Fault

Faults themselves do not cause earthquakes; instead, they are the lines at which plates meet. When the plates press together (compress) or pull apart (are in tension), earthquakes occur. The fault line is essentially a stress concentration. If a rubber band is cut partially through then pulled, the rubber band is most likely to break at the cut (the stress concentration). Similarly, the "break" (stress release or earthquake) occurs along a fault when Figure 2. Formation of oceanic basin. Illustration by Hans & Cassidy. Courtesy of Gale Group.
the plates or rock bodies that meet at the fault press together or pull apart.

Movement along a fault can be vertical (up and down, changing the surface elevation), horizontal (flat at the surface but with one side moving relative to the other), or a combination of motions that inclines at any angle. The angle of inclination of the fault plane measured from the horizontal is called the dip of the fault plane. This movement occurs along a fault surface or fault plane. Any relative vertical motion will produce a hanging wall and a footwall. The hanging wall is the block that rests upon the fault plane, and the footwall is the block upon which you would stand if you were to walk on the fault plane.

Dip-slip faults are those in which the primary motion is parallel to the dip of the fault plane. A normal fault is a dip-slip fault produced by tension that stretches or thins Earth's crust. At a normal fault, the hanging wall moves downward relative to the footwall. Two normal faults are often separated by blocks of rock or land created by the thinning of the crust. When such a block drops down relative to two normal faults dipping toward each other, the block is called a graben. The huge troughs or Figure 3. Thrust fault striking north. The solid square represents the slip vector showing the motion of block A relative to block B. Illustration by Hans & Cassidy. Courtesy of Gale Group.
rift valleys created as plates move apart from each other are grabens. The Rhine Valley of Germany is a graben. An extreme example is the Atlantic Ocean; over 250 million years ago, North America and Africa were a single mass of land that slowly split apart and moved away from each other (a process called divergence), creating a huge graben that became the Atlantic Ocean basin. Two normal faults dipping away from each other can create an uplifted block between them that is called a horst. Horsts look like raised plateaus instead of sunken valleys. If the block between normal faults tilts from one side to the other, it is called a tilted fault block.

A reverse fault is another type of dip-slip fault caused by compression of two plates or masses in the horizontal direction that shortens or contracts the earth's surface. When two crustal masses butt into each other at a reverse fault, the easiest path of movement is upward. The hanging wall moves up relative to the footwall. When the dip is less than (flatter than) 45°, the fault is termed a thrust fault, which looks much like a ramp. When the angle of dip is much less than 45° and the total movement or displacement is large, the thrust fault is called an overthrust fault. In terms of plate movement, the footwall is slipping underneath the hanging wall in a process called subduction.

Strike-slip faults are caused by shear (side-by-side) stress, resulting in a horizontal direction, parallel to the nearly vertical fault plane. Strike-slip faults are common in the sea floor and create the extensive offsets mapped along the mid-oceanic ridges. The San Andreas Fault is perhaps the best-known strike-slip fault, and, because much of its length crosses land, its offsets are easily observed. Strike-slip faults have many other names including lateral, transcurrent, and wrench faults. Strike-slip Figure 4. Strike-slip fault. Illustration by Hans & Cassidy. Courtesy of Gale Group.
faults located along mid-oceanic ridges are called transform faults. As the sea floor spreads, new crust is formed by magma (molten rock) that flows up through the break in the crust. This new crust moves away from the ridge, and the plane between the new crust and the older ridge is the transform fault.

Relative fault movement is difficult to measure because no point on the earth's surface, including sea level is fixed or absolute. Geologists usually measure displacement by relative movement of markers that include veins or dikes in the rock. Sedimentary rock layers are especially helpful in measuring relative uplift over time. Faults also produce rotational movements in which the blocks rotate relative to each other; some sedimentary strata have been rotated completely upside down by fault movements. These beds can also be warped, bent, or folded as the comparatively soft rock tries to resist compressional forces and friction caused by slippage along the fault. Geologists look for many other kinds of evidence of fault activity such as slickensides, which are polished or scratched fault-plane walls, or fault gouge, which is clayey, fine-grained crushed rock caused by compression. Coarse-grained fault gouge is called fault breccia.