Faulting

Faulting occurs when Earth's crust ruptures as a consequence of accumulated stress; it is the most common cause of earthquakes. Along with folds, faults are the most obvious results of deformation within the crust and constitute the field of structural geology. While folds are the result of ductile (plastic) deformation, faults indicate brittle behavior and occur only in the relatively cold surficial layers of Earth, generally down to a depth of about 70 km (kilometers; 40 miles). Although faulting may cause vertical uplift, the stresses that lead to rupture are usually horizontal, related to movements along the boundaries that define the plates that make up Earth's crust. Three types of movement predominate—extension, compression, and lateral shear—and lead to three major types of fault, commonly referred to as normal, reverse (thrust), and strike-slip faults. Because the type of fault is directly related to the orientation of stress in the crust, faults (even small or inactive ones) serve as sensitive indicators of those stresses that existed at the time the fault developed.

In defining faults, it is critical to observe the type of motion on the fault plane. Some faults are vertical, but the majority are inclined at some angle to the surface. The orientation of the fault plane (or any other planar surface in geology) [Page 1086]is defined by strike and dip, the strike being the geographic trend of the plane, given as a compass azimuth, and the dip, which is a direction and angular measure of the inclination from the horizontal, measured perpendicular to strike. In strike-slip faults the displacement is parallel to the strike, while in normal and reverse (dip-slip) faults the displacement is parallel to the dip. Faults that combine components of both strike and dip movement are said to show oblique slip. The side above the fault plane is referred to as the hanging wall, which dates from early mining terminology, while the side below is the footwall. A miner observing an inclined fault in a horizontal tunnel would stand with his feet on the footwall, with the hanging wall suspended over his head (Figure 1). The following definitions are based on the foregoing axioms.