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Ground deformation in volcanic areas is linked to various, often interconnected processes such as magma intrusion, pressurized fluid migration, and thermal expansion effects. The presence of active and extended hydrothermal systems plays a key role and affects the deformation phenomenon in complex ways. In this study, we propose a generalized conceptual and mathematical model, which allows retrieving the flow rate of fluid injection in a volcanic hydrothermal system, assuming a ground deformation data set as input. The basic assumption is that short‐term ground uplift episodes (with characteristic periods of less than 5 years) depend on the injection of volcanic fluids into the hydrothermal system. Then, assuming a deformation field shape independent of time and a linear time‐invariant relation between the amount of injected fluid and the resulting ground deformation, we define a Green's function as the product of spatial and temporal components. The case study is a 3‐D elastic model with permeability and porosity for the Campi Flegrei caldera, Italy. By Green's function, a 2‐km‐long source at 2.4‐km depth, which matches the interferometric synthetic aperture radar deformation, is localized and the amount of injected volcanic fluid in the last 20 years of high‐frequency deformation episodes estimated. In conclusion, we find a good agreement between the measured and estimated temporal deformation patterns and, principally, that fluid injection rates can be retrieved from the deformation field at volcanoes. Key Points: What kind of physical process could produce ground short‐term bradyseismic oscillation in calderas unrest?Linear time invariant approximation applied to multiphase thermo‐fluid dynamical evolution of the Campi Flegrei hydrothermal systemShort‐term deformation source parameters estimated by Green's function‐based inversion of the ground deformation at Campi Flegrei caldera [ABSTRACT FROM AUTHOR]

SUMMARY Campi Flegrei collapse caldera (Italy) is a high-risk volcanic area located close to Naples and includes part of the densely populated city. This area is characterised by large up and down ground displacements. The last large uplift episode caused 3.5 m of cumulative vertical displacement at the centre of the town of Pozzuoli, during the period 1969-1984. Up and down ground movements in this area often occur without intercurring eruptions and are similar to what is observed at other calderas worldwide. Here, however, they appear more evident and amplified. Understanding the mechanism of such movements is crucial for hazard assessment and eruption forecast, mainly due to this densely populated area. This paper presents a detailed model for ground displacements due to deep fluid injection in shallower layers. Such a model explains in a natural way the occurrence of uplift and subsidence without eruptions. We show that it is possible to fit observed ground deformation in this area with a thermofluid dynamical model. The model obtained is also consistent with other observations like microgravity changes, changes in CO2 flux, etc. Here, we suggest that significant uplift and subsidence at calderas can be due to effects of deep fluid injections other than magma. At Campi Flegrei, however, a partial magmatic contribution at the origin of the observed episodes cannot be excluded. [ABSTRACT FROM AUTHOR]