Surface traces of the Minnivellir, Réttarnes and Tjörvafit seismic faults in the South Iceland Seismic Zone: Segmentation, lengths and magnitude of related earthquakes

Abstract

Accurate GPS mapping and field measurements have been carried out along the surface traces of three N–S right-lateral historical and prehistorical seismic strike-slip faults in the South Iceland Seismic Zone: the Minnivellir, Tjörvafit, and Réttarnes faults. We used these data and those previously collected on the Selsund and Leirubakki faults to characterise the faulting behaviour. This study illustrates a characteristic multi-scale segmentation including, from the longer to the smaller fault elements: (i) fault (corresponding to the total length of the seismic fault), (ii) fault segments, (iii) arrays of fractures and (iv) individual fractures. The most distinctive feature of such right-lateral fault is its left-stepping en-échelon arrangement at various scales, with push-up structures accommodating the motion in the relay zones. This study also highlights the role of the nature and thickness of the uppermost layer (lava flow, thin or thick grassy soil) with respect to the type and length of fractures at the surface. Our mapping reveals complex fault geometry which, however, is consistent with the general left-lateral transform behaviour of the South Iceland Seismic Zone. Some left-lateral conjugate fault branches are present along these major right-lateral faults and participate to the deformation. The estimation of the strike-slip offsets (1.3–2 m) based on the analysis of push-up structures allow to infer the total rupture lengths (45–70 km) for these faults and to estimate the magnitudes (7.0–7.2) of the related earthquakes, bringing confirmation that events significantly larger than those of June 2000 (Ms = 6.6) have occurred in the SISZ, which plays an important role for the seismic hazard assessment.