Ground deformation in Askja, Iceland: Its source and possible relation to flow of the mantle plume

Abstract

The volcano Askja in east-central Iceland erupted several times between 1920 and 1930, following a very large caldera-forming eruption in 1875. The last eruption occurred in 1961. Measurements of the vertical component of ground deformation in Askja were commenced in 1966 and made once each year, 1966 to 1972 and 1983 to 1987. Distance measurements with a geodimeter were first made in 1982. The measurements have shown alternating uplift and subsidence of the central part of the Askja caldera. Two periods of uplift and three periods of subsidence have been identified during the 21 years of observations. Observed vertical displacements agree fairly well with a single point source model at a depth of 1.5–3.5 km below the surface, near the center of the Askja caldera. Deviation from a single point source model can be explained by a secondary source of subsidence beneath the site of the 1961 eruption. This secondary source has been diminishing in intensity and appears to have been exhausted by 1983–1987. Askja has been subsiding at a rate of 6–10 cm per year during the period 1983–1987, but in 1970–1972 rapid inflation was observed possibly as much as 20 cm per year. The period of no levelings, 1972 to 1983, is characterized by deflation, but at a slower average rate than observed in 1983–1987. This suggests continuation of the 1970–1972 inflation into 1973 or 1974.