INFERENCES ON THE REGIONAL STRESS FIELD FROM THE STUDY OF SECONDARY EARTHQUAKE FRACTURES

M.E. Belardinelli(*), M. Bonafede(**) and A. Gudmundsson(***)

*   Istituto Nazionale di Geofisica - Via di Vigna Murata 605, I-00182 Roma
**  Dipartimento di Fisica, Viale Berti-Pichat 8, I-40127 Bologna 
*** Nordic Volcanological Institute, Grensasvegur 50, IS-108 Reykjavik

ABSTRACT.

Most earthquakes in the South Iceland Seismic Zone occur on NNE-trending dextral and ENE-trending sinistral strike-slip faults. The largest earthquakes generated by these faults reach Ml=7.1-7.5 and occur at average intervals of 80 years. Many of the earthquake fractures rupture the surface in basaltic (pahoehoe) lava flows of Holocene age. The resulting rupture zones display complex en-echelon patterns of secondary structures including arrays of (mostly) NE-trending fractures and hillocks (push-ups). The field data indicate that the arrays consist of both mixed-mode cracks and pure mode-I cracks, concentrated in a narrow belt trending in the direction of the strike-slip faults in the Pleistocene bedrock buried by the Holocene lava flows. For the dextral faults, the angle between the strike of the fault array and the strike of individual secondary fractures ranges over several tens of degrees, but is commonly 10-30°. In order to improve our understanding of the genetic relationship between the secondary earthquake fractures and the underlying faults, the stress field induced by slip on a buried dextral fault was computed 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 secondary earthquake fractures of the arrays were preliminarily considered as pure mode-I cracks opening in the near surface layer in the direction of the maximum (tensile) principal stress, determined by the superposition of the earthquake-induced stress field (seismic stress change) and an uniform regional stress field (prestress). 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 results indicate that if the arrays consist of pure tension (mode-I) fractures, the angle between the strike of the hidden fault and each tension fracture must be between 22.5° (if the prestress dominates with respect to the seismic stress) and 45° (if the prestress is negligible), assuming that faulting occurs according to the Coulomb-Navier failure criterion. If the arrays consist of mixed-mode cracks, the angle between the fault strike and individual cracks is lower than 22.5°, this value being attained if the seismic stress dominates. We suggest that all fractures, being narrowly concentrated near the fault strike, form as a consequence of slip-induced local stresses during major earthquakes, small angle fractures being predominantly mixed-mode cracks, while higher angle fractures may be pure mode-I cracks. The role played by the regional prestress field is found to be significantly dependent on the rigidity contrast between the shallow layer and the basement rock. Useful inferences on the regional stress field can be extracted from such modeling.