Task 1.1: The South Iceland seismic zone (SISZ)

We carried out a structural study in this area, including analysis of aerial photographs, local observation of major faults and collection of minor fault slip data in outcrops, as well as an analysis of the focal mechanisms of earthquakes (Bergerat et al. 1998; Bergerat et al. 1999; Bergerat and Angelier 2000). At the regional scale, the main fault trends are approximately NNE-SSW and NE-SW. ENE-WSW, NW-SE and WNW-ESE trending faults are also detected in aerial photographs and in the field. All these faults are normal or strike-slip in character. Some of the historical major earthquake fractures are observed in the post-glacial lava flows in the SISZ: most are right-lateral and trend roughly N-S. We analyzed more than 700 minor faults at 25 sites. Most sites are located in rocks of Upper Pliocene-Pleistocene age. Inversion of fault-slip datasets enabled us to reconstruct local paleostress tensors, hence to define the major tectonic regimes which have prevailed in the SISZ. Two main groups of faulting mechanisms reveal two distinct stress regimes, with perpendicular directions of extension, NW-SE (primary) and NE-SW (secondary). Both groups, however, display inhomogeneous datasets, related to extensional and to strike-slip faulting. The primary stress regime is in agreement with both the general behaviour of the SISZ as a left-lateral transform zone and the opening of the rift segments. The secondary stress regime, incompatible with the primary stress regime, is interpreted in terms of stress permutations. A population of 231 double-couple focal mechanisms (M>1 and depth>2 km) was also analyzed in terms of stress states. The results show great similarity in terms of stress directions. Figure 31 gives an example of a characteristic fault-slip data site and a comparison with focal mechanisms of earthquakes.

 

Figure 31: Comparison between the main stress regimes as indicated by fault-slip datasets and focal mechanisms of earthquakes in the SISZ, based on two case examples (after Bergerat and Angelier 2000). (A) Striated faults at site Króksholt. (B) Selected nodal planes and slip vectors of earthquakes throughout the SISZ for the years 1991-1994 and 1996, at crustal depths larger than 2 km, with magnitudes larger than 1.

The present-day stress field mainly inferred from analyses of earthquake focal mechanisms is consistent with the present behaviour of the SISZ as a left-lateral transform zone. However, the proportion of strike-slip faulting within the present-day seismic activity (71%) is significantly higher than that revealed by the geological observation of Quaternary faults (50%). This contrast is interpreted in terms of development and evolution of the transform fault zone. Figure 32 schematically presents a comparison between the paleostress results obtained from faulting analysis and the present day activity based on focal mechanisms of earthquakes analysis. Stress permutations between different regimes are shown as couples of open arrows. The paleostress fields identified in the Upper Pliocene-Pleistocene formations of the SISZ reflect both the previous behaviour of the area, when it was located inside the rift zone, and its present behaviour as a transform zone, thus illustrating the local evolution from rifting to transform motion.

 

Figure: The different stress tensors obtained from analyses of sets of fault-slip data and earthquake focal mechanisms in the SISZ (after Bergerat et al. 1999). Size of diagrams proportional to numbers of data. Three-, four-, and five-branched stars: regional axes of $\sigma _{3}$, $\sigma _{2}$ and $\sigma _{1}$ respectively (schematic). Couples of black arrows: directions of compression (convergent) or extension (divergent). Couples of open arrows: stress permutations ( $\sigma _{1}$/ $\sigma _{2}$ or $\sigma _{2}$/ $\sigma _{3}$ modes) between the different regimes.