.
How Geologists Identify Activity
Geologists determine the activity of a fault primarily through a specialized field of study called paleoseismology, which involves investigating the geological record of ancient earthquakes. The most direct method is trenching, where a trench is carefully excavated across the visible trace of a fault on the surface. The exposed wall of the trench reveals the layers of soil and sediment that have been offset by past earthquakes.
Within the trench wall, scientists look for features like fault scarps (small offsets in the ground surface) and colluvial wedges (fan-shaped deposits that accumulate at the base of a scarp after a rupture). By observing how the fault cuts through and displaces various layers, geologists determine the number and magnitude of past slip events.
Dating materials within these layers, such as organic matter, using radiocarbon dating provides the absolute age of the offset layers and the timing of prehistoric earthquakes. This dating allows geologists to establish the fault’s recurrence interval—the average time between major ruptures—and confirm if the last event occurred within the defined active timeframe.
Real-World Implications for Seismic Hazard
The identification and mapping of active faults is a primary step in assessing seismic hazard for populated areas and infrastructure. Data gathered from paleoseismic studies directly informs seismic hazard maps, which estimate the probability and intensity of future ground shaking. This is relevant for facilities such as dams, power plants, and hospitals, which require a detailed understanding of local fault behavior.
Regulations often prohibit the construction of habitable structures directly across the trace of a known active fault to mitigate the risk of surface rupture. By quantifying the expected movement rate and the potential size of future earthquakes, geologists provide engineers with the necessary parameters to design buildings and infrastructure that can withstand the expected ground motion. The recurrence interval determined for an active fault indicates how frequently a community should prepare for a significant seismic event.