Abstract
Ground tilt measurements demonstrate that Askja is in a state of unrest, and that in the period 1988–1991 a maximum 48±3 μrad tilt occurred down towards the centre of the caldera. This is consistent with 126 mm of deflation at the centre of the caldera with a 2.5–3.0 km depth to the source of deformation. The volume of the subsidence bowl is 6.2x106 m3. When combined with high precision microgravity measurements, the overall change in sub-surface mass may be quantified. After correction for the observed elevation change using the free air gradient of gravity measured for each station, the total decrease in mass is estimated to be less than 109 kg. A small residual ground inflation and net gravity increase in the southeastern part of the caldera may be caused by dyke intrusion in this region. The minimum dimensions of such an intrusion or complex of intrusions are 1 m width, up to 100 m deep and up to several hundred metres thick.
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