Numerical models of restless caldera volcanoes

Abstract

Caldera volcanoes undergo cycles of inflation and deflation on both long and short timescales. This study uses the Distinct Element Method to model numerically a pressurized magma reservoir within a brittle host rock in order to investigate the effects on these cycles of: (i) host rock strength, (ii) strain localization, (iii) fault reactivation and (iv) surface deformation. Additionally, the critical over- or under-pressure within the magma reservoir - i.e. the pressure prior to reservoir roof failure leading to eruption or collapse - is determined. The model results indicate that with increasing number of inflation/deflation cycles, the host rock weakens and strain is increasingly localized onto faults that are reactivated during both inflation and deflation. Consequently, both the critical over- and under-pressure magnitudes for eruption or collapse decrease from cycle to cycle. The relationships of surface uplift and subsidence with magma reservoir over- and under-pressure, respectively, display hysteresis behavior. This means that the magnitude of elevation change as a function of the change of magma pressure differs for inflating and deflating magma reservoirs. These results provide new insight into the relationships between structural development, elevation change, pressure change, and the generation of potential eruptive pathways at caldera volcanoes.