Geometry and segmentation mechanisms of the surface traces associated with the 1912 Selsund Earthquake, Southern Iceland

The Selsund fault is a major N-S seismic fault of the E-W South Iceland Seismic Zone (SISZ), a left-lateral transform zone in Iceland. The Selsund earthquake, May 6th 1912, was the first instrumentally recorded earthquake (M = 7) in Iceland. To characterise the geometry of the surface deformation, we carried out field measurements and kinematic GPS mapping along the 9-km-long fault traces. A co-seismic displacement of about 2.4 m along the fault has been determined from the analysis of push-up structures. Empirical relationships suggest a rupture length of about 44 km to produce such a displacement. The pattern of surface traces reveals left-stepping en-échelon segmentation at three different scales: (1) segment, (2) array of fractures and (3) individual fracture. The segments make an average angle of 10-30° clockwise with respect to the general N-S trend of the Selsund fault. The angle between the fractures arrays and the segments is 4-10° clockwise. The individual fracture trends show a clockwise rotation of 20-30° with respect to that of the fracture array. Based on mechanical considerations, we propose a mechanism for the development process of en-échelon ruptures. Faults and fractures nucleate above a buried fault and propagate from their nucleation points. The spacing of the nucleation points depends on the depth and rheological properties of the upper layers. Interaction between propagating cracks allows maximum overlapping, as a function of the crack separation. The final crack length thus depends on the initial separation imposed by the initial distance between nucleation points, and hence by the thickness, mechanical behaviour and discontinuities of the upper faulted layers.