Multi-Phase Fluid Circulation and Ground Deformation: a New Perspective on Bradyseismic Activity at the Phlegrean Fields (Italy)

Abstract

Like many caldera structures around the world, Phlegrean Fields caldera (Italy) periodically undergoes volcanic unrest, characterized by seismic activity and vertical ground displacement (bradyseism). These bradyseismic crises are usually interpreted as the product of pressure increment at the magma chamber level, but the existence of an important hydrothermal system in the area suggests that the migration of hot hydrothermal fluids should also play a role in the process. A complete description of the processes requires therefore a coupled analysis, accounting for both the flow of hot, multi-phase fluids and for the deformation of the porous medium. Here we applied the coupled thermo-hydro-mechanical model (TOUGH-FLAC) to study the role of hydrothermal circulation in ground deformation episodes at the Phlegrean Fields. Based on previous modeling results, we calculated the effects of an increased magmatic degassing on the deformation of a shallow elastic porous medium. Our results show that a short period of increased injection of deep fluid into the shallow hydrothermal system can represent a potential trigger for bradyseismic events, even in absence of a new magmatic recharge. Results also provided interesting insights on the complex interaction between fluid migration and ground deformation when phase changes are allowed. The comparison between these preliminary results and field data shows that the model captures both the temporal evolution of ground displacement and the compositional variations of fumarolic gas recently observed at the Phlegrean Fields.