Deflation during caldera unrest: constraints on subsurface processes and hazard prediction from gravity–height data

Abstract

We have evaluated published gravity–height (Δg/Δh) data on Campi Flegrei, Kilauea, Askja and Krafla, in order to discriminate between subsurface processes during caldera subsidence. With respect to end member gravity–height correlations, such as the free air gradient (FAG) and the Bouguer corrected free air (BCFAG), Δg/Δh gradients must be interpreted in terms of subsurface mass redistribution, density changes or some combination of these. Δg/Δh gradients during subsidence plot (1) along or below the BCFAG, (2) between the BCFAG and the FAG or (3) along or above the FAG. We have evaluated each of these three regions in terms of subsurface processes during volcano subsidence. We have interpreted Δg/Δh gradients as possible indicators of precursors of volcanic activity and propose that gravity–height surveys may help to detect precursors of caldera collapse caused by magma drainage. In this context, the 1875 eruption of Askja in Iceland has been re-interpreted in terms of the beginning of the eruptive episode being induced by roof collapse of an evacuating magma chamber. Based on other examples of recent volcanic roof collapses, we evaluate the contribution of gravity–height surveys in assessing volcanic risks during caldera subsidence. Caldera-forming eruptions are environmentally and economically the most devastating volcanic events. Inflation is usually considered to be an important precursor to activity. Here, we show that deflation may be associated with the trigger mechanism for caldera-forming explosive eruptions.