A fault is a fracture or zone of fractures in the Earth's crust along which relative displacement of the two sides has occurred. Faults are the primary sources of earthquakes, and their geometry, slip rate, and mechanical properties govern the size, frequency, and characteristics of seismic events.
A fault is a planar or curved zone of weakness in the Earth's crust along which the two blocks on either side have moved relative to each other. Faults form in response to tectonic stresses and accommodate the gradual accumulation of strain that is eventually released as earthquakes. They range in scale from millimeters in laboratory specimens to thousands of kilometers in length, such as the San Andreas Fault in California or the North Anatolian Fault in Turkey. Faults are the fundamental source of shallow earthquakes and the primary control on seismic hazard in most regions.
Faults are classified by their sense of slip. Strike-slip faults accommodate horizontal motion β either left-lateral or right-lateral, depending on whether the opposite block moves left or right when viewed across the fault. Dip-slip faults accommodate vertical motion β normal faults occur in extensional regimes, where the crust is pulled apart, while reverse or thrust faults occur in compressional regimes, where the crust is shortened. Oblique-slip faults combine both horizontal and vertical motion. The dip angle, the dip direction, and the rake of the slip vector together define the fault's orientation in three-dimensional space. Faults that reach the surface are called surface-rupturing faults and produce visible scarps and offset features; blind faults terminate below the surface and may only be identified through geophysical imaging or by the earthquakes they generate.
Faults are characterized by their slip rate, recurrence interval, and seismic behavior. A fault's slip rate (typically measured in mm/year by GPS or geological methods) constrains how rapidly strain accumulates. The recurrence interval is the average time between large earthquakes on a given fault segment, estimated from paleoseismic studies, historical records, or geodetic measurements. Some faults exhibit characteristic earthquakes β repeating similar-size events at quasi-regular intervals β while others produce a wider range of magnitudes. Some faults creep slowly without producing large earthquakes, while others are fully locked between major events. The distinction matters enormously for hazard assessment: a fault with a 500-year recurrence interval for magnitude 7 events poses a very different risk than one with a 5,000-year recurrence interval, even if their long-term slip rates are similar.