The structure of seismogenic strike-slip faults in the eastern part of the Reykjanes Peninsula Oblique Rift, SW Iceland

Abstract

A considerable part of the seismic risk to the capital area of Iceland is provided by a series of strike-slip faults at the mid-Atlantic plate boundary within a distance range of 15–35 km. About two thirds of the Icelandic population lives within this distance range from the source areas of M 6–6.5 earthquakes. The structure of the plate boundaries in SW-Iceland is relatively complex. Three branches meet in the Hengill Triple Junction, the oblique rift of the Reykjanes Peninsula, the South Iceland Seismic Zone (a transform zone) and the Western Volcanic Zone (a rift zone). The Reykjanes Peninsula oblique rift has an over-all trend of 70°, highly oblique with respect to the spreading direction, 101° in this region. It contains both volcanic systems and seismogenic strike-slip faults. The fissure swarms of individual volcanic systems contain normal faults and fissures, with a NE-trend, also quite oblique to the plate boundary. The fissure swarms fade out towards the NE and SW as they extend into the plates on either side. Overprinting this pattern of volcano-tectonic structures are sets of parallel, northerly striking transcurrent faults that generate the largest earthquakes in this zone. Their surface expressions are en echelon fracture arrays and push-up structures. The sense of displacement is right-lateral. In the 15 km long section of the rift studied here we find evidence for at least six faults of this type. The length of individual faults may exceed 12 km. The distance between them varies from 1 to 5 km, and together they form a bookshelf-type fault system taking up the left-lateral component of plate movements across the oblique rift zone.