In his classic 1965 GSA Bulletin paper “Origin of ‘Reverse Drag’ on the Downthrown Side of Normal Faults” Hamblin presented a conceptual model linking the formation of reverse drag (the down-warping of hanging wall strata toward a normal fault) to slip on faults with listric (curved, concave up) cross-sectional profiles. Although this model has been widely accepted, some authors have noted that reverse drag may also form in response to slip on planar faults that terminate at depth. A universal explanation for the origin of reverse drag, a common element of extensional terranes, thus remains elusive almost 50 years after Hamblin’s seminal paper on the subject.
In order to better understand the patterns of deformation around normal faults and their relationship to fault geometry, we have implemented a suite of numerical models with a range of cross-sectional fault geometries from perfectly planar to highly listric. These models suggest that the key to inferring subsurface fault geometry from surface deformation patterns lies in the footwall rather than the hanging wall. The width of up warping in the footwall is sensitive to the fault curvature while the width of down-warping in the hanging wall is sensitive to the depth to detachment or lower fault tip, but relatively insensitive to fault shape.
By applying these numerical models in an inverse sense we can infer subsurface geometry of faults based on patterns of deformation from single earthquakes or accumulated over millions of years. Earthquake examples from the Basin and Range (USA), Greece, and Italy suggest that active faults may have a range of down-dip profiles from nearly planar to listric. Geologic examples from the western margin of the Colorado Plateau (USA) reveal that faults that have previously been interpreted as listric due to the presence of reverse drag folds may actually be planar.