Unrest at Askja Caldera; Evidence from gravity and ground-deformation data

The combination of a divergent plate boundary and a mantle plume makes Iceland a unique place to study active volcanism. Askja caldera, situated in the northeast of Iceland, has been in a state of unrest for decades. This unrest occurs as a result of processes in a deep-lying magma chamber. Simultaneous measurements of ground deformation and gravity change at a few key stations may be used to identify magma chamber processes before conventional eruption precursors become apparent. Microgravity measurements have been made at Askja since 1988 using LaCoste &Romberg meters. By multiplying the elevation change by the observed free air gravity gradient, the predicted gravity changes can be calculated. These are then compared with the observed height-corrected gravity changes and any residual gravity change is interpreted in terms of sub-surface mass changes. Previous results (1988-1991) were interpreted with the use of a (g/(h diagram. Steep gradients for stations in the south-east of the caldera indicated a dyke intrusion, while data in other areas plotted below the Bouguer Corrected Free Air Gradient, corresponding to magma drainage. Preliminary results from 2002 suggest a net microgravity increase at Askja from 1988 to 2002. The observed microgravity change is less than the predicted gravity change. This implies an on-going sub-surface mass decrease. This could be explained by magma drainage from the chamber. Drainage removes magma, resulting in mass decrease and thus gravity decrease. Evolution of the (g/(h gradients at different locations can give considerable information about future activity. All areas measured at Askja show a change from a steeper gradient to a less steep gradient. This would suggest enhanced sub-surface mass decrease with time, which can be interpreted in terms of an increasing rate of magma drainage. Possible implications of this include renewed activity along other parts of the rift system.