Implications of fracturing mechanisms and fluid pressure on earthquakes and fault slip data in the east Iceland rift zone

Comparison of the eroded off-rift zone left inactive by plate motion with the inner active seismic rift zone allows us to constrain the fracturing mechanisms. In eastern off-rift zone, we measured 423 fault slips (including normal and strike-slip faults). Inversion of fault slip data reveals the parallelism of the minimum stress (σ3) computed for the normal and for the strike-slip faulting, and consistency with the direction of plate divergence. North of the Vatnajökull, in the active rift zone, we analysed 14,250 earthquakes recorded by the Icelandic Meteorological Office (IMO) between 2004 and 2009, especially northeast of the Askja volcano. Two main average focal depths were determined at 5 km (Herðubreið table mountain) and 15 km (Upptyppingar hyaloclastite ridge). The double couple focal mechanisms determined by IMO revealed that more than half of the mechanisms are strike-slip. Faulting type both in active and off-rift zone reveals an unusual importance of strike-slip regime in such an extensional tectonic context. This can be explained by stress permutations (σ1/σ2). Similarities in terms of stress orientations and type of faulting are observed both in the old and present-day rift zones. We assume that the seismic events may be generated by rapid deep magma intrusion, also associated with shallower hydrothermal activities. We thus propose that the presence of fluids, shear failure (double couple focal mechanism) and stress permutations in both active and off-rift zones are closely linked. Assuming a Drucker–Prager failure criterion, we evaluated analytically that a state near of lithostatic pore pressure is a necessary condition for shear failure at shallow (5 km) and deep (15 km) depth, in a simple context of crustal extension that allows for stress permutations. However, processes favouring stress permutations cannot be further constrained from our observations since the dynamics of fluid, materiel heterogeneity and post-glacial rebound can also play a significant role.

► The mechanisms of fracturing in the rift zone and off-rift zone (corresponding to the old rift zone) in east Iceland have been characterised by focal mechanisms of earthquakes and fault slip data analyses, respectively. The main direction of extension in both cases is close to ESE–WNW.
► Mechanisms of fracturing in both zones illustrates a remarkable contrast between normal faulting and unusual importance of strike slip faulting in such a typical rifting zone.
► Stress permutation can explain how the coexistence of both normal and strike slip faulting can occur in a constant tectonic setting (switching between σ1/σ2).
► The distribution of the 14,250 seismic events analysed in the active rift zone occur at two distinct mean depth: 5 and 15 km around Herðubreið region and Upptyppingar region respectively. Regarding the nature of seismic events, they may be generated by rapid deep magma intrusion and shallow hydrothermal activities.
► A state of near lithostatic fluid pore pressure may be a necessary condition for shear failure at depths at which seismic events cluster. However processes favouring stress permutation cannot be further constrained from our observations since the dynamics of fluid, material heterogeneity and post-glacial rebound can also play a significant role.