We have analysed the seismicity that occurred at Campi Flegrei caldera (Italy) in the period 1982--1984, during an unrest episode. Earthquake locations describe a system of inward dipping ring faults. Focal mechanisms of such events show a normal dip component, which is not in agreement with the differential uplift of the inner caldera, as defined by static ground deformations. We have performed a simulation of the stress field generated by overpressure in a magma chamber in presence of lateral discontinuities, using a boundary element method. Lateral discontinuities simulate the ring fault system marking the border of the inner caldera collapse. Results allow us to hypothesize that reverse fault slip on the ring fault is mainly aseismic, and such aseismic movement is able to focus normal fault shear stress along the lateral discontinuities. Aseismic slip on the ring fault in response to static deformation is also supported by the low seismic moment released (M0≅1015 Nm), about two orders of magnitude lower than expected from the shear slip on the discontinuities needed to accomplish the total static surface deformation (1.8 m). Such results have been compared with observations at Rabaul caldera, during a similar unrest episode. In this area, the seismic moment release is in good agreement with shear slip produced on a system of outward dipping ring faults, and seismicity is much more focused on the fault structures. Such a different behaviour can be interpreted, in the framework of our model, as due to the different sign of the stress normal to the ring faults, for inward and outward dip. The comparison between the two areas shed new light about the dynamics of earthquakes in calderas, in terms of the role played by ring fault systems.¿ 1997 American Geophysical Union