Fault core refers to a zone of fault rock that is both cut across and bound by slip surfaces at the center of a mature fault (Figure 1). Depending on the degree of the frictional sliding, fault rock can have various proportions of coarse and fine grained material, which defines property and type of fault rocks.

Figure 1. Outcrop photo of a fault composed of fault core (border marked with black line) as well as surrounding damage zone. Slip surfaces (outlined in blue) can be observed in the fault core.

The fault core, along with other fault components, impacts the permeability of the host rock in complex ways. For more discussion, go to the section 'Porosity and Permeability of Faults.'