In geology, the term fault refers to a planar fracture or discontinuity in a volume of rock across which there has been enough displacement as a result of rock-mass movement. Large faults in the crust of the earth are caused by the action of plate tectonic forces. The largest forms the boundaries between the plates like transform faults or subduction zones. Energy release that is associated with rapid movement on active faults is the cause of many earthquakes.
LEARNING OBJECTIVES
By the end of this topic, you are expected to;
A fault plane refers to the plane representing the fracture surface of a fault. A fault line or a fault trace is a place where the fault can be mapped or seen on the surface. A fault trace also refers to the line that is commonly plotted on geologic maps to represent a fault.
Because faults do not normally have a single clean fracture, geologists use the word fault zone when referring to the zone of complex deformation that is associated with the fault plane.
MECHANISMS OF FAULTING
Owing to friction and the constituent rocks’ rigidity, the two sides of a fault cannot always glide or flow past each other easily, and so occasionally all movement stops. Regions of higher friction along a fault plane, where it becomes locked, are known as asperities.
SLIP, HEAVE, THROW
Slip refers to the relative movement of geological features that are present on either side of a fault plane. The throw of the fault refers to the vertical component of the separation. The heave of a fault refers to the horizontal component.
HANGING WALL AND FOOTWALL
The two sides of a non-vertical fault are known as the footwall and the hanging wall. The hanging wall is found above the fault plane and the footwall is found below it.
FAULT TYPES
Based on the direction of slip, there are three categories of faults;
STRIKE-SLIP FAULTS
It is also referred to as a wrench fault, transcurrent fault, or tear fault. In this fault, the fault surface (plane) is normally near vertical, and the footwall moves laterally either right or left with little vertical motion.
DIP-SLIP FAULTS
These faults can either be normal or reverse. In a normal fault, the hanging wall moves downward, relative to the footwall. A reverse fault is the opposite of a normal fault- the hanging wall moves up relative to the footwall.
OBLIQUE-SLIP FAULTS
A fault with a component of dip-slip and a component of strike-slip is called an oblique-slip fault.
