Stress redistribution following unwelding of near-surface layers in strike-slip fault zones

Major strike-slip earthquakes are often associated with a complex pattern of surface fractures, organized in hierarchic order (en-echelon shear fracture arrays, open tensile fractures and hillocks), which are oriented differently from the strike of the main fault and often extend laterally 1 or 2 km. In the attempt to understand why the main fault remains conned at depth, we have considered the possibility that, during the earthquake, a soft shallow sedimentary layer unwelds from the lower basement rock along a horizontal interface. The stress redistribution provided by the unwelding process in the shallow layer is studied in terms of a dislocation model employing the boundary-element method. We show that a nearly uniform Coulomb failure function develops above the main fault, over a km-wide strip along the fault strike (explaining shear fracture arrays) and positive tensile stresses appear near the surface over a similarly wide strip (explaining open fractures). A parametric study is performed to show when the unwelding process can take place and how the resulting stress redistribution in the shallow layer depends on the depth of the unwelded layer and on the coefficient of friction.