There are two types of faults: strike-slip and dip-slip. Right-lateral (or dextral) and left-lateral (or sinistral) strike-slip faults exist, as do normal and reverse (or thrust) dip-slip faults (Figure 7). A third type of fault, a horizontal one, forms when two objects physically touch but do not move relative to each other; this is called a static friction surface. Buildings, for example, can have their foundation walls settle over time due to building weight or soil movement; however, they remain in place because of the presence of an underlying support system of steel beams and columns that prevent further motion.
Fault zones form where two objects move past each other, such as on a mountain trail or when pushing/pulling a cart. If one object is heavier than the other, it will slide toward the lower ground (or uphill in the case of a pull) until both come to a stop.
In geology, faults are linear segments within Earth's crust where rock layers slide past each other. Faults may be either vertical or horizontal. Vertical faults occur where one layer of rock slides down another layer of rock perpendicular to the direction of gravity. Horizontal faults occur where one layer of rock slides along another layer of rock at a slight angle to the direction of gravity.
Most faults contain both vertical and horizontal elements.
Faults are classified into four types: reversal faults, strike-slip faults, oblique faults, and normal faults. Reversal faults occur where one block of rock is dropped down beneath another layer that was previously above it. For example, if two layers of sedimentary rock are tilted toward each other at a 90-degree angle, then they formed in an area where there was previous exposure to an east-west direction, such as along the coast or in an archipelago region. If one of the layers is thicker than the other, it will drop down beneath the thinner layer.
Strike-slip faults form where two rigid plates collide and push past one another instead of merging together. One plate may be the floor of a trench or basin while the other is a nearby mountain range. The point where these plates meet forms a linear feature called a fault line. Faults can also form where a rock ledge overhangs the edge of a valley wall and cracks slowly spread as more rock falls from above. This type of fault is called an oblique fault. Finally, where one layer of rock is dropped down beneath another that was previously below it, but not directly under it, this is known as a normal fault.
Strike-slip faults are horizontally moving faults that are classed as either right-lateral or left-lateral. Oblique-slip faults are those that exhibit both dip-slip and strike-slip motion. They can be either right-dipping or left-dipping.
Fault types are determined by observing the relative orientation of the fault plane to the surrounding terrain. If the fault plane is leaning the same direction as the surrounding terrain, then it is a right-lateral fault. If it is leaning opposite to the direction of the surrounding terrain, then it is a left-lateral fault. If it is dipping at an angle in both directions, then it is an oblique-slip fault.
Right-lateral (or north-dipping) faults occur where one side of the fault is dropping while the other is rising. Left-lateral (or south-dipping) faults occur where one side of the fault is rising while the other is dropping. Oblique-slip faults cross their entire width at a constant angle with respect to the horizontal surface. They can also curve back on themselves forming reverse faults. Strike-slip faults do not curve back on themselves because they require two separate surfaces with different dips to form a loop.
Normal faulting, reverse faulting, strike-slip faulting, and oblique faulting are the four forms of faulting. When normal or reverse faults have some strike-slip movement, and strike-slip faults have some normal or reverse movement, oblique movement occurs. All forms of faulting can occur simultaneously within the same region.
Faulting can also be horizontal or vertical. Horizontal faulting is found in areas where the rock layers tilt toward each other (like the side of a hill). Vertical faulting is when the layers dip downward (as if they were the floor of a room with layers of rock as walls).
Both horizontal and vertical faulting can occur simultaneously within the same region. For example, up on the surface you might find both horiztonal and vertical faulting within the same rock layer.