![]() ![]() strike-slip, where the offset is predominantly horizontal, parallel to the fault trace.īased on the direction of slip, faults can be categorized as: Distinguishing between these two fault types is important for determining the stress regime of the fault movement.įaults are mainly classified in terms of the angle that the fault plane makes with the earth's surface, known as the dip, and the direction of slip along the fault plane. In a reverse fault, the hanging wall displaces upward, while in a normal fault the hanging wall displaces downward. These terms are important for distinguishing different dip-slip fault types: reverse faults and normal faults. This terminology comes from mining: when working a tabular ore body, the miner stood with the footwall under his feet and with the hanging wall above him. The hanging wall occurs above the fault plane and the footwall occurs below it. The two sides of a non-vertical fault are known as the hanging wall and footwall. In practice, it is usually only possible to find the slip direction of faults, and an approximation of the heave and throw vector. The direction and magnitude of heave and throw can be measured only by finding common intersection points on either side of the fault (called a piercing point). Drag folding is a zone of folding close to a fault that likely arises from frictional resistance to movement on the fault. The vector of slip can be qualitatively assessed by studying any drag folding of strata, which may be visible on either side of the fault. Microfault showing a piercing point (the coin's diameter is 18 mm (0.71 in)) In measuring the horizontal or vertical separation, the throw of the fault is the vertical component of the separation and the heave of the fault is the horizontal component, as in "Throw up and heave out". ![]() A fault's sense of slip is defined as the relative motion of the rock on each side of the fault concerning the other side. ![]() Slip is defined as the relative movement of geological features present on either side of a fault plane. Normal fault and drag folds (eastern flanks of the Bighorn Mountains, Wyoming, US) The fault plane is the steeply leftward-dipping line in the centre of the photo, which is the plane along which the rock layers to the left have slipped downwards, relative to the layers to the right of the fault. A fault in ductile rocks can also release instantaneously when the strain rate is too great. Strain occurs accumulatively or instantaneously, depending on the liquid state of the rock the ductile lower crust and mantle accumulate deformation gradually via shearing, whereas the brittle upper crust reacts by fracture – instantaneous stress release – resulting in motion along the fault. Stress builds up when a fault is locked, and when it reaches a level that exceeds the strength threshold, the fault ruptures and the accumulated strain energy is released in part as seismic waves, forming an earthquake. The regions of higher friction along a fault plane, where it becomes locked, are called asperities. A second normal fault is at the right.ĭue to friction and the rigidity of the constituent rocks, the two sides of a fault cannot always glide or flow past each other easily, and so occasionally all movement stops. The light layer of rock shows the displacement. See also: Fault mechanics Normal fault in La Herradura Formation, Morro Solar, Peru. Prolonged motion along closely spaced faults can blur the distinction, as the rock between the faults is converted to fault-bound lenses of rock and then progressively crushed. However, the term is also used for the zone of crushed rock along a single fault. Ī fault zone is a cluster of parallel faults. A fault trace is also the line commonly plotted on geologic maps to represent a fault. A fault trace or fault line is a place where the fault can be seen or mapped on the surface. Ī fault plane is the plane that represents the fracture surface of a fault. Faults may also displace slowly, by aseismic creep. Energy release associated with rapid movement on active faults is the cause of most earthquakes. ![]() Large faults within Earth's crust result from the action of plate tectonic forces, with the largest forming the boundaries between the plates, such as the megathrust faults of subduction zones or transform faults. In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. ![]()
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