Now that we have reviewed plate tectonics and the properties of faults, it's time to put the two together to explain how, why, and where faults form.
Our knowledge of the mechanism of fault formation is still primarily limited to theory and extrapolation. We can create conditions in laboratory experiments to simulate the formation of fractures and faults, but we cannot actually watch faults forming or even lengthening beneath the Earth's surface. Still, the principles of fracturing and fracture propagation are understood well enough to convince us that faulting can be described by applying the basic laws of stress dynamics to the Earth's crust. What insight we do have about fault formation in the real world comes primarily from studying seismicity. New fracturing of the crust produces earthquakes in much the same way that earthquakes are generated by movement along previous fractures. From these movements, we can learn about fault formation indirectly.
Understanding why and where faults form, at least in the context of southern California, requires a simple understanding of plate tectonics. While the models we will be using below are greatly simplified, the general ideas hold true for real-world faulting, as we will show later.
Faults can occur in any orientation and at any angle to horizontal (remember that this angle is known as the dip of the fault), but an active fault's orientation and dip are not necessarily random. These properties are influenced by the regional stress field created by tectonic activity.
This happens because slip along a plane occurs much more easily and efficiently at some angles to the forces being applied than it does at others. For example, if you sit still on a horizontal surface, you will not slip off. However, if someone begins tilting the surface you're sitting on, you will eventually slide off, because the pull of gravity will finally overcome the frictional force keeping you in place.
This is similar to what happens along faults when earthquakes occur. A driving force eventually overcomes a resistant force, resulting in slip. If the surface you sat upon was incredibly sticky -- that is, if the coefficient of friction and the resulting frictional force were large enough -- you might not begin to slip until the surface was tilted almost to vertical (90°). There would be a very narrow range in which the "tectonic stress" (gravity) could be released as slip (you, sliding off the surface). Only a "fault" (the interface between you and the surface) within a certain narrow range of orientations would be efficient at releasing the stress caused by the opposition of the driving and resistant forces, so only such a fault would "form" (experience slip). If your friends were sitting on similar surfaces with lesser tilt angles, they would remain stuck, because their "faults" would not be oriented in a way that encourages slip.