What defines a normal fault?

A normal fault is characterized by the motion of the blocks involved. In this type of fault, the hanging wall moves downward relative to the foot wall. This vertical movement occurs due to extensional forces—when tectonic forces pull the crust apart, the sections of rock on either side of the fault are displaced vertically. This movement is what distinguishes normal faults from other types, such as reverse faults, where the hanging wall moves upward.

The concept of the hanging wall and foot wall is critical in understanding fault mechanics; the hanging wall is the block that sits above the fault plane, while the foot wall is the block below it. In normal faults, the downward movement reflects the tension in the Earth's crust, often associated with divergent boundaries, though normal faults can also occur in continental rifting settings.

In contrast to this, the other choices involve movements or characteristics that do not accurately describe a normal fault. While faults certainly can occur at tectonic plate boundaries, this attribute isn't exclusive to normal faults—different types of faults can exist at these boundaries. Moreover, horizontal movement aligns more with strike-slip faults rather than normal faults, and a normal fault specifically emphasizes vertical displacement.