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 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 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.
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.
- Sheriff, R. E; Geldart, L. P (August 1995). Exploration Seismology, 2nd Ed. Cambridge Univ. Press. p. 376, 461. ISBN 9780521468268.
- Boyer, Steven E.; Elliott, David (1982-09-01). "Thrust systems". AAPG Bulletin 66 (9): 1196–1230. http://aapgbull.geoscienceworld.org/content/66/9/1196.PDF version.
- Bally, A. W.; Snelson, S. (1980). Facts and principles of world petroleum occurrence: realms of subsidence. Memoir 6. Canadian Soc. Petr. Geol.. pp. 9–90.