Toward buoyancy-driven flow at Campi Flegrei: coupled phase change and asymmetric geometry

Abstract

Bradyseism at Campi Flegrei is usually interpreted in terms of hydrothermal pressurization and magmatic degassing. Fluid flow, often treated as a passive response to pressure accumulation, is commonly modeled using simplified geometries and homogeneous permeability fields. We introduce a model in which phase transition, structural heterogeneity and geometric asymmetry jointly influence fluid flow and pressure distribution within a heterogeneous subsurface environment. We hypothesize that coupling among phase change, density gradients and flows may follow a mechanism similar to the self-propulsion observed in asymmetric floating bodies like melting ice blocks, where phase change generates buoyancy-driven currents along their inclined surfaces and net motion in the opposite direction. We simulate pressure evolution in a shallow gas-rich reservoir subject to time-dependent forcing and hydraulic relaxation, coupled to buoyancy-enhanced Darcy flow along prescribed preferential pathways. Our numerical simulations, grounded in reported deformation rates and seismicity depths at Campi Flegrei, reproduce temporal variations in uplift and the persistence of spatially localized flow. Within this framework, asymmetric geometry may promote channelized upward transport, while phase change may enhance buoyancy and contribute to pressure redistribution. Our model predicts nonlinear uplift acceleration, shallow localized seismicity and velocity scaling with pressure and buoyancy. Integration with existing multiphase models would enable the examination of how buoyancy-driven flows influence pressure evolution and deformation during volcanic unrest.