Geological Fault
A geological fault is a planar fracture or discontinuity in rock across which significant displacement has occurred due to tectonic stresses. Faults are classified by the direction of relative motion: normal faults (extension, hanging wall moves down), reverse or thrust faults (compression, hanging wall moves up), and strike-slip faults (horizontal shear motion along the fault plane). The sudden release of accumulated elastic strain energy along a fault produces earthquakes, and repeated fault movements over geological time can build mountain ranges, create rift valleys, and shape landscape topography.
Classification of Geological Faults
| Fault Type | Motion Type | Tectonic Setting | Dip Angle | Example |
|---|---|---|---|---|
| Normal Fault | Extensional | Divergent boundary | 45°–90° | Basin and Range, USA |
| Reverse Fault | Compressional | Convergent boundary | 45°–90° | Rocky Mountains thrust |
| Thrust Fault | Compressional (low angle) | Convergent boundary | <45° | Himalayas Main Thrust |
| Strike-Slip (Right-lateral) | Horizontal shear | Transform boundary | Near vertical | San Andreas Fault |
| Strike-Slip (Left-lateral) | Horizontal shear | Transform boundary | Near vertical | North Anatolian Fault |
| Oblique-Slip Fault | Combined dip and strike | Various | Variable | Alpine Fault, NZ |
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Related Terms
Tectonic Plate
A tectonic plate is a massive, irregularly shaped slab of solid rock composed of oceanic or continental crust together with the underlying upper mantle (lithosphere) that moves atop the semi-fluid asthenosphere. Earth's lithosphere is divided into seven major plates and several minor ones that move relative to one another at rates of 2–15 cm per year, driven primarily by mantle convection, slab pull, and ridge push. The movement of tectonic plates is responsible for earthquakes, volcanic activity, mountain building, and the distribution of continents over geological time.
Earthquake Epicenter
The earthquake epicenter is the point on Earth's surface directly above the hypocenter (or focus), which is the underground location where an earthquake rupture begins. The epicenter is located using seismic wave arrival time differences recorded at multiple seismograph stations, with the distance to the epicenter calculated from the S-P wave time interval. The epicenter is the reference point used in earthquake reporting, and ground shaking intensity is generally greatest near the epicenter, decreasing with distance according to attenuation relations.
Seismograph
A seismograph (or seismometer) is an instrument that detects and records ground motion caused by seismic waves, volcanic activity, explosions, or other disturbances. The basic principle relies on inertia: a heavy mass suspended by a spring remains relatively stationary while the instrument frame moves with the ground, and this relative displacement is amplified and recorded as a seismogram. Modern broadband seismographs use electromagnetic sensors or force-balance accelerometers and can detect ground motions as small as 10⁻¹⁰ m over a frequency range of 0.001–50 Hz.
From Old French "faute" and Latin "fallita" (a failing or deficiency), from "fallere" (to fail or deceive). The geological use of "fault" to describe a fracture in rock developed in mining terminology during the 16th–17th centuries in England, where miners encountered rock displacements that "failed" to match on either side of the break.