Gravity and height variations connected with the current rifting episode in Northern Iceland

Abstract

Following a first gravity survey in 1938, high-precision gravity measurements with LaCoste-Romberg gravimeters have been carried out in northern Iceland, in 1965, 1970/ 1971 and 1975, in order to detect eventual secular gravity variations at that supramarine part of a constructive plate boundary. Since 1975, when an active rifting episode started there, gravity and height variations have been observed in annual intervals. These surveys concentrated on the approx. 100 km long and 5 km wide fissure zone associated with the Krafla central volcano, which is the center of the present activity. The observations, which have been performed at an accuracy level of ± 0.01 mGal, resp. ± 0.03 m, give significant information about the long-term behaviour of the vertical mass shifts, occuring during the rifting process. A steep gravity-variation gradient, observed south of the Krafla caldera between 1965 and 1975, may be interpreted as some kind of precursor of the current process. Since 1975 the rifting event is characterized by shifting activity areas, with gravity decrease and elevation at the flanks, and gravity increase and subsidence at narrow central parts of the Krafla fissure zone, the annual variations reaching ± 0.1 mGal, resp. ∓ 0.5 m and more. Since 1978, the activity has concentrated on the Gjástikki region, north of the Krafla volcano, with decreasing variations in the southern and northern parts of the fissure swarm. The average gravity—height-variation ratio calculated for two profiles south and north of Krafla, over the observation periods between 1975 and 1979, amounts to −0.2 mgal/m, but extreme values are found at recently activated areas.