What type of fault in which the hanging wall moves down relative to the foot wall as a result of extension?

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.

• Faults are fractures (cracks) in rocks along which movement has occurred.  Faults are classified according to which direction the rocks moved along the fault: normal faults, reverse faults, thrust faults, and strike-slip faults. Faults also create environmental effects such as the movement of groundwater, and can cause hazards such as rock slides and earthquakes.

In the above left diagram, the rock block is unfaulted, but contains a weakness.  The above right diagram shows the block after faulting.  The fault plane is the plane on which movement occurs.  The footwall is the part of the fault where your feet would be if you stood on the fault plane.  The hanging wall would be above you. 

Normal Faults

• If the hanging wall moves down relative to the footwall, the fault is a normal fault.  Normal faults are caused by tensional stress, or stress that pulls rocks apart..

Faults are the places in the crust where brittle deformation occurs as two blocks of rocks move relative to one another. Normal and reverse faults display vertical, also known as dip-slip, motion. Dip-slip motion consists of relative up-and-down movement along a dipping fault between two blocks, the hanging wall, and footwall. In a dip-slip system, the footwall is below the fault plane and the hanging wall is above the fault plane. A good way to remember this is to imagine a mine tunnel running along a fault; the hanging wall would be where a miner would hang a lantern and the footwall would be at the miner’s feet.

Faulting as a term refers to the rupture of rocks. Such ruptures occur at plate boundaries but can also occur in plate interiors as well. Faults slip along the fault plane. The fault scarp is the offset of the surface produced where the fault breaks through the surface. Slickensides are polished, often grooved surfaces along the fault plane created by friction during the movement.

A joint or fracture is a plane of brittle deformation in the rock created by the movement that is not offset or sheared. Joints can result from many processes, such as cooling, depressurizing, or folding. Joint systems may be regional affecting many square miles.

Normal faults move by a vertical motion where the hanging-wall moves downward relative to the footwall along the dip of the fault. Normal faults are created by tensional forces in the crust. Normal faults and tensional forces commonly occur at divergent plate boundaries, where the crust is being stretched by tensional stresses (see Chapter 2). Examples of normal faults in Utah are the Wasatch Fault, the Hurricane Fault, and other faults bounding the valleys in the Basin and Range province.

Grabens, horsts, and half-grabens are blocks of crust or rock bounded by normal faults (see Chapter 2). Grabens drop down relative to adjacent blocks and create valleys. Horsts rise up relative to adjacent down-dropped blocks and become areas of higher topography. Where occurring together, horsts and grabens create a symmetrical pattern of valleys surrounded by normal faults on both sides and mountains. Half-grabens are a one-sided version of a horst and graben, where blocks are tilted by a normal fault on one side, creating an asymmetrical valley-mountain arrangement. The mountain-valleys of the Basin and Range Province of Western Utah and Nevada consist of a series of full and half-grabens from the Salt Lake Valley to the Sierra Nevada Mountains.

Normal faults do not continue to clear into the mantle. In the Basin and Range Province, the dip of a normal fault tends to decrease with depth, i.e., the fault angle becomes shallower and more horizontal as it goes deeper. Such decreasing dips happen when large amounts of extension occur along very low-angle normal faults, known as detachment faults. The normal faults of the Basin and Range, produced by tension in the crust, appear to become detachment faults at greater depths.

In reverse faults, compressional forces cause the hanging wall to move up relative to the footwall. A thrust fault is a reverse fault where the fault plane has a low dip angle of less than 45°. Thrust faults carry older rocks on top of younger rocks and can even cause the repetition of rock units in the stratigraphic record.

Convergent plate boundaries with subduction zones create a special type of “reverse” fault called a megathrust fault where denser oceanic crust drives down beneath less dense overlying crust. Megathrust faults cause the largest magnitude earthquakes yet measured and commonly cause massive destruction and tsunamis.

Strike-slip faults have side-to-side motion. Strike-slip faults are most commonly associated with transform plate boundaries and are prevalent in transform fracture zones along mid-ocean ridges. In pure strike-slip motion, fault blocks on either side of the fault do not move up or down relative to each other, rather move laterally, side to side. The direction of the strike-slip movement is determined by an observer standing on a block on one side of the fault. If the block on the opposing side of the fault moves left relative to the observer’s block, this is called sinistral motion. If the opposing block moves right, it is dextral motion.

Video showing motion in a strike-slip fault.

Bends along strike-slip faults create areas of compression or tension between the sliding blocks (see Chapter 2). Tensional stresses create transtensional features with normal faults and basins, such as the Salton Sea in California. Compressional stresses create transpressional features with reverse faults and cause small-scale mountain building, such as the San Gabriel Mountains in California. The faults that splay off transpression or transtension features are known as flower structures.

An example of a dextral, right-lateral strike-slip fault is the San Andreas Fault, which denotes a transform boundary between the North American and Pacific plates. An example of a sinistral, left-lateral strike-slip fault is the Dead Sea fault in Jordan and Israel.

Video showing how faults are classified: