Task 1: Inferences on the regional stress field from the study of secondary earthquake fractures

Start: March 1996 (month 1)
End: June 1997 (month 16)
Responsible partner: UBLG.DF
Cooperative partner: NVI

Most earthquakes in the South Iceland seismic zone (SISZ) occur on NNE-trending dextral and ENE-trending sinistral strike-slip faults. 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 $10-30^{\circ}$ (Figure 29). Modeling indicates 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 $^{\circ }$ (if the prestress dominates with respect to the seismic stress) and 45 $^{\circ }$ (if the prestress is negligible), assuming that faulting occurs according to the Coulomb-Navier failure criterion and the prestress is purely deviatoric. If the arrays consist of mixed-mode cracks, the angle between the fault strike and individual cracks is lower than 22.5 $^{\circ }$ , this value being attained if the seismic stress dominates.

**Figure:** *See text for discussion.*
$\begin{figure}% latex2html id marker 2538 \refstepcounter{myfigure} \begin{cente... ...nal/bonafede/figure-1.eps, angle=-90, width=\textwidth }\end{center}\end{figure}$

Modelling suggests 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 modelling. Results have been submitted for publication [], and were discussed in several meetings and workshops [,,,,]. Further results, on the role of rheological properties in triggering large aftershocks have been published [].