The hydrogeology of geological faults is a complex phenomenon given the tendency of faults to act as both conduits and barriers.[1] Appreciations of the hydrogeology of faults have proven to be different depending on the approach, varying whether it is studied through structural geology, hydrogeology, tunneling engineering), petroleum investigations or mining and dam engineering.[1][2]​.

In faults, brittlely deformed rocks alter the flow of water or other underground fluids depending in part on the lithologies; for example if it consists of clastic, igneous or carbonate rocks.[1]​ Fluid movements, which can be understood through the permeability of the medium, can be facilitated or impeded by the existence of a fault zone.[1]​ This is because the different mechanisms that deform rocks in faults alter the porosity and permeability of their environment.[1]​[3]​ The fluids hosted in fault systems are generally water, whether fresh or brackish, or hydrocarbons such as oil and natural gas.[4]​.

Note that permeability (k) and hydraulic conductivity (K) are used interchangeably in this article.

A fault zone can broadly be divided into two zones; a fault core (NF) and a damage zone (ZD) surrounding it.[2][5]​ (Figure 1). The fault core has a measurable thickness, the greater the longer the displacement of the fault, which implies greater deformation.[1]​ The damage zone surrounds the fault core irregularly and can have a width (perpendicular to the fault zone) from a few decimeters to hundreds of meters.[6]​ Within large fault zones there may be multiple fault cores and damage zones.[1]​ Fault cores and damage zones of more recent origin may be overlap with cores and older areas of damage.

There are several processes that can alter the permeability in the cores and damage zones in a fault zone. In general, the permeability of a damage zone exceeds that of the associated fault core by several orders of magnitude, thus causing damage zones to be conduits for groundwater.[7]​ Within the damage zone, permeability tends to decrease as one moves away from the fault core.[7]​.

There are several classifications for fault zones based on their permeability patterns. Some classification categories are interchangeable, while others have different subgroups. Most of the expressions are shown in the following table. Dickerson's classification is the most used as it is easy to understand in a wide range of studies.[2]​.