Reverse drag: Host rock deformation during slip along existing faults

Faults are typically weaker than the surrounding host rocks, hence it is anticipated that loading a faulted rock body will cause slip along existing faults while the fault-bounded blocks should remain undeformed. We present here field, experimental, and numerical observations that profoundly deviate from the above concept. The field observations are from the Negev (southern Israel), on the western side of the Dead Sea rift, and are related to the intra-plate deformation. We found that (1) the host rock along existing faults may undergo significant deformation during slip along these faults; (2) the syn-slip strain may have the opposite sense of shear with respect to the sense of shear along the fault (commonly known as "reverse drag"); (3) the strain of the host rock adjacent to the faults increases with increasing fault slip; (4) this strain is restricted to a region extending 5-10% of the fault length on each side of the fault, and it decreases non-linearly with distance from the fault; and (5) the above deformation features were observed in host rock of elastic, viscous, or plastic rheology.