Reverse and normal drag along a fault

Abstract

An analysis of the theoretical displacement field around a single dip-slip fault at depth reveals that normal and reverse fault drag develop by perturbation flow induced by fault slip. We analytically model the heterogeneous part of the instantaneous displacement field of an isolated two-dimensional mode II fault in an infinite, homogeneous elastic body in response to fault slip. Material on both sides of the fault is displaced and ‘opposing circulation cells’ arise on opposite sides of the fault, with displacement magnitudes increasing towards the center of the fault. Both normal and reverse drag can develop at the fault center depending on the angle between the markers and the fault; normal drag develops there for low angles (<30–40°) and reverse drag for higher angles. A comparison of the theoretical results with published models and natural examples reveals that the characteristics of normal and reverse fault drag are largely insensitive to the scale and rheology of the faulted rocks and that drag forces generated by frictional resistance need not be the primary cause of fault drag. Fault drag has some interesting geometric implications for normal and reverse fault terminology emphasizing the importance for discrimination of vertical separation and throw. Furthermore, our results lead us to propose an alternative model for the formation of rollover anticlines above normal faults.