Task 2.2: Seismotectonic analysis of individual faults

The Tjörnes fracture zone

The study of the Húsavík-Flatey fault in the Tjörnes fracture zone includes a tectonic analysis (see Subsection 3.6.1.2) and also a seismotectonic analysis in order to obtain a reconstruction of the stress patterns in accordance with the mechanisms of the transform zone since about 7 million years to the present (Garcia 1999; Garcia et al. 2000; Garcia et al., in preparation). We carried out an analysis of 669 double-couple earthquake mechanisms (period 1995-1997, magnitude ranging between 1 and 4.8). Three main sets of stress regimes have been identified, each including three individual stress regimes (Figure 35). The major one corresponds to an ENE-WSW trending right-lateral transtension. The two other regimes, less important, correspond to transform-parallel (WNW-ESE) and transform-perpendicular extensions (NNE-SSW).

 

Figure 35: Focal mechanisms of earthquakes characterizing the nine stress regimes in the Tjörnes fracture zone (SSR: strike-slip regime, NR: normal regime, RR: reverse regime) (after Garcia 1999; Garcia et al., in preparation). For the SSR2, SSR3, NR3 and RR4, only the focal mechanisms with M>1.8 are shown, for the other regimes all the focal mechanisms (M>1) are shown.

Considering the angle between the trends of the rift and of the transform zone involves an extension occurring throughout the transform zone. This extension is accommodated by the transtensional regime, as well as, locally, by the extension sub-perpendicular to the transform direction. The extension sub-parallel to the transform direction may express by pull-apart process or by locking of part of the transform fault. The analysis of focal mechanisms of earthquakes indicates that most of these regimes currently occur, invalidating as well the hypothesis of polyphase tectonism (see Subsection 3.6.1.2). It suggests that transform motion along an oceanic fault zone may induce a variety of tectonic regimes. A major point of the transform mechanism is the variation of coupling along the HFF. These changes may correspond, at least partly, to the necessity of extension in this area because of the obliquity of the transform zone versus the rifts zones of N-Iceland and of Kolbeinsey, but the occurrence of magmatism at depth probably plays an important role also in such a phenomenon.

The Leirubakki fault

The Leirubakki fault is one of the large N-S seismic faults of the South Iceland seismic zone, located in its eastern part, a few kilometers west of the 1912 earthquake major earthquake. The Leirubakki rupture occurred earlier during historical times (maybe before the settlement of Iceland). Our work included the reconnaissance of the fault trace based on aerial photograph analysis and field study, a GPS mapping to reconstruct the morphology of the rupture zone, and some measurements of structures along the fault, including orientations and amplitudes (Bergerat et al. 2000; Bergerat et al., in preparation). We thus identify a pattern of dextral segments that connect typical push-up structures, with specific angular relationships between these features and the general trend of the earthquake fault (Figure 36). The fracturing process involved development of near-surface strike-slip segments, oblique relative to the underlying shear zone (Figure 36). We computed the shortening amounts and rock volumes involved in push-up development and estimated the magnitude of the Leirubakki fault to more than 7.

 

Figure 36: The Leirubakki fault in the SISZ. On the left: detailed part of the map (drawn after GPS measurements) showing a typical array of individual fractures and individual push-ups. On the right: schematic view of a push-up and en-echelon fractures in the uppermost part of the crust and a single right-lateral strike-slip fault at depth (after Bergerat et al. 2000; Bergerat et al., in preparation).