Geodetic measurements of surface deformations during the present rifting episode in NE Iceland

Abstract

Iceland is part of the accreting plate boundary between the American and European plates. Since 1965 a number of repeated triangulation‐trilateration surveys spanning the neovolcanic zone in NE Iceland were made in order to detect crustal movements. All the measurements for a single observation epoch are separately adjusted by the method of least squares. Statistical tests are carried out to check the observations and adjustment models for outliers and model errors. To detect significant movements in repeatedly measured geodetic networks with multivariate design, a testing rule is described. After unexpected horizontal contractions of the surface of approximately 0.5 m from 1965 to 1971, enormous deformations have been found since the beginning of a major rifting episode in December 1975. During the period from 1971 to 1980 a horizontal expansion of about 7.5 m and vertical displacements of 3 m were observed across the approximately 3‐km‐wide active rifting zone of the Krafla fissure swarm. In adjacent zones, compressional strains of up to 2×10−4 occurred, rapidly decreasing to a value of 1×10−5 at a distance of 40 km from the rifting axis. After 1980 the pattern of ground deformation has altered. The east‐west shifts have decreased and are superposed by a radial component of motion around the Krafla caldera. West of the Krafla fissure swarm a shear zone is found.