Because strike-slip faults are vertical, they lack hanging walls and footwalls. You have a typical defect if the hanging wall dips relative to the footwall. Normal faults arise in regions that are being extended (stretching). The stretching causes certain layers of rock to break free from above the fault line and fall towards it.
The falling rock creates a pile known as a slump. Slumps can be quite large; some are more than 100 feet deep and 1,000 feet wide. As the rocks fall toward the fault, they re-arrange themselves into another layer called a cross-stratification. This new layer is also horizontal, so it does not stretch when the region is being stretched.
Normal faults can occur on many different types of geologic formations. They are most common on sedimentary rocks like shale or sandstone but can also form on crystalline rocks such as granite or gneiss. Normal faults can be very difficult to detect because they usually do not cause damage to buildings or roads. They may even appear to be useful for farming since they tend to drain land of moisture. However, farmers should not use these faults as water sources as they may contain acid minerals that would harm any plants that were grown in them.
Normal faults are typically less than 10 meters deep.
Vertical compression as the Earth's crust lengthens causes normal dip-slip faults. The hanging wall descends in relation to the footwall. Reverse dip-slip faults are generated by horizontal compressional forces induced by the Earth's crust shortening, or contraction. The hanging wall rises and crosses the footwall.
Compression along a normal fault creates tension in the rock that causes it to bend downward. In contrast, compression along a reverse fault induces compression stress that causes the rock to bend upward.
Reverse faults occur where one side of a valley is dropping more rapidly than the other. If there is no force balancing this drop, then the rock will break and create a reverse fault. For example, if a mountain range was created when two continents collided, then there would be no way for gravity to pull both continents together. Instead, the faster-dropping continent would plunge beneath its slower-moving neighbor.
If a river flows through a valley floor formed by a reverse fault, then we can assume that the rock on each side of the valley is moving away from each other. Because compression stresses rock fibers into parallel layers, the only option left for the rock on opposite sides of the valley to share pressure is with distance. One side will compress more than the other because there is more space between them.
A hanging wall is the steeply sloping portion of a valley wall where erosion has removed most of the rock material.
If the fault occurs in an extension scenario, it is a typical fault because the extension permits the hanging wall to slide down relative to the footwall in reaction to gravity. The third scenario involves rock bodies sliding sideways with regard to one other, as is the case along a transform fault (see Chapter 10). Sliding rocks can move for many miles until they come into contact with another rock body and then stop.
In addition to these three scenarios, faults may also develop due to tectonic activity. Faults resulting from tectonics are called "active" or "moving" faults. Active faults are those that deform over time due to plate movement. Moving faults allow seismic waves to pass through them, indicating that they may contain fluid that dampens these waves.
Finally, some faults do not move but instead develop parallel shear zones along their length. Shear zones can form where two blocks of rock are being pulled apart by gravity, for example, near the top of a mountain range. As long as the upper block is moving away from the lower block, stress will build up and a fault will eventually form.
The type of fault that develops depends on how the hanging wall is attached to the footwall. If the two walls are firmly attached, such as what would happen if they were buried under sedimentary layers and then exposed at the surface, then we call this a "normal" fault.
To properly diagnose a defect, first determine which block is the footwall and which is the hanging wall. The relative motion between the hanging wall and the footwall is then calculated. Every fault that is angled away from the vertical has a hanging wall and a footwall. Faults that are horizontal or nearly so don't require measurement.
Fault blocks can be identified by measuring their height off the floor or ground. If they're less than 2 feet, they're called shallow defects. Those below 4 feet are considered deep. Fault blocks may be of any color or composition, but if they're dry-laid, they'll usually be white or gray.
The next step is to locate the source of the defect. This requires walking around the area looking for signs of damage such as scuffed floors, broken tiles, or cracks in walls. Do not walk on top of damaged tile, because it will only cause more damage to itself and its surrounding area.
After you've taken all the necessary measurements and observed the damage, call in an expert. A geologist can help you identify what type of rock is underneath your building site and suggest ways to prevent future problems. They can also advise you on how to fix certain faults like dry holes or washouts if they occur again. Geologists typically work for building contractors or engineering firms but some operate their own businesses.
Normal faults are precisely the opposite of reverse faults. A reversal fault occurs when the hanging wall rises relative to the footwall. Reverse faults arise in regions that are being compressed (squishing). When the stress in a rock layer becomes great enough, it can no longer resist deformation and instead ruptures, often with dramatic results.
Reverse faults may occur as cracks or fissures form within the hanging wall due to compression. As more and more stress is placed on the rock, the crack or fissure will grow until it reaches through the rock layer and into the underlying material. This creates an opening through which the stressed-outlying rock layers can slip with respect to each other.
The amount by which a reverse fault depresses into the footwall is called the gob depth. The word "gob" comes from the mining term for the excavated material above a normal fault. The word "gobe" is used similarly for a reverse fault's depression.
Reverse faults may also form when two different rock layers are squeezed together under high pressure. If one layer is harder than the other, it will break first, leaving a cavity behind. This is called a "hole-punching" effect and can cause significant damage to buildings and roads constructed over reversed faults.