This task was included in the PRENLAB-1 workprogramme and was not originally included within the PRENLAB-2 workprogramme. The latest developments of this topic, however, lead us to include the main results in the present report. Most 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 (O.F.) and hillocks.
Three spatial scales characterize the surface faulting pattern: the length of the main fault (M.F. 104 m), the arrays here interpreted as surface evidence of secondary faults (102 m) and the individual O.F.(10 m). In order to improve our understanding of the genetic relationship between the O.F. and the M.F. we computed the stress field induced by slip on a buried M.F. using a dislocation model in a layered half-space: the fault surface is assumed to be embedded in the basement rock, topped by a softer near-surface layer. The O.F. were preliminarily considered as pure mode-I cracks opening in the near surface layer in the direction of the maximum (tensile) principal stress. Alternatively, secondary fractures were interpreted, as mixed-mode cracks, slipping at depth as shear cracks and opening near the surface due to low confining pressure. The Coulomb failure function after the earthquake (obtained summing the M.F. stress change and the lithostatic stress) suggests that secondary faulting (S.F.) can be expected to occur in response to the main rupture below the upper soft layer down to few hundreds of meter depth. The total stress change induced by the M.F. and the S.F. (of smaller scale) is shown to yield quantitative explanations of the complex geometry observed in the fault region in terms of simple frictional laws and friction coefficients very close to those measured in the lab (Figure 26).