The South Iceland Seismic Zone is a roughly 70-kilometre transform belt across the lowlands between the Reykjanes and Eastern Volcanic zones. It transfers the offset between two spreading segments of the Mid-Atlantic Ridge as the North American and Eurasian plates pull apart at about 2 centimetres per year.
Although the overall motion is left-lateral, strain is released on a dense array of north-south right-lateral faults known as bookshelf faulting, rather than on one continuous east-west structure. This produces clusters of shallow earthquakes spread across the farmland of the Árnessýsla and Rangárvellir districts.
Historical destructive sequences struck in 1784 and 1896, and modern paired events on 17 and 21 June 2000 (both magnitude ~6.5) damaged farms near Hella and Selfoss. A further magnitude ~6.3 quake in May 2008 near Ölfus reminded planners that Iceland's most populated agricultural region faces repeated moderate earthquakes.