# Dictionary:Fault

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1. A displacement of rocks along a shear surface; see Figures F-2, F-3, and F-4. The surface along which displacement occurs is called the fault plane (often a curved surface and not "plane" in the geometric sense). The dip of the fault plane is the angle that it makes with the horizontal; the angle with the vertical is called the hade; other terms related to faulting are defined in Figures F-2, F-3, and F-4.

The trace of a fault is the line that the fault plane makes with a surface (often the surface of the ground, sometimes a bedding surface). Faults are classified as normal, reverse, or strike-slip, depending on the relative motion along the fault plane; see Figure F-3. A fourth type of fault, associated with plate movement, is a transform fault (q.v.). A hinge or scissors fault is produced by rotation of the blocks across the fault about an axis perpendicular to the fault plane so that throw varies along the fault trace. Primary faults may produce secondary stresses that produce secondary faults (which may be of different type). Thus thrusting may produce tensions that cause secondary normal faults. Faulting and folding are common responses to the same stresses; see Figure F-17. Faulting during sediment deposition (growth faulting) often affects the stratigraphy such that beds may abruptly thicken and become more sandy downthrown at a normal growth fault. Evidences of faults in seismic data[1] are principally by:

(a) abrupt termination of events, (b) diffractions, (c) changes in dip, either flattening or steepening, (d) distortions of dips seen through the fault, a consequence of raypath bending because of velocity changes across a fault, (e) deterioration of data beneath the fault producing a "shadow-zone," (f) changes in the pattern of events across the fault, and (g) occasionally a reflection from the fault plane. Faults (especially small ones) are often en echelon or braided rather than parallel and continuous.

 FIG. F-4. Fault types. (a) A secondary antithetic fault, has throw in the direction opposite to that of the primary fault P and secondary synthetic fault S. (b) Faulting involving rotation that increases the throw of the fault. (c) Growth fault curved (listric) in both plan and cross-section. Fault movement contemporaneous with deposition produces thickening into the fault and a rollover anticline. (d) Development of a duplex structure by thrust faulting (overthrusting or underthrusting)[2]. FIG. F-4. Fault types. (Continued). (e) Wrench (strike-slip) faulting. A wrench fault often has associated secondary wrench, normal, and thrust faults and folding at roughly 30° to the wrench fault. Wrench fault traces are often en echelon rather than continuous. (f) Right-lateral wrench fault. Components of convergence or divergence may produce flower structures (see Figure F-12). (From Sheriff and Geldart, 1995, 369–370.) (g) Transitions from fault to fault may be accompanied by tear faults, folds, or in other ways. (h) Faulting associated with subduction zone. The accretionary wedge is generally built by underthrusting. (From Lowell, 1985.) (i) Transform fault offsetting spreading center. Rift faulting associated with extension is usually asymmetric[3].

2. In gravity or magnetic data, the edge of a thin, roughly horizontal slab with density or susceptibility different from that of horizontally adjacent material.