Smear sourced from shale or clay is thought to provide an across-fault barrier to fluid flow in sedimentary rocks. Permeability changes of this phenomenon were investigated using a triaxial testing machine. Experimental specimens consisting of interlayered siltstone (low initial permeability, ∼10⁻¹⁶ m²) and sandstone (high initial permeability, ∼10⁻¹³ m²) were subjected to 20, 30, and 40 MPa of effective normal stresses on a precut surface, dividing each specimen at a 30° angle to its axis, at axial shortening velocities between 0.14 and 1.41 μm s⁻¹. Permeability was measured by the oscillation method, and permeability changes categorized in three distinct regimes that corresponded to progressively increasing rock deformation: regime 1, rapid reduction due to compaction of the siltstone layer prior to fault movement; regime 2, constant and minimum permeability while the smear developed; and regime 3, permeability recovery caused by smear thinning and loss of smear continuity. The duration of regime 2 and of smear continuity recorded provide measures of the sealing potential produced by the smear. The shale smear factor SSF, defined as (fault throw)/(thickness of low-permeability layer), shows that there may be a relation between seal potential and effective normal stress. Both factors in SSF can reach higher values when effective normal stress is 40 MPa than when that is ≥30 MPa.