Surface deformation and fault displacement associated with an earthquake swarm in Iceland

Repeated precision leveling across a fracture zone in southwest Iceland showed significant surface deformation over a period of three years, 1966 to 1969. Most of the observed deformation is assumed to have taken place during an earthquake swarm in September 1967, originating about 25 km to the southwest of the leveling profile. The observed deformation includes a dip-slip displacement of about 6 mm on the fault Hrafnagjá, and subsidence of up to 8 mm of a narrow zone centered near the same fault. The rate of change of deformation away from the fault shows that the dip-slip displacement is confined to a thin surface layer, probably less than 500 meters thick, and the subsidence can be explained as resulting from tensional displacement of about 7 mm in the fault zone between 400 and 3000 meters depth. During the year 1968 to 1969 the Hrafnagjá fault zone rose slightly, probably 1 to 2 mm, indicating a reversal of the subsidence observed during the previous two years. It is suggested that this uplift is due either to local heating caused by change of flow of ground water and associated thermal expansion or to lateral compression triggered by an earthquake of magnitude 6 that occurred on December 5, 1968, about 30 km east of the leveling profile.