Secondary earthquake fractures generated by a strike-slip fault in the South Iceland Seismic Zone

Most large earthquakes in the South Iceland Seismic Zone occur on N-S trending dextral strike-slip faults. The resulting rupture zones display complex en echelon patterns of secondary structures including NNE trending arrays of (mostly) NE trending open fractures and hillocks. Three spatial scales characterize the surface faulting pattern: the length of the main fault (MF similar to 10(4) m), the arrays of fractures, here interpreted as surface evidence of subordinate faults (SF similar to 10(2) m), and the individual open fractures (OF similar to 10 m). In order to improve our understanding of the genetic relationship between the OF and the MF we computed the stress field induced by slip on a buried MF using a dislocation model in a layered half-space where the fault surface is assumed to be embedded in the basement rock, topped by a softer, tens of meters thick near-surface layer. The OF were modeled as secondary fractures of either of the following types: (1) pure mode I cracks opening in the near-surface layer in the direction of the tensile principal stress or (2) mixed-mode cracks, slipping at depth as shear cracks and opening near the surface due to low confining pressure. Assuming that both types of secondary fractures occur in the SISZ, we can explain their first-order characteristics by reasoning that they are the end product of a hierarchy of deformation produced by the main fault. The Coulomb failure function after the earthquake (obtained by summing the MF stress change and the lithostatic stress) suggests that subordinate faulting (SF) must be expected to occur in response to the main rupture below the upper soft layer, extending to a depth of similar to 100 m. The total stress change induced by the MF and the SF is shown to yield quantitative explanations of the spatial extent, the "double en echelon" arrangement and the orientation of surface fractures observed in the field in terms of a simple frictional model of rupture.